mod12
NONRESIDENT
TRAINING
COURSE
SEPTEMBER 1998
Navy Electricity and
Electronics Training Series
Module 12—Modulation
NAVEDTRA 14184
DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.
Although the words “he,” “him,” and
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DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.
PREFACE
By enrolling in this self-study course, you have demonstrated a desire to improve yourself and the Navy.
Remember, however, this self-study course is only one part of the total Navy training program. Practical
experience, schools, selected reading, and your desire to succeed are also necessary to successfully round
out a fully meaningful training program.
COURSE OVERVIEW: To introduce the student to the subject of Modulation Principles who needs
such a background in accomplishing daily work and/or in preparing for further study.
THE COURSE: This self-study course is organized into subject matter areas, each containing learning
objectives to help you determine what you should learn along with text and illustrations to help you
understand the information. The subject matter reflects day-to-day requirements and experiences of
personnel in the rating or skill area. It also reflects guidance provided by Enlisted Community Managers
(ECMs) and other senior personnel, technical references, instructions, etc., and either the occupational or
naval standards, which are listed in the Manual of Navy Enlisted Manpower Personnel Classifications
and Occupational Standards, NAVPERS 18068.
THE QUESTIONS: The questions that appear in this course are designed to help you understand the
material in the text.
VALUE: In completing this course, you will improve your military and professional knowledge.
Importantly, it can also help you study for the Navy-wide advancement in rate examination. If you are
studying and discover a reference in the text to another publication for further information, look it up.
1998 Edition Prepared by
ETCM Danny K. Krutson
Published by
NAVAL EDUCATION AND TRAINING
PROFESSIONAL DEVELOPMENT
AND TECHNOLOGY CENTER
NAVSUP Logistics Tracking Number
0504-LP-026-8370
i
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Constitution of the United States of
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I proudly serve my country’s Navy
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and commitment.
I am committed to excellence and
the fair treatment of all.”
ii
TABLE OF CONTENTS
CHAPTER
PAGE
1. Amplitude Modulation .............................................................................................
1-1
2. Angle and Pulse Modulation ....................................................................................
2-1
3. Demodulation ...........................................................................................................
3-1
APPENDIX
I. Glossary..................................................................................................................
INDEX
.........................................................................................................................
iii
AI-1
INDEX-1
NAVY ELECTRICITY AND ELECTRONICS TRAINING
SERIES
The Navy Electricity and Electronics Training Series (NEETS) was developed for use by personnel in
many electrical- and electronic-related Navy ratings. Written by, and with the advice of, senior
technicians in these ratings, this series provides beginners with fundamental electrical and electronic
concepts through self-study. The presentation of this series is not oriented to any specific rating structure,
but is divided into modules containing related information organized into traditional paths of instruction.
The series is designed to give small amounts of information that can be easily digested before advancing
further into the more complex material. For a student just becoming acquainted with electricity or
electronics, it is highly recommended that the modules be studied in their suggested sequence. While
there is a listing of NEETS by module title, the following brief descriptions give a quick overview of how
the individual modules flow together.
Module 1, Introduction to Matter, Energy, and Direct Current, introduces the course with a short history
of electricity and electronics and proceeds into the characteristics of matter, energy, and direct current
(dc). It also describes some of the general safety precautions and first-aid procedures that should be
common knowledge for a person working in the field of electricity. Related safety hints are located
throughout the rest of the series, as well.
Module 2, Introduction to Alternating Current and Transformers, is an introduction to alternating current
(ac) and transformers, including basic ac theory and fundamentals of electromagnetism, inductance,
capacitance, impedance, and transformers.
Module 3, Introduction to Circuit Protection, Control, and Measurement, encompasses circuit breakers,
fuses, and current limiters used in circuit protection, as well as the theory and use of meters as electrical
measuring devices.
Module 4, Introduction to Electrical Conductors, Wiring Techniques, and Schematic Reading, presents
conductor usage, insulation used as wire covering, splicing, termination of wiring, soldering, and reading
electrical wiring diagrams.
Module 5, Introduction to Generators and Motors, is an introduction to generators and motors, and
covers the uses of ac and dc generators and motors in the conversion of electrical and mechanical
energies.
Module 6, Introduction to Electronic Emission, Tubes, and Power Supplies, ties the first five modules
together in an introduction to vacuum tubes and vacuum-tube power supplies.
Module 7, Introduction to Solid-State Devices and Power Supplies, is similar to module 6, but it is in
reference to solid-state devices.
Module 8, Introduction to Amplifiers, covers amplifiers.
Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits, discusses wave generation and
wave-shaping circuits.
Module 10, Introduction to Wave Propagation, Transmission Lines, and Antennas, presents the
characteristics of wave propagation, transmission lines, and antennas.
iv
Module 11, Microwave Principles, explains microwave oscillators, amplifiers, and waveguides.
Module 12, Modulation Principles, discusses the principles of modulation.
Module 13, Introduction to Number Systems and Logic Circuits, presents the fundamental concepts of
number systems, Boolean algebra, and logic circuits, all of which pertain to digital computers.
Module 14, Introduction to Microelectronics, covers microelectronics technology and miniature and
microminiature circuit repair.
Module 15, Principles of Synchros, Servos, and Gyros, provides the basic principles, operations,
functions, and applications of synchro, servo, and gyro mechanisms.
Module 16, Introduction to Test Equipment, is an introduction to some of the more commonly used test
equipments and their applications.
Module 17, Radio-Frequency Communications Principles, presents the fundamentals of a radiofrequency communications system.
Module 18, Radar Principles, covers the fundamentals of a radar system.
Module 19, The Technician's Handbook, is a handy reference of commonly used general information,
such as electrical and electronic formulas, color coding, and naval supply system data.
Module 20, Master Glossary, is the glossary of terms for the series.
Module 21, Test Methods and Practices, describes basic test methods and practices.
Module 22, Introduction to Digital Computers, is an introduction to digital computers.
Module 23, Magnetic Recording, is an introduction to the use and maintenance of magnetic recorders and
the concepts of recording on magnetic tape and disks.
Module 24, Introduction to Fiber Optics, is an introduction to fiber optics.
Embedded questions are inserted throughout each module, except for modules 19 and 20, which are
reference books. If you have any difficulty in answering any of the questions, restudy the applicable
section.
Although an attempt has been made to use simple language, various technical words and phrases have
necessarily been included. Specific terms are defined in Module 20, Master Glossary.
Considerable emphasis has been placed on illustrations to provide a maximum amount of information. In
some instances, a knowledge of basic algebra may be required.
Assignments are provided for each module, with the exceptions of Module 19, The Technician's
Handbook; and Module 20, Master Glossary. Course descriptions and ordering information are in
NAVEDTRA 12061, Catalog of Nonresident Training Courses.
v
Throughout the text of this course and while using technical manuals associated with the equipment you
will be working on, you will find the below notations at the end of some paragraphs. The notations are
used to emphasize that safety hazards exist and care must be taken or observed.
WARNING
AN OPERATING PROCEDURE, PRACTICE, OR CONDITION, ETC., WHICH MAY
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CAUTION
AN OPERATING PROCEDURE, PRACTICE, OR CONDITION, ETC., WHICH MAY
RESULT IN DAMAGE TO EQUIPMENT IF NOT CAREFULLY OBSERVED OR
FOLLOWED.
NOTE
An operating procedure, practice, or condition, etc., which is essential to emphasize.
vi
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vii
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viii
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NEETS Module 12
Modulation Principles
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ix
CHAPTER 1
AMPLITUDE MODULATION
LEARNING OBJECTIVES
Learning objectives are stated at the beginning of each chapter. These learning objectives serve as a
preview of the information you are expected to learn in the chapter. The comprehensive check questions
are based on the objectives. By successfully completing the OCC/ECC, you indicate that you have met
the objectives and have learned the information. The learning objectives are listed below.
Upon completion of this chapter, you will be able to:
1.
Discuss the generation of a sine wave by describing its three characteristics: amplitude, phase,
and frequency.
2.
Describe the process of heterodyning.
3.
Discuss the development of continuous-wave (cw) modulation.
4.
Describe the two primary methods of cw communications keying.
5.
Discuss the radio frequency (rf) spectrum usage by cw transmissions.
6.
Discuss the advantages and disadvantages of cw transmissions.
7.
Explain the operation of typical cw transmitter circuitry.
8.
Discuss the method of changing sound waves into electrical impulses.
9.
Describe the rf usage of an AM signal.
10.
Calculate the percent of modulation for an AM signal.
11.
Discuss the difference between high- and low-level modulation.
12.
Describe the circuit description, operation, advantages, and disadvantages of the following
common AM tube/transistor modulating circuits: plate/collector, control grid/base, and
cathode/emitter.
13.
Discuss the advantages and disadvantages of AM communications.
INTRODUCTION TO MODULATION PRINCIPLES
People have always had the desire to communicate their ideas to others. Communications have not
only been desired from a social point of view, but have been an essential element in the building of
civilization. Through communications, people have been able to share ideas of mutual benefit to all
mankind. Early attempts to maintain communications between distant points were limited by several
factors. For example, the relatively short distance sound would carry and the difficulty of hand-carrying
messages over great distances hampered effective communications.
1-1
As the potential for the uses of electricity were explored, scientists in the United States and England
worked to develop the telegraph. The first practical system was established in London, England, in 1838.
Just 20 years later, the final link to connect the major countries with electrical communications was
completed when a transatlantic submarine cable was connected. Commercial telegraphy was practically
worldwide by 1890. The telegraph key, wire lines, and Morse code made possible almost instantaneous
communications between points at great distances. Submarine cables solved the problems of transoceanic
communications, but communications with ships at sea and mobile forces were still poor.
In 1897 Marconi demonstrated the first practical wireless transmitter. He sent and received messages
over a distance of 8 miles. By 1898 he had demonstrated the usefulness of wireless telegraph
communications at sea. In 1899 he established a wireless telegraphic link across the English Channel. His
company also established general usage of the wireless telegraph between coastal light ships (floating
lighthouses) and land. The first successful transatlantic transmissions were achieved in 1902. From that
time to the present, radio communication has grown at an extraordinary rate. Early systems transmitted a
few words per minute with doubtful reliability. Today, communications systems reliably transmit
information across millions of miles.
The desire to communicate directly by voice, at a higher rate of speed than possible through basic
telegraphy, led to further research. That research led to the development of MODULATION. Modulation
is the ability to impress intelligence upon a TRANSMISSION MEDIUM, such as radio waves. A
transmission medium can be described as light, smoke, sound, wire lines, or radio-frequency waves. In
this module, you will study the basic principles of modulation and DEMODULATION (removing
intelligence from the medium).
In your studies, you will learn about modulation as it applies to radio-frequency communications. To
modulate is to impress the characteristics (intelligence) of one waveform onto a second waveform by
varying the amplitude, frequency, phase, or other characteristics of the second waveform. First, however,
you will review the characteristics and generation of a sine wave. This review will help you to better
understand the principles of modulation. Then, an important principle called HETERODYNING (mixing
two frequencies across a nonlinear impedance) will be studied and applied to modulation. Nonlinear
impedance will be discussed in the heterodyning section. You will also study several methods of
modulating a radio-frequency carrier. You will come to a better understanding of the demodulation
principle by studying the various circuits used to demodulate a modulated carrier.
Q-1. What is modulation?
Q-2. What is a transmission medium?
Q-3. What is heterodyning?
Q-4. What is demodulation?
SINE WAVE CHARACTERISTICS
The basic alternating waveform for all complex waveforms is the sine wave. Therefore, an
understanding of sine wave characteristics and how they can be acted upon is essential for you to
understand modulation. You may want to review sine waves in chapter 1 of NEETS, Module 2,
Introduction to Alternating Current and Transformers at this point.
1-2
GENERATION OF SINE WAVES
Since numbers represent individual items in a group, arrows can be used to represent quantities that
have magnitude and direction. This may be done by using an arrow and a number, as illustrated in figure
1-1, view (A). The number represents the magnitude of force and the arrow represents the direction of the
force.
Figure 1-1A.—Vectors representing magnitude and direction.
View (B) illustrates a simpler method of representation. In this method, the length of the arrow is
proportional to the magnitude of force, and the direction of force is indicated by the direction of the
arrow. Thus, if an arrow 1-inch long represents 50 pounds of force, then an arrow 2-inches long would
represent 100 pounds of force. This method of showing both magnitude and direction is called a
VECTOR. To more clearly show the relationships between the amplitude, phase, and frequency of a sine
wave, we will use vectors.
+
Figure 1-1B.—Vectors representing magnitude and direction.
1-3
Vector Applied to Sine-Wave Generation
As covered, in NEETS, Module 2, Introduction to Alternating Current and Transformers, an
alternating current is generated by rotating a coil in the magnetic field between two magnets. As long as
the magnetic field is uniform, the output from the coil will be a sine wave, as shown in figure 1-2. This
wave shape is called a sine wave because the voltage of the coil depends on its angular position in the
magnetic field.
Figure 1-2.—Sine-wave generator.
This relationship can be expressed mathematically by the formula:
1-4
You should recall that the trigonometric ratio (inset in figure 1-3) for the sine in a right triangle (a
triangle in which one angle is 90 degrees) is:
When an alternating waveform is generated, the coil is represented by a vector which has a length
that is equal to the maximum output voltage (Emax). The output voltage at any given angle can be found by
applying the above trigonometric function. Because the output voltage is in direct relationship with the
sine of the angle !, it is commonly called a sine wave.
You can see this relationship more clearly in figure 1-3 where the coil positions in relation to time
are represented by the numbers 0 through 12. The corresponding angular displacements, shown as !, are
shown along the horizontal time axis. The induced voltages (V1 through V12) are plotted along this axis.
Connecting the induced voltage points, shown in the figure, forms a sine-wave pattern. This relationship
can be proven by taking any coil position and applying the trigonometric function to an equivalent right
triangle. When the vector is placed horizontally (position 0), the angle ! is 0 degrees. Since e = Emax sine
!, and the sine of 0 degrees is 0, the output voltage is 0 volts, as shown below:
1-5
Figure 1-3.—Generation of sine-wave voltage.
At position 2, the sine of 60 degrees is 0.866 and an output of 86.6 volts is developed.
This relationship is plotted through 360 degrees of rotation. A continuous line is drawn through the
successive points and is known as 1 CYCLE of a sine wave. If the time axis were extended for a second
revolution of the vector plotted, you would see 2 cycles of the sine wave. The 0-degree point of the
second cycle would be the same point as the 360-degree point of the first cycle.
Q-5. What waveform is the basis of all complex waveforms?
Q-6. What is the purpose of using vectors?
Q-7. What is the trigonometric ratio for the sine of an angle?
1-6
Q-8. What is the mathematical formula for computing the output voltage from a moving coil in a
magnetic field?
AMPLITUDE
A sine wave is used to represent values of electrical current or voltage. The greater its height, the
greater the value it represents. As you have studied, a sine wave alternately rises above and then falls
below the reference line. That part above the line represents a positive value and is referred to as a
POSITIVE ALTERNATION. That part of the cycle below the line has a negative value and is referred to
as a NEGATIVE ALTERNATION. The maximum value, above or below the reference line, is called the
PEAK AMPLITUDE. The value at any given point along the reference line is called the
INSTANTANEOUS AMPLITUDE.
PHASE
PHASE or PHASE ANGLE indicates how much of a cycle has been completed at any given instant.
This merely describes the angle that exists between the starting point of the vector and its position at that
instant. The number of degrees of vector rotation and the number of degrees of the resultant sine wave
that have been completed will be the same. For example, at time position 2 of figure 1-3, the vector has
rotated to 60 degrees and 60 degrees of the resultant sine wave has been completed. Therefore, both are
said to have an instantaneous phase angle of 60 degrees.
FREQUENCY
The rate at which the vector rotates determines the FREQUENCY of the sine wave that is generated;
that is, the faster the vector rotates, the more cycles completed in a given time period. The basic time
period used is 1 second. If a vector completes one revolution per 1 second, the resultant sine wave has a
frequency 1 cycle per second (1 hertz). If the rate of rotation is increased to 1,000 revolutions per second,
the frequency of the sine wave generated will be 1,000 cycles per second (1 kilohertz).
PERIOD
Another term that is important in the discussion of a sine wave is its duration, or PERIOD. The
period of a cycle is the elapsed time from the beginning of a cycle to its completion. If the vector shown
in figure 1-3 were to make 1 revolution per second, each cycle of the resultant sine wave would have a
period of 1 second. If it were rotating at a speed of 1,000 revolutions per second, each revolution would
require 1/1,000 of a second and the period of the resultant sine wave would be 1/1,000 of a second. This
illustrates that the period is related to the frequency. As the number of cycles completed in 1 second
increases, the period of each cycle will decrease proportionally. This relationship is shown in the
following formulas:
1-7
WAVELENGTH
The WAVELENGTH of a sine wave is determined by its physical length. During the period a wave
is being generated, its leading edge is moving away from the source at 300,000,000 meters per second.
The physical length of the sine wave is determined by the amount of time it takes to complete one full
cycle. This wavelength is an important factor in determining the size of equipments used to generate and
transmit radio frequencies.
To help you understand the magnitude of the distance a wavefront (the initial part of a wave) travels
during 1 cycle, we will compute the wavelengths (l;) of several frequencies. Consider a vector that rotates
at 1 revolution per second. The resultant sine wave is transmitted into space by an antenna. As the vector
moves from its 0-degree starting position, the wavefront begins to travel away from the antenna. When
the vector reaches the 360-degree position, and the sine wave is completed, the sine wave is stretched out
over 300,000,000 meters. The reason the sine wave is stretched over such a great distance is that the
wavefront has been moving away from the antenna at 300,000,000 meters per second. This is shown in
the following example:
If a vector were rotating at 1,000 revolutions per second, its period would be 0.001 second. By
applying the formula for wavelength, you would find that the wavelength is 300,000 meters:
Since we normally know the frequency of a sine wave instead of its period, the wavelength is easier
to find using the frequency:
1-8
Thus, for a sine wave with a frequency of 1,000,000 hertz (1 megahertz), the wavelength would be
300 meters, as shown below:
The higher the frequency, the shorter the wavelength of a sine wave. This important relationship
between frequency and wavelength is illustrated in table 1-1.
Table 1-1.—Radio frequency versus wavelength
FREQUENCY
WAVELENGTH
300,000 MHz
METRIC
.001 m
U.S.
.04 in
30,000 MHz
.01 m
.39 in
3,000 MHz
.1 m
3.94 in
300 MHz
1m
39.37 in
30 MHz
10 m
10.93 yd
3 MHz
100 m
109.4 yd
300 kHz
1 km
.62 mi
30 kHz
10 km
6.2 mi
3 kHz
100 km
62 mi
EHFSHFUHF-VHF--HF---MF---LF---VLF-----
Q-9. What is the instantaneous amplitude of a sine wave?
Q-10. What term describes how much of a cycle has been completed?
Q-11. What determines the frequency of a sine wave?
Q-12. What is the period of a cycle?
1-9
Q-13. How do you calculate the wavelength of a sine wave?
HETERODYNING
Information waveforms are produced by many different sources and are generally quite low in
frequency. A good example is the human voice. The frequencies involved in normal speech vary from one
individual to another and cover a wide range. This range can be anywhere from a low of about 90 hertz
for a deep bass to as high as 10 kilohertz for a high soprano.
The most important speech frequencies almost entirely fall below 3 kilohertz. Higher frequencies
merely help to achieve more perfect sound production. The range of frequencies used to transmit voice
intelligence over radio circuits depends on the degree of FIDELITY (the ability to faithfully reproduce the
input in the output) that is desired. The minimum frequency range that can be used for the transmission of
speech is 500 to 2,000 hertz. The average range used on radiotelephone circuits is 250 to 2,750 hertz.
Frequencies contained within the human voice can be transmitted over telephone lines without
difficulty, but transmitting them via radio circuits is not practical. This is because of their extremely long
wavelengths and the fact that antennas would have to be constructed with long physical dimensions to
transmit or radiate these wavelengths. Generally, antennas have radiating elements that are 1/4, 1/2, 1, or
more full wavelengths of the frequency to be radiated. The wavelengths of voice frequencies employed on
radiotelephone circuits range from 1,200,000 meters at 250 hertz to 109,090 meters at 2,750 hertz. Even a
quarter-wave antenna would require a large area, be expensive to construct, and consume enormous
amounts of power.
As studied in NEETS, Module 10, Introduction to Wave Propagation, Transmission Lines, and
Antennas, radio frequencies do not have the limitations just described for voice frequencies. Radio waves,
given a suitable antenna, can often radiate millions of miles into space. Several methods of modulation
can be used to impress voices frequencies onto radio waves for transmission through space.
In the modulation process, waves from the information source are impressed onto a radio-frequency
sine wave called a CARRIER. This carrier is sufficiently high in frequency to have a wavelength short
enough to be radiated from an antenna of practical dimensions. For example, a carrier frequency of 10
megahertz has a wavelength of 30 meters, as shown below:
Construction of an antenna related to that wavelength does not cause any problems.
An information wave is normally referred to as a MODULATING WAVE. When a modulating wave
is impressed on a carrier, the voltages of the modulating wave and the carrier are combined in such a
manner as to produce a COMPLEX WAVE (a wave composed of two or more parts). This complex wave
1-10
is referred to as the MODULATED WAVE and is the waveform that is transmitted through space. When
the modulated wave is received and demodulated, the original component waves (carrier and modulating
waves) are reproduced with their respective frequencies, phases, and amplitudes unchanged.
Modulation of a carrier can be achieved by any of several methods. Generally, the methods are
named for the sine-wave characteristic that is altered by the modulation process. In this module, you will
study AMPLITUDE MODULATION, which includes CONTINUOUS-WAVE MODULATION. You
will also learn about two forms of ANGLE MODULATION (FREQUENCY MODULATION and
PHASE MODULATION). A special type of modulation, known as PULSE MODULATION, will also be
discussed. Before we present the methods involved in developing modulation, you need to study a process
that is essential to the modulation of a carrier, known as heterodyning.
To help you understand the operation of heterodyning circuits, we will begin with a discussion of
LINEAR and NONLINEAR devices. In linear devices, the output rises and falls directly with the input. In
nonlinear devices, the output does not rise and fall directly with the input.
LINEAR IMPEDANCE
Whether the impedance of a device is linear or nonlinear can be determined by comparing the
change in current through the device to the change in voltage applied to the device. The simple circuit
shown in view (A) of figure 1-4 is used to explain this process.
Figure 1-4A.—Circuit with one linear impedance.
First, the current through the device must be measured as the voltage is varied. Then the current and
voltage values can be plotted on a graph, such as the one shown in view (B), to determine the impedance
of the device. For example, assume the voltage is varied from 0 to 200 volts in 50-volt steps, as shown in
view (B). At the first 50-volt point, the ammeter reads 0.5 ampere. These ordinates are plotted as point a
in view (B). With 100 volts applied, the ammeter reads 1 ampere; this value is plotted as point b. As these
steps are continued, the values are plotted as points c and d. These points are connected with a straight
line to show the linear relationship between current and voltage. For every change in voltage applied to
the device, a proportional change occurs in the current through the device. When the change in current is
proportional to the change in applied voltage, the impedance of the device is linear and a straight line is
developed in the graph.
1-11
Figure 1-4B.—Circuit with one linear impedance.
The principle of linear impedance can be extended by connecting two impedance devices in series,
as shown in figure 1-5, view (A). The characteristics of both individual impedances are determined as
explained in the preceding section. For example, assume voltmeter V1 shows 50 volts and the ammeter
shows 0.5 ampere. Point a in view (B) represents this ordinate. In the same manner, increasing the voltage
in increments of 50 volts gives points b, c, and d. Lines Z1 and Z2 show the characteristics of the two
impedances. The total voltage of the series combination can be determined by adding the voltages across
Z1 and Z2. For example, at 0.5 ampere, point a (50 volts) plus point e (75 volts) produces point i (125
volts). Also, at 1 ampere, point b plus point f produces point j. Line Z1 + Z2 represents the combined
voltage-current characteristics of the two devices.
Figure 1-5A.—Circuit with two linear impedances.
1-12
Figure 1-5B.—Circuit with two linear impedances.
View (A) of figure 1-6 shows two impedances in parallel. View (B) plots the impedances both
individually (Z1 and Z2) and combined (Z1 x Z2)/(Z1 + Z2). Note that Z1 and Z2 are not equal. At 100
volts, Z1 has 1 ampere of current plotted at point b and Z2 has 0.5 ampere plotted at point f. The
coordinates of the equivalent impedance of the parallel combination are found by adding the current
through Z1 to the current through Z2. For example, at 100 volts, point b is added to point f to determine
point j (1.5 amperes).
1-13
Figure 1-6.—Circuit with parallel linear impedances.
Positive or negative voltage values can be used to plot the voltage-current graph. Figure 1-7 shows
an example of this situation. First, the voltage versus current is plotted with the battery polarity as shown
in view (A). Then the battery polarity is reversed and the remaining voltage versus current points are
plotted. As a result, the line shown in view (C) is obtained.
Figure 1-7A.—Linear impedance circuit.
1-14
Figure 1-7B.—Linear impedance circuit.
Figure 1-7C.—Linear impedance circuit
The battery in view (A) could be replaced with an ac generator, as shown in view (B), to plot the
characteristic chart. The same linear voltage-current chart would result. Current flow in either direction is
directly proportional to the change in voltage.
In conclusion, when dc or sine-wave voltages are applied to a linear impedance, the current through
the impedance will vary directly with a change in the voltage. The device could be a resistor, an air-core
inductor, a capacitor, or any other linear device. In other words, if a sine-wave generator output is applied
to a combination of linear impedances, the resultant current will be a sine wave which is directly
proportional to the change in voltage of the generator. The linear impedances do not alter the waveform of
the sine wave. The amplitude of the voltage developed across each linear component may vary, or the
phase of the wave may shift, but the shape of the wave will remain the same.
1-15
NONLINEAR IMPEDANCE
You have studied that a linear impedance is one in which the resulting current is directly
proportional to a change in the applied voltage. A nonlinear impedance is one in which the resulting
current is not directly proportional to the change in the applied voltage. View (A) of figure 1-8 illustrates
a circuit which contains a nonlinear impedance (Z), and view (B) shows its voltage-current curve.
Figure 1-8A.—Nonlinear impedance circuit.
Figure 1-8B.—Nonlinear impedance circuit.
As the applied voltage is varied, ammeter readings which correspond with the various voltages can
be recorded. For example, assume that 50 volts yields 0.4 milliampere (point a), 100 volts produces 1
milliampere (point b), and 150 volts causes 2.2 milliamperes (point c). Current through the nonlinear
impedance does not vary proportionally with the voltage; the chart is not a straight line. Therefore, Z is a
nonlinear impedance; that is, the current through the impedance does not faithfully follow the change in
voltage. Various combinations of voltage and current for this particular nonlinear impedance may be
obtained by use of this voltage-current curve.
1-16
COMBINED LINEAR AND NONLINEAR IMPEDANCES
The series combination of a linear and a nonlinear impedance is illustrated in view (A) of figure 1-9.
The voltage-current charts of Z1 and Z2 are shown in view (B). A chart of the combined impedance can
be plotted by adding the amount of voltage required to produce a particular current through linear
impedance Z1 to the amount of voltage required to produce the same amount of current through nonlinear
impedance Z2. The total will be the amount of voltage required to produce that particular current through
the series combination. For example, point a (25 volts) is added to point c (50 volts) which yields point e
(75 volts); and point b (50 volts) is added to point d (100 volts) which yields point f (150 volts).
Intermediate points may be determined in the same manner and the resultant characteristic curve (Z1 +
Z2) is obtained for the series combination.
Figure 1-9A.—Combined linear and nonlinear impedances.
Figure 1-9B.—Combined linear and nonlinear impedances.
1-17
You should see from this graphic analysis that when a linear impedance is combined with a
nonlinear impedance, the resulting characteristic curve is nonlinear. Some examples of nonlinear
impedances are crystal diodes, transistors, iron-core transformers, and electron tubes.
AC APPLIED TO LINEAR AND NONLINEAR IMPEDANCES
Figure 1-10 illustrates an ac sine-wave generator applied to a circuit containing several linear
impedances. A sine-wave voltage applied to linear impedances will cause a sine wave of current through
them. The wave shape across each linear impedance will be identical to the applied waveform.
Figure 1-10.—Sine wave generator applied to several impedances.
The amplitude, on the other hand, may differ from the amplitude of the applied voltage. Furthermore,
the phase of the voltage developed by any of the impedances may not be identical to the phase of the
voltage across any of the other impedances or the phase of the applied voltage. If an impedance is a
reactive component (coil or capacitor), voltage or current may lead or lag, but the wave shape will remain
the same. In a linear circuit, the output of the generator is not distorted. The frequency remains the same
throughout the entire circuit and no new frequencies are generated.
View (A) of figure 1-11 illustrates a circuit that contains a combination of linear and nonlinear
impedances with a sine wave of voltage applied. Impedances Z2, Z3, and Z4 are linear; and Z1 is
nonlinear. The result of a linear and nonlinear combination of impedances is a nonlinear waveform. The
curve Z, shown in view (B), is the nonlinear curve for the circuit of view (A). Because of the nonlinear
impedance, current can flow in the circuit only during the positive alternation of the sine-wave generator.
If an oscilloscope is connected, as shown in view (A), the waveform across Z3 will not be a sine wave.
Figure 1-12, view (A), illustrates the sine wave from the generator and view (B) shows the waveform
across the linear impedance Z3. Notice that the nonlinear impedance Z1 has eliminated the negative half
cycles.
1-18
Figure 1-11A.—Circuit with nonlinear impedances.
Figure 1-11B.—Circuit with nonlinear impedances.
Figure 1-12A.—Waveform in a circuit with nonlinear impedances.
1-19
Figure 1-12B.—Waveform in a circuit with nonlinear impedances.
The waveform in view (B) is no longer identical to that of view (A) and the nonlinear impedance
network has generated HARMONIC FREQUENCIES. The waveform now consists of the fundamental
frequency and its harmonics. (Harmonics were discussed in NEETS, Module 9, Introduction to
Wave-Generation and Wave-Shaping Circuits.)
TWO SINE WAVE GENERATORS IN LINEAR CIRCUITS
A circuit composed of two sine-wave generators, G1 and G2, and two linear impedances, Z1 and Z2,
is shown in figure 1-13. The voltage applied to Z1 and Z2 will be the vector sum of the generator
voltages. The sum of the individual instantaneous voltages across each impedance will equal the applied
voltages.
Figure 1-13.—Two sine-wave generators with linear impedances.
If the two generator outputs are of the same frequency, then the waveform across Z1 and Z2 will be a
sine wave, as shown in figure 1-14, views (A) and (B). No new frequencies will be created. Relative
amplitude and phase will be determined by the relative values and types of the impedances.
1-20
Figure 1-14A.—Waveforms across two nonlinear impedances.
Figure 1-14B.—Waveforms across two nonlinear impedances.
If the two sine wave generators are of different frequencies, then the sum of the instantaneous values
will appear as a complex wave across the impedances, as shown in figure 1-15, views (A) and (B). To
determine the wave shape across each individual impedance, assume only one generator is connected at a
1-21
time and compute the sine-wave voltage developed across each impedance for that generator input. Then,
combine the instantaneous voltages (caused by each generator input) to obtain the complex waveform
across each impedance. The nature of the impedance (resistive or reactive) will determine the shape of the
complex waveform. Because the complex waveform is the sum of two individual sine waves, the
composite waveform contains only the two original frequencies.
Figure 1-15A.—Sine-wave generators with different frequencies and linear impedances.
Figure 1-15B.—Sine-wave generators with different frequencies and linear impedances.
Linear impedances may alter complex waveforms, but they do not produce new frequencies. The
output of one generator does not influence the output of the other generator.
TWO SINE WAVE GENERATORS AND A COMBINATION OF LINEAR AND NONLINEAR
IMPEDANCES
Figure 1-16 illustrates a circuit that contains two sine-wave generators (G1 and G2), linear
impedance Z1, and nonlinear impedance Z2, in series. When a single sine-wave voltage is applied to a
1-22
combined linear and nonlinear impedance circuit, the voltages developed across the impedances are
complex waveforms.
Figure 1-16.—Sine-wave generators with a combination of impedances.
When two sine wave voltages are applied to a circuit, as in figure 1-16, nonlinear impedance Z2
reshapes the two sine-wave inputs and their harmonics, resulting in a very complex waveform.
Assume that nonlinear impedance Z2 will allow current to flow only when the sum of the two
sine-wave generators (G1 and G2) has the polarity indicated. The waveforms present across the linear
impedance will appear as a varying waveform. This will be a complex waveform consisting of:
•
a dc level
•
the two fundamental sine wave frequencies
•
the harmonics of the two fundamental frequencies
•
the sum of the fundamental frequencies
•
the difference between frequencies
The sum and difference frequencies occur because the phase angles of the two fundamentals are
constantly changing. If generator G1 produces a 10-hertz voltage and generator G2 produces an 11-hertz
voltage, the waveforms produced because of the nonlinear impedance will be as shown in the following
list:
•
a 10-hertz voltage
•
an 11-hertz voltage
•
harmonics of 10 hertz and 11 hertz (the higher the harmonic, the lower its strength)
•
the sum of 10 hertz and 11 hertz (21 hertz)
•
the difference between 10 hertz and 11 hertz (1 hertz)
1-23
Figure 1-17 illustrates the relationship between the two frequencies (10 and 11 hertz). Since the
waveforms are not of the same frequency, the 10 hertz of view (B) and the 11 hertz of view (A) will be in
phase at some points and out of phase at other points. You can see this by closely observing the two
waveforms at different instants of time. The result of the differences in phase of the two sine waves is
shown in view (C). View (D) shows the waveform that results from the nonlinearity in the circuit.
Figure 1-17A.—Frequency relationships.
Figure 1-17B.—Frequency relationships.
Figure 1-17C.—Frequency relationships.
Figure 1-17D.—Frequency relationships.
The most important point to remember is that when varying voltages are applied to a circuit which
contains a nonlinear impedance, the resultant waveform contains frequencies which are not present at the
input source.
The process of combining two or more frequencies in a nonlinear impedance results in the
production of new frequencies. This process is referred to as heterodyning.
1-24
SPECTRUM ANALYSIS
The heterodyning process can be analyzed by using SPECTRUM ANALYSIS (the display of
electromagnetic energy arranged according to wavelength or frequency). As shown in figure 1-18,
spectrum analysis is an effective way of viewing the energy in electronic circuits. It clearly shows the
relationships between the two fundamental frequencies (10 and 11 hertz) and their sum (21 hertz) and
difference (1 hertz) frequencies. It also allows you to view the BANDWIDTH (the amount of the
frequency spectrum that signals occupy) of the signal you are studying.
Figure 1-18.—Spectrum analysis of heterodyned signal.
TYPICAL HETERODYNING CIRCUIT
Two conditions must be met in a circuit for heterodyning to occur. First, at least two different
frequencies must be applied to the circuit. Second, these signals must be applied to a nonlinear
impedance. These two conditions will result in new frequencies (sum and difference) being produced.
Any one of the frequencies can be selected by placing a frequency-selective device (such as a tuned tank
circuit) in series with the nonlinear impedance in the circuit.
Figure 1-19 illustrates a basic heterodyning circuit. The diode D1 serves as the nonlinear impedance
in the circuit. Generators G1 and G2 are signal sources of different frequencies. The primary of T1, with
its associated capacitance, serves as the frequency-selective device.
Figure 1-19.—Typical heterodyning circuit.
1-25
The principles of this circuit are similar to those of the block diagram circuit of figure 1-16. Notice in
figure 1-19 that the two generators are connected in series. Therefore, the resultant waveform of their
combined frequencies will determine when the cathode of D1 will be negative with respect to the anode,
thereby controlling the conduction of the diode. The new frequencies that are generated by applying these
signals to nonlinear impedance D1 are the sum and difference of the two original frequencies. The
frequency-selective device T1 may be tuned to whichever frequency is desired for use in later circuit
stages. Heterodyning action takes place, intentionally or not, whenever these conditions exist.
Heterodyning (MIXING) circuits are found in most electronic transmitters and receivers. These
transmitter and receiver circuits will be explained in detail later in this module.
Q-14. Define the heterodyne principle.
Q-15. What is a nonlinear impedance?
Q-16. What is spectrum analysis?
Q-17. What two conditions are necessary for heterodyning to take place?
AMPLITUDE-MODULATED SYSTEMS
Amplitude modulation refers to any method of varying the amplitude of an electromagnetic carrier
frequency in accordance with the intelligence to be transmitted by the carrier. The CARRIER frequency is
a radio-frequency wave suitable for modulation by the intelligence to be transmitted. One form of this
method of modulation is simply to interrupt the carrier in accordance with a prearranged code.
CONTINUOUS WAVE (CW)
The "on-off" KEYING of a continuous wave (cw) carrier frequency was the principal method of
modulating a carrier in the early days of electrical communications. The intervals of time when a carrier
either was present or absent conveyed the desired intelligence. This is still used in modern
communications. When applied to a continuously oscillating radio-frequency source, on-off keying is
referred to as cw signaling. This type of communication is sometimes referred to as an interrupted
continuous wave (icw).
Development
The use of a cw transmitter can be very simple. All that is required for the transmitter to work
properly is a device to generate the oscillations, a method of keying the oscillations on and off, and an
antenna to radiate the energy. Continuous wave was the first type of modulation used. It is still
extensively used for long-range communications. When Marconi and others were attempting the transfer
of intelligence between two points, without reliance on a conducting path, they employed the use of a
practical coding system known as Morse code. You probably know that Morse code is a system of on-off
keying developed for telegraph that is capable of passing intelligence over wire at an acceptable rate.
Morse code consists only of periods of signal and no-signal.
Figure 1-20 is the International Morse code used with telegraphy and cw modulation. Each character
in the code is made up of a series of elements referred to as DOTS or DASHES. These are short (dot) and
long (dash) bursts of signal separated by intervals of no signal. The dot is the basic time element of the
code. The dash has three times the duration of a dot interval. The waveforms for both are shown in figure
1-21. The elements within each character are separated by intervals of no signal with a time duration of
one dot. The characters are separated by a no-signal interval equal in duration to one dash. Each interval
1-26
during which signal is present is called the MARKING interval, and the period of no signal is called the
SPACING interval. Figure 1-22 shows the relationships between the rf carrier view (A), the on-off keying
waveform view (B), and the resultant carrier wave view (C).
Figure 1-20.—International Morse code.
Figure 1-21.—Dot and dash in radiotelegraph code.
1-27
Figure 1-22A.—Essential elements of ON-OFF keying.
Figure 1-22B.—Essential elements of ON-OFF keying.
Figure 1-22C.—Essential elements of ON-OFF keying.
1-28
Keying Methods
Keying a transmitter causes an rf signal to be radiated only when the key contacts are closed. When
the key contacts are open, the transmitter does not radiate energy. Keying is accomplished in either the
oscillator or amplifier stage of a transmitter. A number of different keying systems are used in Navy
transmitters.
In most Navy transmitters, the hand telegraph key is at a low-voltage potential with respect to
ground. A keying bar is usually grounded to protect the operator. Generally, a keying relay, with its
contacts in the center-tap lead of the filament transformer, is used to key the equipment. Because one or
more stages use the same filament transformer, these stages are also keyed. A class C final amplifier,
when operated with fixed bias, is usually not keyed. This is because no output occurs when no excitation
is applied in class C operation. Keying the final amplifier along with the other stages is not necessary in
this case.
OSCILLATOR KEYING.—Two methods of OSCILLATOR KEYING are shown in figure 1-23.
In view (A) the grid circuit is closed at all times. The key (K) opens and closes the negative side of the
plate circuit. This system is called PLATE KEYING. When the key is open, no plate current can flow and
the circuit does not oscillate. In view (B), the cathode circuit is open when the key is open and neither
grid current nor plate current can flow. Both circuits are closed when the key is closed. This system is
called CATHODE KEYING. Although the circuits of figure 1-23 may be used to key amplifiers, other
keying methods are generally employed because of the high values of plate current and voltage
encountered.
Figure 1-23A.—Oscillator keying.
Figure 1-23B.—Oscillator keying.
1-29
BLOCKED-GRID KEYING.—Two methods of BLOCKED-GRID KEYING are shown in figure
1-24. The key in view (A) shorts cathode resistor R1 allowing normal plate current to flow. With the key
open, reduced plate current flows up through resistor R1 making the end connected to grid resistor Rg
negative. If R1 has a high enough value, the bias developed is sufficient to cause cutoff of plate current.
Depressing the key short-circuits R1. This increases the bias above cutoff and allows the normal flow of
plate current. Grid resistor Rg is the usual grid-leak resistor for normal biasing.
Figure 1-24A.—Blocked-grid keying.
Figure 1-24B.—Blocked-grid keying.
The blocked-grid keying method in view (B) affords a complete cutoff of plate current. It is one of
the best methods for keying amplifier stages in transmitters. In the voltage divider with the key open, twothirds of 1,000 volts, or 667 volts, is developed across the 200-kilohm resistor; one-third of 1,000 volts, or
333 volts, is developed across the 100-kilohm resistor. The grid bias is the sum of 100 volts and 333
volts, or 433 volts. This sum is below cutoff and no plate current flows. The plate voltage is 667 volts.
With the key closed, the 200-kilohm resistor drops 1,000 volts. The plate voltage becomes 1,000 volts at
the same time the grid bias becomes 100 volts. Grid bias is reduced enough so that the triode amplifier
will conduct only on the peaks of the drive signal.
1-30
When greater frequency stability is required, the oscillator should not be keyed, but should remain in
continuous operation; other transmitter circuits may be keyed. This procedure keeps the oscillator tube at
a normal operating temperature and offers less chance for frequency variations to occur each time the key
is closed.
KEYING RELAYS.—In transmitters using a crystal-controlled oscillator, the keying is almost
always in a circuit stage following the oscillator. In large transmitters (75 watts or higher), the ordinary
hand key cannot accommodate the plate current without excessive arcing.
WARNING
BECAUSE OF THE HIGH PLATE POTENTIALS USED, OPERATING A HAND
KEY IN THE PLATE CIRCUIT IS DANGEROUS. A SLIGHT SLIP OF THE HAND
BELOW THE KEY KNOB COULD RESULT IN SEVERE SHOCK OR, IN THE
CASE OF DEFECTIVE RF PLATE CHOKES, A SEVERE RF BURN.
In larger transmitters, some local low-voltage supply, such as a battery, is used with the hand key to
open and close a circuit through the coils of a KEYING RELAY. The relay contacts open and close the
keying circuit of the amplifier. A schematic diagram of a typical relay-operated keying system is shown
in figure 1-25. The hand key closes the circuit from the low-voltage supply through the coil (L) of the
keying relay. The relay armature closes the relay contacts as a result of the magnetic pull exerted on the
armature. The armature moves against the tension of a spring. When the hand key is opened, the relay coil
is deenergized and the spring opens the relay contacts.
Figure 1-25.—Relay-operated keying system.
KEY CLICKS.—Ideally, cw keying a transmitter should instantly start and stop radiation of the
carrier completely. However, the sudden application and removal of power causes rapid surges of current
which may cause interference in nearby receivers. Even though such receivers are tuned to frequencies far
removed from that of the transmitter, interference may be present in the form of "clicks" or "thumps."
KEY-CLICK FILTERS are used in the keying systems of radio transmitters to prevent such interference.
Two types of key-click filters are shown in figure 1-26.
1-31
Figure 1-26A.—Key-click filters.
Figure 1-26B.—Key-click filters.
The capacitors and rf chokes in figure 1-26, views (A) and (B), prevent surges of current. In view
(B), the choke coil causes a lag in the current when the key is closed, and the current builds up gradually
instead of instantly. The capacitor charges as the key is opened and slowly releases the energy stored in
the magnetic field of the inductor. The resistor controls the rate of charge of the capacitor and also
prevents sparking at the key contacts by the sudden discharge of the capacitor when the key is closed.
MACHINE KEYING.—The speed with which information can be transmitted using a hand key
depends on the keying ability of the operator. Early communicators turned to mechanized methods of
keying the transmitters to speed transmissions. More information could be passed in a given time by
replacing the hand-operated key with a keying device capable of reading information from punched tape.
Using this method, several operators could prepare tapes at their normal operating speed. The tapes could
then be read through the keying device at a higher rate of speed and more information could be
transmitted in a given amount of time.
Spectrum Analysis
Continuous-wave transmission has the disadvantage of being a relatively slow transmission method.
Still, it has several advantages. Some of the advantages of cw transmission are a high degree of clarity
under severe noise conditions, long-range operation, and narrow bandwidth. A highly skilled operator can
pick out and read a cw signal even though it has a high degree of background noise or interference. Since
only a single-carrier frequency is being transmitted, all of the transmitter power can be concentrated in the
intelligence. This concentration of power gives the transmission a greater range. The use of spectrum
1-32
analysis (figure 1-27) illustrates the transmitted frequency characteristics of a cw signal. Because the cw
signal is a pure sine wave, it occupies only a single frequency in the rf spectrum and the system is
relatively simple.
Figure 1-27.—Carrier-wave signal spectrum analysis.
Q-18. What is amplitude modulation?
Q-19. What are the three requirements for cw transmission?
Q-20. Name two methods of oscillator keying.
Q-21. State the method used to increase the speed of keying in a cw transmitter.
Q-22. Name three advantages of cw transmission.
Single-Stage Transmitters
A simple, single-tube cw transmitter can be made by coupling the output of an oscillator directly to
an antenna (figure 1-28). The primary purpose of the oscillator is to develop an rf voltage which has a
constant frequency and is immune to outside factors which may cause its frequency to shift. The output of
this simple transmitter is controlled by placing a telegraph key at point K in series with the voltage
supply. Since the plate supply is interrupted when the key is open, the circuit oscillates only as long as the
key is closed. Although the transmitter shown uses a Colpitts oscillator, any of the oscillators previously
described in NEETS, Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits can be
used.
Figure 1-28.—Simple electron-tube transmitter.
1-33
Capacitors C2 and C3 can be GANGED (mechanically linked together) to simplify tuning. Capacitor
C1 is used to tune (resonate) the antenna to the transmitter frequency. CA is the effective capacitance
existing between the antenna and ground. This antenna-to-ground capacitance is in parallel with the
tuning capacitors, C2 and C3. Since the antenna has capacitance, any change in its length or position,
such as that caused by swaying of the antenna, changes the value of CA and causes the oscillator to
change frequency. Because these frequency changes are undesirable for reliable communications, the
multistage transmitter was developed to increase reliability.
Multistage Transmitters
The simple, single-tube transmitter, shown in figure 1-28, is rarely used in practical equipment. Most
of the transmitters you will see use a number of tubes or stages. The number used depends on the
frequency, power, and application of the equipment. For your study, the following three categories of cw
transmitters are discussed: (1) master oscillator power amplifier (mopa) transmitters, (2) multistage,
high-power transmitters, (3) high- and very-high frequency transmitters.
The mopa is both an oscillator and a power amplifier. Power-amplifying stages and frequencymultiplying stages must be used to increase power and raise the frequency from those achievable in a
mopa. The main difference between many low- and high-power transmitters is in the number of poweramplifying stages that are used. Similarly, the main difference between many high- and very-high
frequency transmitters is in the number of frequency-multiplying stages used.
MASTER OSCILLATOR POWER AMPLIFIER.—For a transmitter to be stable, its oscillator
must not be LOADED DOWN. This means that its antenna (which can present a varying impedance)
must not be connected directly to the oscillator circuit. The rf oscillations must be sent through another
circuit before they are fed to the antenna for good frequency stability to be obtained. That additional
circuit is an rf power amplifier. Its purpose is to raise the amplitude of rf oscillations to the required
output power level and isolate the oscillator from the antenna. Any transmitter consisting of an oscillator
and a single-amplifier stage is called a master oscillator power amplifier transmitter (mopa), as shown in
figure 1-29.
Figure 1-29.—Block diagram of a master oscillator power amplifier transmitter (mopa).
Most mopa transmitters have only one tube amplifier in the power-amplifier stage. However, the
oscillator may not produce sufficient power to drive a power-amplifier tube to the power output level
required for the antenna. In such cases, the power-amplifier stages are designed to use two or more
amplifiers which can be driven by the oscillator. Two or more amplifiers can be connected in parallel
(with similar elements of each tube connected) or in a push-pull arrangement. In a push-pull amplifier, the
grids are fed equal rf voltages that are 180 degrees out of phase.
The main advantage of a mopa transmitter is that the power-amplifier stage isolates the oscillator
from the antenna. This prevents changes in antenna-to-ground capacitance from affecting the oscillator
1-34
frequency. A second advantage is that the rf power amplifier is operated so that a small change in the
voltage applied to its grid circuit will produce a large change in the power developed in its plate circuit.
Rf power amplifiers require that a specific amount of power be fed into the grid circuit. Only in this
way can the tube deliver an amplified power output. However, the stable oscillator can produce only
limited amounts of power. Therefore, the mopa transmitter is limited in the amount of power it can
develop. This is one of the disadvantages of the mopa transmitter. Another disadvantage is that it often is
impractical for use at very- and ultra-high frequencies. The reason is that the stability of self-excited
oscillators decreases rapidly as the operating frequency increases. Circuit tuning capacitances are small at
high frequencies and stray capacitances adversely affect frequency stability.
MULTISTAGE HIGH-POWER TRANSMITTERS.—The power amplifier of a high-power
transmitter may require far more driving power than can be supplied by an oscillator. Therefore, one or
more low-power intermediate amplifiers may be inserted between the oscillator and the final power
amplifier to boost power to the antenna. In some types of equipment, a VOLTAGE AMPLIFIER, called a
BUFFER is used between the oscillator and the first intermediate amplifier. The ideal buffer is operated
class A and is biased negatively to prevent grid current flow during the excitation cycle. Therefore, it does
not require driving power from, nor does it load down, the oscillator. The purpose of the buffer is to
isolate the oscillator from the following stages and to minimize changes in oscillator frequency that occur
with changes in loading. A buffer is required when keying takes place in an intermediate or final amplifier
operating at comparatively high power. Look at the block diagrams of several medium-frequency
transmitters in figure 1-30. The input and output powers are given for each stage. You should be able to
see that the power output rating of a transmitter can be increased by adding amplifier tubes capable of
delivering the power required.
Figure 1-30.—Block diagram of several medium-frequency transmitters.
1-35
HF AND VHF TRANSMITTERS.—Oscillators are too unstable for direct frequency control in
very- and ultra-high frequency transmitters. Therefore, these transmitters have oscillators operating at
comparatively low frequencies, sometimes as low as 1/100 of the output frequency. The oscillator
frequency is raised to the required output frequency by passing it through one or more FREQUENCY
MULTIPLIERS. Frequency multipliers are special rf power amplifiers which multiply the input
frequency. In practice, the MULTIPLICATION FACTOR (number of times the input frequency is
multiplied) is seldom larger than five in any one stage. The block diagram of a typical VHF transmitter,
designed for continuous tuning between 256 and 288 megahertz, is shown in figure 1-31.
Figure 1-31.—Block diagram of a vhf transmitter.
The stages which multiply the frequency by two are DOUBLERS; those which multiply by four are
QUADRUPLERS. The oscillator is tunable from 4 to 4.5 megahertz. The multiplier stages increase the
frequency by multiplying successively by 4, 4, 2, and 2, for a total factor of 64. In high-power, highfrequency transmitters, one or more intermediate amplifiers may be used between the last frequency
multiplier and the power amplifier.
Q-23. Name a disadvantage of a single-stage cw transmitter.
Q-24. What is the purpose of the power-amplifier stage in a master oscillator power amplifier cw
transmitter?
Q-25. What is the purpose of frequency-multiplier stages in a VHF transmitter?
AMPLITUDE MODULATION
The telegraph and radiotelegraph improved man's ability to communicate by allowing speedy
passage of information between two distant points. However, it failed to satisfy one of man's other
communications needs; that is, the ability to hear and be heard, by voice, at a great distance. In an effort
to improve on the telegraph, Alexander Graham Bell developed the principles on which modern
communications are built. He developed the modulation of an electric current by complex waveforms, the
demodulation of the resulting wave, and recovery of the original waveform. This section will examine the
process of varying an electric current in amplitude at an audio frequency.
1-36
Microphones
If an rf carrier is to convey intelligence, some feature of the carrier must be varied in accordance
with the information to be transmitted. In the case of speech intelligence, sound waves must be converted
to electrical energy.
A MICROPHONE is an energy converter that changes sound energy into electrical energy. A
diaphragm in the microphone moves in and out in accordance with the compression and rarefaction of the
atmosphere caused by sound waves. The diaphragm is connected to a device that causes current flow in
proportion to the instantaneous pressure delivered to it. Many devices can perform this function. The
particular device used in a given application depends on the characteristics desired, such as sensitivity,
frequency response, impedance matching, power requirements, and ruggedness.
The SENSITIVITY or EFFICIENCY of a microphone is usually expressed in terms of the electrical
power level which the microphone delivers to a matched-impedance load compared to the sound level
being converted. The sensitivity is rated in dB and must be as high as possible. A high microphone output
requires less gain in the amplifiers used with the microphone. This keeps the effects of thermal noise,
amplifier hum, and noise pickup at a minimum.
For good quality sound reproduction, the electrical signal from the microphone must correspond in
frequency content to the original sound waves. The microphone response should be uniform, or flat,
within its frequency range and free from the electrical or mechanical generation of new frequencies.
The impedance of a microphone is important in that it must be matched to the microphone cable and
to the amplifier input as well as to the amplifier input load. Exact impedance matching is not always
possible, especially in the case where the impedance of the microphone increases with an increase in
frequency. A long microphone cable tends to seriously attenuate the high frequencies if the microphone
impedance is high. This attenuation is caused by the increased capacitive action of the line at higher
frequencies. If the microphone has a low impedance, a lower voltage is developed in the microphone, and
more voltage is available at the load. Because many microphone lines used aboard ship are long, lowimpedance microphones must be used to preserve a sufficiently high voltage level- over the required
frequency range.
The symbol used to represent a microphone in a schematic diagram is shown in figure 1-32. The
schematic symbol identifies neither the type of microphone used nor its characteristics.
Figure 1-32.—Microphone schematic symbol.
CARBON MICROPHONE.—Operation of the SINGLE-BUTTON CARBON MICROPHONE
figure 1-33, view (A) is based on varying the resistance of a pile of carbon granules located within the
microphone. An insulated cup, referred to as the button, holds the loosely piled granules. It is so mounted
that it is in constant contact with the thin metal diaphragm. Sound waves striking the diaphragm vary the
pressure on the button which varies the pressure on the pile of carbon granules. The dc resistance of the
carbon granule pile is varied by this pressure. This varying resistance is in series with a battery and the
1-37
primary of a transformer. The changing resistance of the carbon pile produces a corresponding change in
the current of the circuit. The varying current in the transformer primary produces an alternating voltage
in the secondary. The transformer steps up the voltage and matches the low impedance of the microphone
to the high impedance of the first amplifier. The voltage across the secondary may be as high as 25 volts
peak. The impedance of this type of microphone varies from 50 to 200 ohms. This effect is caused by the
pressure of compression and rarefaction of sound waves, discussed in chapter 1 of NEETS, Module 10,
Introduction to Wave Propagation, Transmission Lines, and Antennas.
Figure 1-33A.—Carbon microphones. SINGLE-BUTTON CARBON MICROPHONE.
The DOUBLE-BUTTON CARBON MICROPHONE is shown in figure 1-33, view (B). Here, one
button is positioned on each side of the diaphragm so that an increase in resistance on one side is
accompanied by a simultaneous decrease in resistance on the other. Each button is in series with the
battery and one-half of the transformer primary. The decreasing current in one-half of the primary and the
increasing current in the other half produces an output voltage in the secondary winding. The output
voltage is proportional to the sum of the primary winding signal components. This action is similar to that
of push-pull amplifiers.
Figure 1-33B.—Carbon microphones. DOUBLE-BUTTON CARBON MICROPHONE.
1-38
One disadvantage of carbon microphones is that of a constant BACKGROUND HISS (hissing noise)
which results from random changes in the resistance between individual carbon granules. Other
disadvantages are reduced sensitivity and distortion that may result from the granules packing or sticking
together. The carbon microphone also has a limited frequency response. Still another disadvantage is a
requirement for an external voltage source.
The disadvantages, however, are offset by advantages that make its use in military applications
widespread. It is lightweight, rugged, and can produce an extremely high output.
CRYSTAL MICROPHONE.—The CRYSTAL MICROPHONE uses the PIEZOELECTRIC
EFFECT of Rochelle salt, quartz, or other crystalline materials. This means that when mechanical stress is
placed upon the material, a voltage electromagnetic force (EMF) is generated. Since Rochelle salt has the
largest voltage output for a given mechanical stress, it is the most commonly used crystal in microphones.
View (A) of figure 1-34 is a crystal microphone in which the crystal is mounted so that the sound waves
strike it directly. View (B) has a diaphragm that is mechanically linked to the crystal so that the sound
waves are indirectly coupled to the crystal.
Figure 1-34A.—Crystal microphones.
Figure 1-34B.—Crystal microphones.
1-39
A crystal microphone has a high impedance and does not require an external voltage source. It can
be connected directly into the input circuit of a high-gain amplifier. However, because its output is low,
several stages of high-gain amplification are required. Crystal microphones are delicate and must be
handled with care. Exposure to temperatures above 52 degrees Celsius (125 degrees Fahrenheit) may
permanently damage the crystal unit. Crystals are also soluble in water and other liquids and must be
protected from moisture and excessive humidity.
DYNAMIC MICROPHONE.—A cross section of the DYNAMIC or MOVING-COIL
MICROPHONE is shown in figure 1-35. A coil of fine wire is mounted on the back of the diaphragm and
located in the magnetic field of a permanent magnet. When sound waves strike the diaphragm, the coil
moves back and forth cutting the magnetic lines of force. This induces a voltage into the coil that is an
electrical reproduction of the sound waves.
Figure 1-35.—Dynamic microphone.
The sensitivity of the dynamic microphone is almost as high as that of the carbon type. It is
lightweight and requires no external voltage. The dynamic microphone is rugged and can withstand the
effects of vibration, temperature, and moisture. This microphone has a uniform response over a frequency
range that extends from 40 to 15,000 hertz. The impedance is very low (generally 50 ohms or less). A
transformer is required to match its impedance to that of the input of an af amplifier.
MAGNETIC MICROPHONE.—The MAGNETIC or MOVING-ARMATURE MICROPHONE
(figure 1-36) consists of a coil wound on an armature that is mechanically connected to the diaphragm
with a driver rod. The coil is located between the pole pieces of the permanent magnet. Any vibration of
the diaphragm vibrates the armature at the same rate. This varies the magnetic flux in the armature and
through the coil.
1-40
Figure 1-36.—Magnetic microphone action.
When the armature is in its resting position (midway between the two poles), the magnetic flux is
established across the air gap. However, no resultant flux is established in the armature. When a
compression wave strikes the diaphragm, the armature is deflected to the right. Most of the flux continues
to move in the direction of the arrows. However, some flux now flows from the north pole of the magnet
across the reduced gap at the upper right, down through the armature, and around to the south pole of the
magnet.
When a rarefaction wave occurs at the diaphragm, the armature is deflected to the left. Some flux is
now directed from the north pole of the magnet, up through the armature, through the reduced gap at the
upper left, and back to the south pole.
The vibrations of the diaphragm cause an alternating flux in the armature which induces an
alternating voltage in the coil. This voltage has the same waveform as that of the sound waves striking the
diaphragm.
The magnetic microphone is very similar to the dynamic microphone in terms of impedance,
sensitivity, and frequency response. However, it is more resistant to vibration, shock, and rough handling
than other types of microphones.
Changing sound waves into electrical impulses is the first step in voice communications. It is
common to all the transmission media you will study in the remainder of this chapter. We will discuss the
various types of modulation that arc used to transfer this information to a transmission medium in the
following sections.
Q-26. What is a microphone?
Q-27. What special electromechanical effect is the basis for carbon microphone operation?
Q-28. What is a major disadvantage of a carbon microphone?
Q-29. What property of a crystalline material is used in a crystal microphone?
Q-30. What is the difference between a dynamic microphone and a magnetic microphone?
1-41
AM TRANSMITTER PRINCIPLES
In this section we will describe the methods used to apply voice signals (intelligence) to a carrier
wave by the process of amplitude modulation (AM).
An AM transmitter can be divided into two major sections according to the frequencies at which they
operate, radio-frequency (rf) and audio-frequency (af) units. The rf unit is the section of the transmitter
used to generate the rf carrier wave. As illustrated in figure 1-37, the carrier originates in the master
oscillator stage where it is generated as a constant-amplitude, constant-frequency sine wave. The carrier is
not of sufficient amplitude and must be amplified in one or more stages before it attains the high power
required by the antenna. With the exception of the last stage, the amplifiers between the oscillator and the
antenna are called INTERMEDIATE POWER AMPLIFIERS (ipa). The final stage, which connects to the
antenna, is called the FINAL POWER AMPLIFIER (fpa).
Figure 1-37.—Block diagram of an AM transmitter.
The second section of the transmitter contains the audio circuitry. This section of the transmitter
takes the small signal from the microphone and increases its amplitude to the amount necessary to fully
modulate the carrier. The last audio stage is the MODULATOR. It applies its signal to the carrier in the
final power amplifier. In this way, intelligence is included in the radiated rf waveform.
The Modulated Wave
The frequencies present in a signal can be conveniently represented by a graph of the frequency
spectrum, shown in figure 1-38. In this graph, each individual frequency is portrayed as a vertical line.
The position of the line along the horizontal axis indicates the frequency of the signal. The height of the
frequency line is proportional to the amplitude of the signal. The rf spectrum in figure 1-38 shows the
frequencies present when heterodyning occurs between frequencies of 5 and 100 kilohertz.
1-42
Figure 1-38.—Radio-frequency spectrum.
Radiating energy at audio frequencies (discussed earlier in this chapter) is not practical. The
heterodyning principle, however, makes possible the conversion of an af signal (intelligence) to an rf
signal (with af intelligence) which can be radiated or transmitted through space.
Look again at figure 1-38. The sum and difference frequencies are located very near the rf signal
(100 kilohertz), while the audio signal (5 kilohertz) is spaced a considerable distance away. Because of
this frequency separation, the audio frequency can be easily removed by filter circuits, leaving just three
radio frequencies of 95, 100, and 105 kilohertz. These three radio frequencies are radiated through space
to the receiving station. At the receiver, the process is reversed. The frequency of 95 kilohertz, for
example, is heterodyned with the frequency of 100 kilohertz and the sum and difference frequencies are
again produced. (A similar process occurs between the frequencies of 100 and 105 kilohertz.) Of the
resultant frequencies (95, 100, 105, and 5 kilohertz), all are filtered out except the 5 kilohertz difference
frequency. This frequency, which is identical to the original 5 kilohertz audio applied at the transmitter, is
retained and amplified. Thus, the 5 kilohertz audio tone appears to have been radiated through space
from the transmitter to the receiver.
In the process just described, the 100 kilohertz frequency is referred to as the CARRIER
FREQUENCY, and the sum and difference frequencies are referred to as SIDE FREQUENCIES. Since
the sum frequency appears above the carrier frequency, it is referred to as the UPPER SIDE
FREQUENCY. The difference frequency appears below the carrier and is referred to as the LOWER
SIDE FREQUENCY.
When a carrier is modulated by voice or music signals, a large number of sum and difference
frequencies are produced. All of the sum frequencies above the carrier are spoken of collectively as the
UPPER SIDEBAND. All the difference frequencies below the carrier, also considered as a group, are
called the LOWER SIDEBAND.
If the carrier and the modulating signal are constant in amplitude, the sum and difference frequencies
will also be constant in amplitude. However, when the carrier and sidebands are combined in a single
impedance and viewed simultaneously with an oscilloscope, the resultant waveform appears as shown in
figure 1-39. This resultant wave is called the MODULATION ENVELOPE. The modulation envelope
has the same frequency as the carrier. However, it rises and falls in amplitude with the continual phase
shift between the carrier and sidebands. This causes these signals to first aid and then oppose one another.
These cyclic variations in the amplitude of the envelope have the same frequency as the audio-modulating
1-43
voltage. The audio intelligence is actually contained in the spacing or difference between the carrier and
sideband frequencies.
Figure 1-39.—Formation of the modulation envelope.
BANDWIDTH OF AN AM WAVE.—An ideal carrier wave contains a single frequency and
occupies very little of the frequency spectrum. When the carrier is amplitude modulated, sideband
frequencies are created both above and below the carrier frequency. This causes the signal to use up a
greater portion of the frequency spectrum. The amount of space in the frequency spectrum required by the
signal is called the BANDWIDTH of the signal.
The bandwidth of a modulated wave is a function of the frequencies contained in the modulating
signal. For example, when a 100-kilohertz carrier is modulated by a 5-kilohertz audio tone, sideband
frequencies are created at 95 and 105 kilohertz. This signal requires 10 kilohertz of space in the spectrum.
If the same 100-kilohertz carrier is modulated by a 10-kilohertz audio tone, sideband frequencies will
appear at 90 and 110 kilohertz and the signal will have a bandwidth of 20 kilohertz. Notice that as the
modulating signal becomes higher in frequency, the bandwidth required also becomes greater. As
illustrated by the above examples, the bandwidth of an amplitude-modulated wave at any instant is two
times the highest modulating frequency applied at that time. Thus, if a 400-kilohertz carrier is modulated
with 3, 5, and 8 kilohertz simultaneously, sideband frequencies will appear at 392, 395, 397, 403, 405,
and 408 kilohertz. This signal extends from 392 to 408 kilohertz and has a bandwidth of 16 kilohertz,
twice the highest modulating frequency of 8 kilohertz.
Musical instruments produce complex sound waves containing a great number of frequencies. The
frequencies produced by a piano, for example, range from approximately 27 to 4,200 hertz with harmonic
frequencies extending beyond 10 kilohertz. Modulating frequencies of up to 15 kilohertz must be
included in the signal to transmit a musical passage with a high degree of fidelity. This requires a
bandwidth of at least 30 kilohertz to prevent attenuation of higher-order harmonic frequencies.
If the signal to be transmitted contains voice frequencies only, and fidelity is of minor importance,
the bandwidth requirement is much smaller. A baritone voice includes frequencies of approximately 100
to 350 hertz, or 250 hertz. Intelligible voice communications can be carried out as long as the
communications system retains audio frequencies up to several thousand hertz. Comparing the conditions
1-44
for transmitting voice signals with those for transmitting music reveals that much less spectrum space is
required for voice communications.
Radio stations in the standard broadcast band are assigned carrier frequencies by the Federal
Communications Commission (FCC). When two stations are located near each other, their carriers must
be spaced some minimum distance apart in the radio spectrum. Otherwise, the sideband frequencies of
one station will interfere with sideband frequencies of the other station. The standard AM broadcast band
starts at 535 kilohertz and ends at 1,605 kilohertz. Carrier assignments start at 540 kilohertz and continue
in a succession of 10-kilohertz increments until the upper limit of the broadcast band is reached. This adds
up to a total of 107 carrier assignments, or CHANNELS, over the entire broadcast band. If stations were
assigned to all 107 channels (in a given geographical area), each station would be allotted a channel width
of 10 kilohertz. This leaves 5 kilohertz on each side of each carrier for sidebands. Since interference
between such closely spaced stations would be nearly impossible to prevent, the FCC avoids assigning
adjacent channels to stations in the same area. As a consequence of this policy, one or more vacant
channels normally exist between stations in the broadcast band. In the interest of better fidelity, the
stations are permitted to use modulating frequencies higher than 5 kilohertz as long as no interference
with other stations is produced.
Q-31. What are the two major sections of a typical AM transmitter?
Q-32. When 100 kilohertz and 5 kilohertz are heterodyned, what frequencies are present?
Q-33. What is the upper sideband of an AM transmission?
Q-34. Where is the intelligence in an AM transmission located?
Q-35. What determines the bandwidth of an AM transmission?
ANALYSIS OF AN AM WAVE.—A significant amount of information concerning the basic
principles of amplitude modulation can be obtained from a study of the properties of the modulation
envelope.
A carrier wave which has been modulated by voice or music signals is accompanied by two
sidebands; each sideband contains individual frequencies that vary continuously. Since a wave of this
nature is nearly impossible to analyze, you can assume in the following sections that the modulating
signal, unless otherwise qualified, is a single-frequency, constant-amplitude sine wave.
PERCENT OF MODULATION IN AN AM WAVE.—The degree of modulation is defined in
terms of the maximum permissible amount of modulation. Thus, a fully modulated wave is said to be
100-PERCENT MODULATED. The modulation envelope in figure 1-40, view (A), shows the conditions
for 100-percent sine-wave modulation. For this degree of modulation, the peak audio voltage must be
equal to the dc supply voltage to the final power amplifier. Under these conditions, the rf output voltage
will reach 0 on the negative peak of the modulating signal; on the positive peak, it will rise to twice the
amplitude of the unmodulated carrier.
1-45
Figure 1-40A.—Conditions for 100-percent modulation.
When analyzed, the modulation envelope consists of the unmodulated rf carrier voltage plus the
combined voltage of the two sidebands. The combined sideband voltages are approximately equal to the
rf carrier voltage since each sideband frequency contains one-half the carrier voltage, as shown in view
(B). This condition is known as 100-percent modulation and the maximum modulated rf voltage is twice
the carrier voltage. The audio-modulating voltage can be increased beyond the amount required to
produce 100-percent modulation. When this happens, the negative peak of the modulating signal becomes
larger in amplitude than the dc plate-supply voltage to the final power amplifier. This causes the final
plate voltage to be negative for a short period of time near the negative peak of the modulating signal. For
the duration of the negative plate voltage, no rf energy is developed across the plate tank circuit and the rf
output voltage remains at 0, as shown in figure 1-41, view (A).
Figure 1-40B.—Conditions for 100-percent modulation.
1-46
Figure 1-41A.—Overmodulation conditions.
Look carefully at the modulation envelope in view (A). It shows that the negative peak of the
modulating signal has effectively been limited. If the signal were demodulated (detected in the receiver),
it would have an appearance somewhat similar to a square wave. This condition, known as
OVERMODULATION, causes the signal to sound severely distorted (although this will depend on the
degree of overmodulation).
Overmodulation will generate unwanted (SPURIOUS) sideband frequencies. This effect can easily
be detected by tuning a receiver near, but somewhat outside the desired frequency. You would likely be
able to tune to one or more of these undesired sideband frequencies, but the reception would be severely
distorted, possibly unintelligible. (Without overmodulation, no such unwanted sideband frequencies
would exist and you would be able to tune only to the desired frequency.) These unwanted frequencies
will appear for a considerable range both above and below the desired channel. This effect is sometimes
called SPLATTER. These spurious frequencies, shown in view (B), cause interference with other stations
operating on adjacent channels. You should clearly understand that overmodulation, and its attendant
distortion and interference is to be avoided.
Figure 1-41B.—Overmodulation conditions.
In addition to the above problems, overmodulation also causes abnormally large voltages and
currents to exist at various points within the transmitter. Therefore, sufficient overload protection by
1-47
circuit breakers and fuses should be provided. When this protection is not provided, the excessive
voltages can cause arcing between transformer windings and between the plates of capacitors, which will
permanently destroy the dielectric material. Excessive currents can also cause overheating of tubes and
other components.
Ideally, you will want to operate a transmitter at 100-percent modulation so that you can provide the
maximum amount of energy in the sideband. However, because of the large and rapid fluctuations in
amplitude that these signals normally contain, this ideal condition is seldom possible. When the
modulator is properly adjusted, the loudest parts of the transmission will produce 100-percent modulation.
The quieter parts of the signal then produce lesser degrees of modulation.
To measure degrees of modulation less than 100 percent, you can use a MODULATION FACTOR
(M) to indicate the relative magnitudes of the rf carrier and the audio-modulating signal. Numerically, the
modulation factor is:
To illustrate this use of the equation, assume that a carrier wave with a peak amplitude of 400 volts is
modulated by a 3-kilohertz sine wave with a peak amplitude of 200 volts. The modulation factor is
figured as follows:
If the modulation factor were multiplied by 100, the resultant quantity would be the PERCENT OF
MODULATION (%M):
1-48
By using the correct equation, you can determine the percent of modulation from the modulation
envelope pattern. This method is useful when the percent of modulation is to be determined using the
pattern on the screen of an oscilloscope. For example, assume that your oscilloscope is connected to the
output of a modulator circuit and produces the screen pattern shown in figure 1-42. According to the
setting of the calibration control, each large division on the vertical scale is equal to 200 volts. By using
this scale, you can see that the peak carrier amplitude (unmodulated portion) is 400 volts. The peak
amplitude of the carrier is designated as eo in figure 1-42.
Figure 1-42.—Computing percent of modulation from the modulation envelope.
The amplitude of the audio-modulating voltage can also be determined from amplitude variations in
the envelope pattern. Notice that the peak-to-peak variations in envelope amplitude (emax − e min) is equal
to 400 volts on the scale. Note then that the peak amplitude of the audio voltage is 200 volts. If these rf
and audio voltage values are inserted into the equation, the pattern in figure 1-42 is found to represent
50-percent modulation.
1-49
If Em and Ec in the equation are assumed to represent peak-to-peak values, the following formula
results:
Since the peak-to-peak value of E m in figure 1-42 is emax − emin, we can substitute as follows:
Also, since the peak-to-peak value of the carrier Ec is 2 times eo, we can subsititute 2eo for Ec as
follows:
Linear vertical distance represents voltage on the screen of a cathode-ray tube. Vertical distance units
can be used in place of voltage in equations. Thus, if only the percent of modulation is required, the
oscilloscope need not be calibrated and the actual circuit voltages are not required. In figure 1-42, emax
represents 600 volts (3 large divisions); emin is 200 volts (1 division); and eo is 400 volts (2 divisions).
Using the equation and the dimensions of the screen pattern, you can figure the percent of modulation as
follows:
When eo of the equation is difficult to measure, an alternative solution can be obtained with the
equation below:
VECTOR ANALYSIS OF AN AM WAVE.—You studied earlier in this chapter that the
modulation envelope results when the instantaneous sums of the carrier and sideband voltages are plotted
with respect to time. An attempt to add these three voltages, point-by-point, would prove to be a huge
task. The same end result can be obtained by using a rotating vector to represent each of the three
1-50
frequencies in the composite envelope. In the following analysis, vectors will be scaled to indicate the
peak voltage value of the frequencies they represent.
The analysis has been simplified further by using a frequency of 8 hertz to represent the carrier
frequency. Each cycle of the carrier then requires 1/8 of a second to complete 360 degrees. The carrier
will be 100-percent modulated by a sine wave having a frequency of 1 hertz, thereby producing sideband
frequencies of 7 and 9 hertz.
Envelope Development from Vectors.—The modulating signal, upper sideband, carrier, and lower
sideband waveforms are illustrated in views (A) through (D), respectively, in figure 1-43. Notice that the
vertical lines passing through the figure divide each waveform into segments of 1/8 of a second each.
These lines also coincide with the starting and ending points of each cycle of the carrier wave.
Figure 1-43.—Formation of the modulation envelope by the addition of vectors representing the carrier and
sidebands.
1-51
During the first 1/8 of a second (T1 to T2), the carrier wave completes exactly 1 cycle, or 360
degrees, as shown in view (C). The upper sideband, which has a frequency of 9 hertz, will complete each
cycle in less than 1/8 of a second. Therefore, during the time required for the carrier to complete 1 cycle
of 360 degrees, the upper sideband [view (B)] is able to complete 1 cycle of 360 degrees plus an
additional 45 degrees of the next cycle, for a total of 405 degrees.
The lower sideband [view (D)] has a frequency of 7 hertz and cannot complete an entire cycle in 1/8
of a second. During the time interval required for the carrier wave to progress through 360 degrees, the
lower sideband frequency of 7 hertz can complete only 315 degrees, 45 degrees short of a full cycle.
Keeping these factors in mind, you should be able to see that the phase angles between the two
sideband frequencies, and between each sideband frequency and the carrier frequency, will continually
shift. At an instant in time (T3), the carrier and sidebands will be in phase [view (E)], causing the
envelope amplitude [view (F)] to be twice the amplitude of the carrier. At another instant in time (T7), the
sidebands are out of phase with the carrier [view (E)], causing complete cancellation of the rf voltage.
The envelope amplitude will become 0 at this point. You should see that, although the carrier and
sideband frequencies have constant amplitudes, the ever-changing phase differences between them causes
the modulation envelope to vary continuously in amplitude.
The vector analysis of the modulation envelope will be developed with the aid of figure 1-44. In
figure 1-44, view (A), a vertical vector (C) has been drawn to represent the carrier wave in figure 1-43. At
T1 in figure 1-43, the upper and lower sideband frequencies are of opposite phase with respect to each
other, and 90 degrees out of phase with respect to the carrier. This condition is illustrated in figure 1-44,
view (A), by sideband vectors U and L drawn in opposite directions along the horizontal axis. Since the
upper sideband U is equal in amplitude but opposite in phase to lower sideband L, the two sideband
voltages cancel one another; the amplitude of the envelope at T1 is equal to the amplitude of the carrier.
The same vector diagram is shown on a smaller scale in figure 1-43, view (E).
Figure 1-44A.—Vector diagrams for T1 and T2.
During the 1/8 of a second time interval between T1 and T2, all three vectors rotate in a
counterclockwise direction at a velocity determined by their respective frequencies. The vector
representing the carrier, for example, has made one complete rotation of 360 degrees and is back in its
original position, as shown in figure 1-44, view (B). The upper sideband frequency, however, will
complete 405 degrees in this same 1/8 of a second. Notice in view (B) that vector U has made one
complete counterclockwise rotation of 360 degrees, plus an additional 45 degrees for a total rotation of
1-52
405 degrees. Vector L, representing the lower sideband, rotates at a velocity less than that of either the
carrier or the upper sideband. In 1/8 of a second, vector L completes only 315 degrees, which is 45
degrees short of one complete rotation. At the end of 1/8 of a second, the three vectors have advanced to
the positions shown in view (B).
Figure 1-44B.—Vector diagrams for T1 and T2.
The resultant vector in view (B) is obtained by adding vector U to vector L. Since each sideband has
one-half the amplitude of the carrier, and the two sidebands differ in phase by 90 degrees, the amplitude
of the resultant vector can be computed. This computation (not shown) would show the resultant vector to
have an amplitude that is approximately 70 percent that of the carrier. Thus, at T2 the amplitude of the
modulation envelope is about 1.7 times the amplitude of the carrier. This condition is shown in figure
1-43, view (F).
By a similar procedure, vector diagrams can be constructed for time intervals T3 through T9. This
has been done in figure 1-43, view (E). From these nine individual vector diagrams, the complete
modulation envelope in figure 1-43, view (F), can be constructed.
Notice in particular the vector diagrams for T3 and T7. At T3, all three waves, and therefore all three
vectors, are in phase. The modulation envelope at this instant must, therefore, be equal to twice the
amplitude of the carrier since each sideband frequency has one-half the amplitude of the carrier.
At T7, the two sideband frequencies are in phase with each other but 180 degrees out of phase with
the carrier. This causes the combined sideband voltage to cancel the carrier voltage, and the modulation
envelope becomes 0 at that instant. Note that for the transmitter output to be 0 at T7, both the carrier and
sideband frequencies must be present. If any one of these three frequencies were missing, complete
cancellation would not occur and rf energy would be present in the output.
Although this vector analysis was made for frequencies of 7, 8, and 9 hertz, the same description
could be applied to the frequencies actually present at the output of a transmitter.
Modulation Level of an AM Wave
As stated earlier, the modulating signal can be introduced into any active element of a tube. In
addition to the various arrangements possible within a single stage, the modulating signal can also be
applied to any of the rf stages in the transmitter. For example, the modulating signal could be applied to
the control grid or plate of one of the intermediate power amplifiers.
A modulator circuit is usually placed into one of two categories, high- or low-level modulation.
Circuits are categorized according to the level of the carrier wave at the point in the system where the
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modulation is applied. The FCC defines HIGH-LEVEL MODULATION in the Code of Federal
Regulations as "modulation produced in the plate circuit of the last radio stage of the system." This same
document defines LOW-LEVEL MODULATION as "modulation produced in an earlier stage than the
final."
Q-36. What is percent of modulation?
Q-37. With a single modulating tone, what is the amplitude of the sideband frequencies at 100-percent
modulation?
Q-38. What is the formula for percent of modulation?
Q-39. What is high-level modulation?
MODULATION SYSTEMS
To complete your understanding of AM modulation, we are now going to analyze the operation of a
typical plate modulator. Detailed circuit descriptions will be used to give you an understanding of a basic
AM plate modulator. In addition, we will cover basic circuit descriptions for cathode and grid electrontube modulators and for base, emitter, and collector transistor modulators in this chapter.
Plate Modulator
Figure 1-45 is a basic plate-modulator circuit. Plate modulation permits the transmitter to operate
with high efficiency. It is the simplest of the modulators available and is also the easiest to adjust for
proper operation. The modulator is coupled to the plate circuit of the final rf amplifier through the
modulation transformer. For 100-percent modulation, the modulator must supply enough power to cause
the plate voltage of the final rf amplifier to vary between 0 and twice the dc operating plate voltage. The
modulator tube (V2) is a power amplifier biased so that it operates class A. The final rf power amplifier
(V1) is biased in the nonlinear portion of its operating range (class C). This provides for efficient
operation of V1 and produces the necessary heterodyning action between the rf carrier and the af
modulating frequencies.
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Figure 1-45.—Plate-modulation circuit.
PLATE MODULATOR CIRCUIT OPERATION.—Figure 1-46, views (A) through (E), shows
the waveforms associated with the plate-modulator circuit shown in figure 1-45. Refer to these two
figures throughout the following discussion.
1-55
Figure 1-46.—Plate-modulator waveforms.
The rf power amplifier (V1) acts as a class C amplifier when no modulation is present in the plate
circuit. V2 is the modulator which transfers the modulating voltage to the plate circuit of V1. Let's see
how this circuit produces a modulated rf output.
View (A) of figure 1-46 shows the plate supply voltage for V1 as a constant dc value (Eb) at time 1
with no modulating signal applied. V1 is biased at cutoff at this time. The incoming rf carrier [view (A)]
is applied to the grid of V1 by transformer T1 and causes the plate circuit current to PULSE (SURGE)
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each time the grid is driven positive. These rf pulses are referred to as current pulses and are shown in
view (C). The plate tank output circuit (T3) is shocked into oscillation by these current pulses and the rf
output waveform shown in view (E) is developed. The rf plate voltage waveform is shown in view (D).
An audio-modulating voltage applied to the grid of V2 is amplified by the modulator and coupled to
the plate of V1 by modulation transformer T2. The secondary of T2 is in series with the plate-supply
voltage (E) of V1. The modulating voltage will either add to or subtract from the plate voltage of V1. This
is shown in view (A) at time 2 and time 3. At time 2 in view (A), the plate supply voltage for V1
increases to twice its normal value and the rf plate current pulses double, as shown in view (C). At time 3
in view (A), the supply voltage is reduced to 0 and the rf plate current decreases to 0, as shown in view
(C). These changes in rf plate current cause rf tank T3 voltage to double at time 2 and to decrease to 0 at
time 3, as shown in view (E). This action results in the modulation envelope shown in view (E) that
represents 100-percent modulation. This is transformer-coupled out of tank circuit T3 to an antenna.
Because of the oscillating action of tank circuit T3, V1 has to be rated to handle at least four times its
normal plate supply voltage (Eb), as shown by the plate voltage waveform in view (D).
Heterodyning the audio frequency intelligence from the modulator (V2) with the carrier in the plate
circuit of the final power amplifier (V1) requires a large amount of audio power. All of the power or
voltage that contains the intelligence must come from the modulator stage. This is why plate modulation
is called high-level modulation.
The heterodyning action in the plate modulator effectively changes an audio frequency to a different
part of the frequency spectrum. This action allows antennas and equipment of practical sizes to be used to
transmit the intelligence. Now, let's look at several other typical modulators.
Collector-Injection Modulator
The COLLECTOR-INJECTION MODULATOR is the transistor equivalent of the electron-tube AM
plate modulator. This transistor modulator can be used for low-level or relatively high-level modulation.
It is referred to as relatively high-level modulation because, at the present time, transistors are limited in
their power-handling capability. As illustrated in figure 1-47, the circuit design for a transistor collectorinjection modulator is very similar to that of a plate modulator. The collector-injection modulator is
capable of 100-percent modulation with medium power-handling capabilities.
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Figure 1-47.—Collector-injection modulator.
In figure 1-47, the rf carrier is applied to the base of modulator Q1. The modulating signal is applied
to the collector in series with the collector supply voltage through T3. The output is then taken from the
secondary of T2. With no modulating signal, Q1 acts as an rf amplifier for the carrier frequency. When
the modulation signal is applied, it adds to or subtracts from the collector supply voltage. This causes the
rf current pulses of the collector to vary in amplitude with the collector supply voltage. These collector
current pulses cause oscillations in the tank circuit (C4 and the primary of T2). The tank circuit is tuned to
the carrier frequency. During periods when the collector current is high, the tank circuit oscillates
strongly. At times when the collector current is small, or entirely absent, little or no energy is supplied to
the tank and oscillations become weak or die out. Thus, the modulation envelope is developed as it was in
a plate modulator.
As transistor technology continues to develop, higher power applications of transistor collectorinjection modulation will be employed. Plate and collector-injection modulation are the most commonly
used types of modulation because the modulating signal can be applied in the final stages of rf
amplification. This allows the majority of the rf amplifier stages to be operated class C for maximum
efficiency. The plate and collector-injection modulators also require large amounts of af modulating
power since the modulator stage must supply the power contained in the sidebands.
Q-40. For what class of operation is the final rf power amplifier of a plate-modulator circuit biased?
Q-41. The modulator is required to be what kind of a circuit stage in a plate modulator?
Q-42. How much must the fpa plate current vary to produce 100-percent modulation in a plate
modulator?
1-58
Q-43. The collector-injection modulator is similar to what type of tube modulator?
Control-Grid Modulator
In cases when the use of a minimum of af modulator power is desired, a form of low-level
modulation is necessary. The CONTROL-GRID MODULATOR is used widely in portable and mobile
equipment to reduce size and power requirements. It is also used in extremely high-power, wideband
equipment such as television transmitters where high-level or plate modulation is difficult and costly to
achieve. Figure 1-48 is a basic schematic for a typical control-grid modulator.
Figure 1-48.—Control-grid modulator.
The control-grid modulator uses a variation of grid bias (at the frequency of the modulating signal)
to vary the instantaneous plate voltage and current. These variations cause modulation of the carrier
frequency. The carrier frequency is introduced through coupling capacitor Cc. The modulating frequency
is introduced in series with the grid bias through T1. As the modulating signal increases and decreases
(positive and negative), it will add to or subtract from the bias on rf amplifier V1. This change in bias
causes a corresponding change in plate voltage and current. These changes in plate voltage and current
add vectorially to the carrier frequency and provide a modulation envelope in the same fashion as does
the plate modulator. Since changes in the plate circuit of the rf amplifier are controlled by changes in the
grid bias, the gain of the tube requires only a low-level modulating signal. Even when the input signals
are at these low levels, occasional modulation voltage peaks will occur that will cause V1 to saturate. This
creates distortion in the output. Care must be taken to bias the rf amplifier tube for maximum power out
1-59
while maintaining minimum distortion. The power to develop the modulation envelope comes from the rf
amplifier. Because the rf amplifier has to be capable of supplying this additional power, it is biased for
(and driven by the carrier frequency at) a much lower output level than its rating. This reduced efficiency
is necessary during nonmodulated periods to provide the tube with the power to develop the sidebands.
Compared to plate modulation, grid modulation is less efficient, produces more distortion, and
requires the rf power amplifier to supply all the power in the output signal. Grid modulation has the
advantage of not requiring much power from the modulator.
Base-Injection Modulator
The BASE-INJECTION MODULATOR is similar to the control-grid modulator in electron-tube
circuits. It is used to produce low-level modulation in equipment operating at very low power levels.
In figure 1-49, the bias on Q1 is established by the voltage divider R1 and R2. With the rf carrier
input at T1, and no modulating signal, the circuit acts as a standard rf amplifier. When a modulating
signal is injected through C1, it develops a voltage across R1 that adds to or subtracts from the bias on
Q1. This change in bias changes the gain of Q1, causing more or less energy to be supplied to the
collector tank circuit. The tank circuit develops the modulation envelope as the rf frequency and af
modulating frequency are mixed in the collector circuit. Again, this action is identical to that in the plate
modulator.
Figure 1-49.—Base-injection modulator.
Because of the extremely low-level signals required to produce modulation, the base-injection
modulator is well suited for use in small, portable equipment, such as "walkie-talkies," and test
equipment.
Cathode Modulator
Another low-level modulator, the CATHODE MODULATOR, is generally employed where the
audio power is limited and distortion of the grid-modulated circuit cannot be allowed. The cathode
1-60
modulator varies the voltage of the cathode to produce the modulation envelope. Since the cathode is in
series with the grid and plate circuits, you should be able see that changing the cathode voltage will
effectively change the voltage of the other tube elements. By properly controlling the voltages on the
tube, you can cause the cathode modulator to operate in a form of plate modulation with high efficiency.
Usually, the cathode modulator is designed to perform about midway between plate and grid modulator
levels, using the advantages of each type. When operated between the two levels, the modulator provides
a more linear output with moderate efficiency and a modest audio power requirement.
In figure 1-50, the rf carrier is applied to the grid of V1 and the modulating signal is applied in series
with the cathode through T1. Since the modulating signal is effectively in series with the grid and plate
voltage, the level of modulating voltage required will be determined by the relationships of the three
voltages. The modulation takes place in the plate circuit with the plate tank developing the modulation
envelope, just as it did in the plate modulator.
Figure 1-50.—Cathode modulator.
Emitter-Injection Modulator
This is the transistor equivalent of the cathode modulator. The EMITTER-INJECTION
MODULATOR has the same characteristics as the base-injection modulator discussed earlier. It is an
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extremely low-level modulator that is useful in portable equipment. In emitter-injection modulation, the
gain of the rf amplifier is varied by the changing voltage on the emitter. The changing voltage is caused
by the injection of the modulating signal into the emitter circuitry of Q1, as shown in figure 1-51. Here
the modulating voltage adds to or subtracts from transistor biasing. The change in bias causes a change in
collector current and results in a heterodyning action. The modulation envelope is developed across the
collector-tank circuit.
Figure 1-51.—Emitter-injection modulator.
Q-44. When is a control-grid modulator used?
Q-45. What type of modulator is the cathode modulator (low- or high-level)?
Q-46. What causes the change in collector current in an emitter-injection modulator?
You have studied six methods of amplitude modulation. These are not the only methods available,
but they are the most common. All methods of AM modulation use the same theory of heterodyning
across a nonlinear device. AM modulation is one of the easiest and least expensive types of modulation to
achieve. The primary disadvantages of AM modulation are susceptibility to noise interference and the
inefficiency of the transmitter. Power is wasted in the transmission of the carrier frequency because it
contains no AM intelligence. In the next chapter, you will study other forms of modulation that have been
developed to overcome these disadvantages.
SUMMARY
Now that you have completed this chapter, a short review of what you have learned is in order. The
following summary will refresh your memory of amplitude modulation, its basic principles, and typical
circuitry used to generate this modulation.
The SINE WAVE is the basis for all complex waveforms and is generated by moving a coil through
a magnetic field.
1-62
AMPLITUDE (instantaneous voltage) of a coil is found by the formula:
PHASE or PHASE ANGLE is the angle that exists between the starting position of a vector
generating the sine wave and its position at a given instant.
FREQUENCY is the rate at which the vector rotates.
HETERODYNING is the process of mixing two different frequencies across a nonlinear impedance
to give the ORIGINAL frequencies, a SUM frequency, and a DIFFERENCE frequency.
1-63
CONTINUOUS-WAVE MODULATION is the basic form of rf communications. It is essentially
on-off keying of an rf carrier.
HAND-OPERATED and MACHINE KEYING are two types of cw keying. PLATE, CATHODE,
and BLOCKED-GRID KEYING are circuits commonly used in hand-operated and machine keying.
KEYING RELAYS are used for safety and to handle the current requirements in high-power
transmitters.
1-64
KEY-CLICK FILTERS are used to prevent interference in cw transmitters.
Although it is a relatively slow transmission method, CW COMMUNICATIONS is highly reliable
under severe noise conditions for long-range operation.
SINGLE-STAGE CW TRANSMITTERS can be made by coupling the output of an oscillator to
an antenna.
1-65
MULTISTAGE CW TRANSMITTERS are used to improve frequency stability and increase
output power.
A MICROPHONE is an energy converter that changes sound energy into electrical energy.
A CARBON MICROPHONE uses carbon granules and an external battery supply to generate af
voltages from sound waves.
1-66
A CRYSTAL MICROPHONE uses the piezoelectric effect to generate an output voltage.
A DYNAMIC MICROPHONE uses a coil of fine wire mounted on the back of a diaphragm
located in the magnetic field of a permanent magnet.
1-67
A MAGNETIC MICROPHONE uses a moving armature in a magnetic field to generate an output.
The FREQUENCY SPECTRUM of a modulated wave can be conveniently illustrated in graph
form as frequency versus amplitude.
1-68
The MODULATION ENVELOPE is the waveform observed when the CARRIER, UPPER
SIDEBAND, and LOWER SIDEBAND are combined in a single impedance and observed as time versus
amplitude.
The BANDWIDTH of an rf signal is the amount of space in the frequency spectrum used by the
signal.
PERCENT OF MODULATION is a measure of the relative magnitudes of the rf carrier and the af
modulating signal.
1-69
HIGH-LEVEL MODULATION is modulation produced in the plate circuit of the last radio stage
of the system.
LOW-LEVEL MODULATION is modulation produced in an earlier stage than the final power
amplifier.
The PLATE MODULATOR is a high-level modulator. The modulator tube must be capable of
varying the plate-supply voltage of the final power amplifier. It must vary the plate voltage so that the
plate current pulses will vary between 0 and nearly twice their unmodulated value to achieve 100-percent
modulation.
1-70
A COLLECTOR-INJECTION MODULATOR is a transistorized version of the plate modulator.
It is classified as a high-level modulator, although present state-of-the-art transistors limit them to
medium-power applications.
A CONTROL-GRID MODULATOR is a low-level modulator that is used where a minimum of af
modulator power is desired. It is less efficient than a plate modulator and produces more distortion.
1-71
A BASE-INJECTION MODULATOR is used to produce low-level modulation in equipment
operating at very low power levels. It is often used in small portable equipment and test equipment.
The CATHODE MODULATOR is a low-level modulator employed where the audio power is
limited and the inherent distortion of the grid modulator cannot be tolerated.
1-72
The EMITTER-INJECTION MODULATOR is an extremely low-level modulator that is useful in
portable equipment.
The primary disadvantages of AM modulation are susceptibility to NOISE INTERFERENCE and
the INEFFICIENCY of the transmitter.
1-73
ANSWERS TO QUESTIONS Q1. THROUGH Q46.
A-1. Modulation is the impressing of intelligence on a transmission medium.
A-2. May be anything that transmits information, such as light, smoke, sound, wire lines, or
radio-frequency waves.
A-3. Mixing two frequencies across a nonlinear impedance.
A-4. The process of recovering intelligence from a modulated carrier.
A-5. The sine wave.
A-6. To represent quantities that have both magnitude and direction.
A-7. Sine ! = opposite side ÷ hypotenuse.
A-8. e = Emax sine !.
A-9. The value at any given point on the sine wave.
A-10. Phase or phase angle.
A-11. The rate at which the vector which is generating the sine wave is rotating.
A-12. The elapsed time from the beginning of cycle to its completion.
A-13. Wavelength = rate of travel × period.
A-14. Process of combining two signal frequencies in a nonlinear device.
A-15. An impedance in which the resulting current is not proportional to the applied voltage.
A-16. The display of electromagnetic energy that is arranged according to wavelength or frequency.
A-17. At least two different frequencies applied to a nonlinear impedance.
A-18. Any method of modulating an electromagnetic carrier frequency by varying its amplitude in
accordance with the intelligence.
A-19. A method of generating oscillations, a method of turning the oscillations on and off (keying), and
an antenna to radiate the energy.
A-20. Plate keying and cathode keying.
A-21. Machine keying.
A-22. A high degree of clarity even under severe noise conditions, long-range operation, and narrow
bandwidth.
A-23. Antenna-to-ground capacitance can cause the oscillator frequency to vary.
A-24. To isolate the oscillator from the antenna and increase the amplitude of the rf oscillations to the
required output level.
A-25. To raise the low frequency of a stable oscillator to the vhf range.
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A-26. An energy converter that changes sound energy into electrical energy.
A-27. The changing resistance of carbon granules as pressure is applied to them.
A-28. Background hiss resulting from random changes in the resistance between individual carbon
granules.
A-29. The piezoelectric effect.
A-30. A dynamic microphone has a moving coil and the magnetic microphone has a moving armature.
A-31. Rf and af units.
A-32. 100 kilohertz, 5 kilohertz, 95 kilohertz, and 105 kilohertz.
A-33. All of the sum frequencies above the carrier.
A-34. The intelligence is contained in the spacing between the carrier and sideband frequencies.
A-35. The highest modulating frequency.
A-36. The depth or degree of modulation.
A-37. One-half the amplitude of the carrier.
A-38.
A-39. Modulation produced in the plate circuit of the last radio stage of the system.
A-40. Class C.
A-41. Power amplifier.
A-42. Between 0 and nearly two times its unmodulated value.
A-43. Plate modulator.
A-44. In cases when the use of a minimum of af modulator power is desired.
A-45. Low-level.
A-46. Gain is varied by changing the voltage on the emitter.
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CHAPTER 2
ANGLE AND PULSE MODULATION
LEARNING OBJECTIVES
Upon completion of this chapter you will be able to:
1. Describe frequency-shift keying (fsk) and methods of providing this type of modulation.
2. Describe the development of frequency modulation (fm) and methods of frequency modulating a
carrier.
3. Discuss the development of phase modulation (pm) and methods of phase modulating a carrier.
4. Describe phase-shift keying (psk), its generation, and application.
5. Discuss the development and characteristics of pulse modulation.
6. Describe the operation of the spark gap and thyratron modulators.
7. Discuss the characteristics of a pulse train that may be varied to provide communications
capability.
8. Describe pulse-amplitude modulation (pam) and generation.
9. Describe pulse-duration modulation (pdm) and generation.
10. Describe pulse-position modulation (ppm) and generation.
11. Describe pulse-frequency modulation (pfm) and generation.
12. Describe pulse-code modulation (pcm) and generation.
INTRODUCTION
In chapter 1 you learned that modulation of a carrier frequency was necessary to allow fast
communications between two points. As the volume of transmissions increased, a need for more reliable
methods of communication was realized. In this chapter you will study angle modulation and pulse
modulation. These two types of modulation have been developed to overcome one of the main
disadvantages of amplitude modulation - susceptibility to noise interference. In addition, a special
application of pulse type modulation for ranging and detection equipment will be discussed.
2-1
ANGLE MODULATION
ANGLE MODULATION is modulation in which the angle of a sine-wave carrier is varied by a
modulating wave. FREQUENCY MODULATION (fm) and PHASE MODULATION (pm) are two types
of angle modulation. In frequency modulation the modulating signal causes the carrier frequency to vary.
These variations are controlled by both the frequency and the amplitude of the modulating wave. In phase
modulation the phase of the carrier is controlled by the modulating waveform. Let’s study these
modulation methods for an understanding of their similarities and differences.
FREQUENCY-MODULATION SYSTEMS
In frequency modulation an audio signal is used to shift the frequency of an oscillator at an audio
rate. The simplest form of this is seen in FREQUENCY-SHIFT KEYING (fsk). Frequency-shift keying is
somewhat similar to continuous-wave keying (cw) in AM transmissions.
Frequency-Shift Keying
Consider figure 2-1, views (A) through (D). View (A) is a radio frequency (rf) carrier which is
actually several thousand or million hertz. View (B) represents the intelligence to be transmitted as
MARKS and SPACES. Recall that in cw transmission, this intelligence was applied to the rf carrier by
interrupting the signal, as shown in view (C). The amplitude of the rf alternated between maximum and 0
volts. By comparing views (B) and (C), you can see the mark/space intelligence of the Morse code
character on the rf. The spacing of the waveform in view (D) is an example of the same intelligence as it
is applied to the frequency instead of the amplitude of the rf. This is simple frequency-shift keying of the
same Morse code character.
Figure 2-1A.—Comparison of ON-OFF and frequency-shift keying. RF CARRIER (CW) (EACH CYCLE
REPRESENTS SEVERAL THOUSAND OR SEVERAL MILLION CYCLES).
Figure 2-1B.—Comparison of ON-OFF and frequency-shift keying. AMPLITUDE VARYING (ON-OFF)
MODULATING WAVE (MORSE CODE CHARACTER "N").
2-2
Figure 2-1C.—Comparison of ON-OFF and frequency-shift keying. TRANSMITTED ON-OFF KEYED CW
SIGNAL.
Figure 2-1D.—Comparison of ON-OFF and frequency-shift keying. TRANSMITTED FREQUENCY-SHIFT
KEYED SIGNAL (FSK).
In fsk the output is abruptly changed between two differing frequencies by opening and closing the
key. This is shown in view (D). For illustrative purposes, the spacing frequency in view (D) is shown as
double the marking frequency. However, in practice the difference is usually less than 1,000 hertz, even
when operating at several megahertz. You should also note that the limit of frequency shift is determined
without reference to the amplitude of the keying signal in the fsk system. The frequency shift may be set
at plus or minus 425 hertz from the allocated channel frequency. The total shift between mark and space
would be 850 hertz. Either the mark or space may use the higher of the two frequencies. The upper
frequency of the transmitted signal is usually the spacing interval and the lower frequency is the marking
interval.
COMPARING FSK AND CW SIGNALS.—A comparison of on-off keyed cw (figure 2-2), (view
(A), view (B), view (C)), and fsk (figure 2-3), (view (A), view (B), view (C)), signals will show clearly
the principal features of fsk and give us a basis on which frequency modulation can be discussed. Let's
use views (A), (B), and (C) of both figures to show the Morse code character "F" for an example. Figures
2-2 and 2-3 are graphic drawings of the two types of keying. Time and amplitude are known dimensions
of AM; but to explain fsk properly, we have added the third dimension of frequency.
2-3
Figure 2-2A.—Comparison of AM and fm receiver response to an AM signal. THREE-DIMENSIONAL
REPRESENTATION OF AN ON-OFF KEYED CW TELEGRAPH SIGNAL.
Figure 2-2B.—Comparison of AM and fm receiver response to an AM signal. RESPONSE OF AM
DEMODULATOR TO SIGNAL.
Figure 2-2C.—Comparison of AM and fm receiver response to an AM signal.RESPONSE OF FM
DISCRIMINATOR TO SIGNAL.
2-4
Figure 2-3A.—Comparison of AM and fm receiver response to an fm signal. THREE-DIMENSIONAL
REPRESENTATION OF A FREQUENCY-SHIFT KEYED TELEGRAPH SIGNAL.
Figure 2-3B.—Comparison of AM and fm receiver response to an fm signal. RESPONSE OF AM
DEMODULATOR TO SIGNAL.
Figure 2-3C.—Comparison of AM and fm receiver response to an fm signal. RESPONSE OF FM
DISCRIMINATOR TO SIGNAL.
CW SIGNALS.—Since cw signals are of essentially constant frequency, there is no variation along
the frequency axis in view (A) of figure 2-2. The complete intelligence is carried as variations in the
amplitude of the signal. To receive the intelligence carried by such a signal, the receiving equipment must
be able to scan the signal along the time and amplitude axes, which carry the information. When scanned
along the time and amplitude axes [shown in view (B)], the intelligence appears as large changes in
amplitude. If the circuit were perfect, these variations would be from 0 amplitude to some maximum
value (established by transmitter power, distance, and so forth) depending on whether the key were open
or closed. However, interfering components of energy caused by atmospherics, interfering stations, and
2-5
electrical machinery appear as additional variations along the amplitude axis. When these amplitude
variations approach or exceed the variation caused by the keyed intelligence, the signal is blanked out by
interference. We have all heard this happen on our AM radios during storms or when near operating
machinery.
View (C) of figure 2-2 represents the same signal when scanned along the time and frequency axes
as it would be in an fm receiver. Variations in signal amplitude have no effect on the frequency and no
intelligence can be received. Note that the noise and interference components have also been suppressed
so that they have little effect on the received signal. Thus, if the intelligence variations were impressed as
changes along the frequency axis, and the receiving equipment were designed to respond to this type of
signal, then the effects of noise and interference would be practically eliminated. Frequency-shift keyed
circuits fulfill these conditions.
FSK SIGNALS.—In fsk the rf signal is shifted in frequency (not amplitude) between "key-open"
and "key-closed" conditions. The signal amplitude remains essentially constant. View (A) of figure 2-3
represents the letter "F" keyed as a shift in frequency between mark and space. The normal frequency
condition with the key open is a space. Recall that this may be either the lower or higher frequency. When
the key is closed, the frequency instantly changes to the mark value and remains constant during the
marking interval. Opening the key again returns the frequency to the space frequency. Midway between
the mark and space frequencies is the assigned channel frequency.
Also shown in view (A) is the variation along the amplitude axis caused by the same noise and
interference mentioned earlier. The right-hand portion of view (A) illustrates the elimination of this noise
by the receiving equipment. View (B) clearly shows that scanning the signal along the amplitude and time
axes reproduces no amplitude variations from signal interference. However, if the scanning is
accomplished along the frequency and time axes, the intelligence is reproduced, as shown in view (C). By
this system, the intelligence can be recovered at the receiving station in its original form; it will be nearly
unaffected by conditions in the radio path other than fading. As a matter of fact, fsk resists the effects of
fading better than cw.
FREQUENCY-SHIFT KEYING.—In its simplest form, frequency-shift keying of a transmitter
can be accomplished by shunting a capacitor (or an inductor) and key (in series) across the oscillator
circuit. By locking the normal key of the transmitter and operating only the oscillator circuit key, you can
change the oscillator frequency. The shift in frequency between mark (key-closed) and space (key-open)
conditions is determined by the effect of the additional capacitance (or inductance) on the oscillator
frequency. The frequency multiplication factor in the transmitter amplifiers must be taken into
consideration when determining the oscillator frequency shift. Thus, if the desired shift is the
conventional 850 hertz at the transmission frequency, and this frequency is four times the oscillator
frequency (that is, doubled in two stages), then the effect of the additional capacitance (or inductance) on
the oscillator must be limited to 212.5 hertz as shown below:
2-6
Frequency-shift keyers are, of course, more complicated than this simple illustration would seem to
show, but the basic principles are the same. Still, the keyer does change the oscillator frequency by a
certain number of cycles. Further, this change must be correlated with the multiplication factor of the
transmitter to cause the desired shift between mark and space frequencies.
METHODS OF FREQUENCY SHIFTING.—Frequency-shift keyers operate on either of two
general principles. First, the keyer may take the output of the transmitter's master oscillator and modulate
it with the output of another oscillator that is frequency-shift keyed. This action will result in two
frequencies that are used to excite the first amplifier stage of the transmitter. This system is illustrated in
view (A) of figure 2-4. View (B) illustrates the second method of frequency-shift keyer operation. In this
method the transmitter's master oscillator is itself shifted in frequency by the mark and space impulses
from the keyer unit.
Figure 2-4A.—Two methods of frequency-shift keying (fsk). FREQUENCY-SHIFT KEYING BY MODULATING
MASTER OSCILLATOR OUTPUT.
2-7
Figure 2-4B.—Two methods of frequency-shift keying (fsk). FREQUENCY-SHIFT KEYING IN MASTER
OSCILLATOR CIRCUIT.
ADVANTAGES OF FSK OVER AM.—Frequency-shift keying is used in all single-channel,
radiotelegraph systems that use automatic printing systems. The advantage of fsk over on-off keyed cw is
that it rejects unwanted signals (noise) that are weaker than the desired signal. This is true of all fm
systems. Also, since a signal is always present in the fsk receiver, automatic volume control methods may
be used to minimize the effects of signal fading caused by ionospheric variations. The amount of inherent
signal-to-noise ratio improvement of fsk over AM is approximately 3 to 4 dB. This improvement is
because the signal energy of fsk is always present while signal energy is present for only one-half the time
in AM systems. Noise is continuously present in both fsk and AM, but is eliminated in fsk reception.
Under the rapid fading and high-noise conditions that commonly exist in the high frequency (hf) region,
fsk shows a marked advantage over AM. Overall improvement is sometimes expressed as the RATIO OF
TRANSMITTED POWERS required to give equivalent transmission results over the two systems. Such a
ratio varies widely, depending on the prevailing conditions. With little fading, the ratio may be entirely
the result of the improvement in signal-to-noise ratio and may be under 5 dB. However, under severe
fading conditions, large amounts of power often fail to give good results for AM transmission. At the
same time, fsk may be satisfactory at nominal power. The power ratio (fsk versus AM) would become
infinite in such a case.
Another application of fsk is at low and very low frequencies (below 300 kilohertz). At these
frequencies, keying speeds are limited by the "flywheel" effect of the extremely large capacitance and
inductance of the antenna circuits. These circuits tend to oscillate at their resonant frequencies.
Frequency-shifting the transmitter and changing the antenna resonance by the same keying impulses will
result in much greater keying speeds. As a result, the use of these expensive channels is much more
efficient.
Q-1.
What are the two types of angle modulation?
Q-2.
Name the modulation system in which the frequency alternates between two discrete values in
response to the opening and closing of a key?
Q-3.
What is the primary advantage of an fsk transmission system?
Frequency Modulation
In frequency modulation, the instantaneous frequency of the radio-frequency wave is varied in
accordance with the modulating signal, as shown in view (A) of figure 2-5. As mentioned earlier, the
amplitude is kept constant. This results in oscillations similar to those illustrated in view (B). The number
2-8
of times per second that the instantaneous frequency is varied from the average (carrier frequency) is
controlled by the frequency of the modulating signal. The amount by which the frequency departs from
the average is controlled by the amplitude of the modulating signal. This variation is referred to as the
FREQUENCY DEVIATION of the frequency-modulated wave. We can now establish two clear-cut rules
for frequency deviation rate and amplitude in frequency modulation:
Figure 2-5.—Effect of frequency modulation on an rf carrier.
•
AMOUNT OF FREQUENCY SHIFT IS PROPORTIONAL TO THE AMPLITUDE OF THE
MODULATING SIGNAL
(This rule simply means that if a 10-volt signal causes a frequency shift of 20 kilohertz, then a
20-volt signal will cause a frequency shift of 40 kilohertz.)
•
RATE OF FREQUENCY SHIFT IS PROPORTIONAL TO THE FREQUENCY OF THE
MODULATING SIGNAL
(This second rule means that if the carrier is modulated with a 1-kilohertz tone, then the carrier is
changing frequency 1,000 times each second.)
Figure 2-6 illustrates a simple oscillator circuit with the addition of a condenser microphone (M) in
shunt with the oscillator tank circuit. Although the condenser microphone capacitance is actually very
low, the capacitance of this microphone will be considered near that of the tuning capacitor (C). The
frequency of oscillation in this circuit is, of course, determined by the LC product of all elements of the
circuit; but, the product of the inductance (L) and the combined capacitance of C and M are the primary
frequency components. When no sound waves strike M, the frequency is the rf carrier frequency. Any
excitation of M will alter its capacitance and, therefore, the frequency of the oscillator circuit. Figure 2-7
illustrates what happens to the capacitance of the microphone during excitation. In view (A), the audiofrequency wave has three levels of intensity, shown as X, a whisper; Y, a normal voice; and Z, a loud
voice. In view (B), the same conditions of intensity are repeated, but this time at a frequency twice that of
view (A). Note in each case that the capacitance changes both positively and negatively; thus the
frequency of oscillation alternates both above and below the resting frequency. The amount of change is
determined by the change in capacitance of the microphone. The change is caused by the amplitude of the
sound wave exciting the microphone. The rate at which the change in frequency occurs is determined by
2-9
the rate at which the capacitance of the microphone changes. This rate of change is caused by the
frequency of the sound wave. For example, suppose a 1,000-hertz tone of a certain loudness strikes the
microphone. The frequency of the carrier will then shift by a certain amount, say plus and minus 40
kilohertz. The carrier will be shifted 1,000 times per second. Now assume that with its loudness
unchanged, the frequency of the tone is changed to 4,000 hertz. The carrier frequency will still shift plus
and minus 40 kilohertz; but now it will shift at a rate of 4,000 times per second. Likewise, assume that at
the same loudness, the tone is reduced to 200 hertz. The carrier will continue to shift plus and minus 40
kilohertz, but now at a rate of 200 times per second. If the loudness of any of these modulating tones is
reduced by one-half, the frequency of the carrier will be shifted plus and minus 20 kilohertz. The carrier
will then shift at the same rate as before. This fulfills all requirements for frequency modulation. Both the
frequency and the amplitude of the modulating signal are translated into variations in the frequency of the
rf carrier.
Figure 2-6.—Oscillator circuit illustrating frequency modulation.
Figure 2-7A.—Capacitance change in an oscillator circuit during modulation. CHANGE IN INTENSITY OF
SOUND WAVES CHANGES CAPACITY.
2-10
Figure 2-7B.—Capacitance change in an oscillator circuit during modulation. AT A FREQUENCY TWICE THAT
OF (A), THE CAPACITY CHANGES THE SAME AMOUNT, BUT TWICE AS OFTEN.
Figure 2-8 shows how the frequency shift of an fm signal goes through the same variations as does
the modulating signal. In this figure the dimension of the constant amplitude is omitted. (As these
remaining waveforms are presented, be sure you take plenty of time to study and digest what the figures
tell you. Look each one over carefully, noting everything you can about them. Doing this will help you
understand this material.) If the maximum frequency deviation is set at 75 kilohertz above and below the
carrier, the audio amplitude of the modulating wave must be so adjusted that its peaks drive the frequency
only between these limits. This can then be referred to as 100-PERCENT MODULATION, although the
term is only remotely applicable to fm. Projections along the vertical axis represent deviations in
frequency from the resting frequency (carrier) in terms of audio amplitude. Projections along the
horizontal axis represent time. The distance between A and B represents 0.001 second. This means that
carrier deviations from the resting frequency to plus 75 kilohertz, then to minus 75 kilohertz, and finally
back to rest would occur 1,000 times per second. This would equate to an audio frequency of 1,000 hertz.
Since the carrier deviation for this period (A to B) extends to the full allowable limits of plus and minus
75 kilohertz, the wave is fully modulated. The distance from C to D is the same as that from A to B, so
the time interval and frequency are the same as before. Notice, however, that the amplitude of the
modulating wave has been decreased so that the carrier is driven to only plus and minus 37.5 kilohertz,
one-half the allowable deviation. This would correspond to only 50-percent modulation if the system
were AM instead of fm. Between E and F, the interval is reduced to 0.0005 second. This indicates an
increase in frequency of the modulating signal to 2,000 hertz. The amplitude has returned to its maximum
allowable value, as indicated by the deviation of the carrier to plus and minus 75 kilohertz. Interval G to
H represents the same frequency at a lower modulation amplitude (66 percent). Notice the GUARD
BANDS between plus and minus 75 kilohertz and plus and minus 100 kilohertz. These bands isolate the
modulation extremes of this particular channel from that of adjacent channels.
2-11
Figure 2-8.—Frequency-modulating signal.
PERCENT OF MODULATION.—Before we explain 100-percent modulation in an fm system,
let's review the conditions for 100-percent modulation of an AM wave. Recall that 100-percent
modulation for AM exists when the amplitude of the modulation envelope varies between 0 volts and
twice its normal unmodulated value. At 100-percent modulation there is a power increase of 50 percent.
Because the modulating wave is not constant in voice signals, the degree of modulation constantly varies.
In this case the vacuum tubes in an AM system cannot be operated at maximum efficiency because of
varying power requirements.
In frequency modulation, 100-percent modulation has a meaning different from that of AM. The
modulating signal varies only the frequency of the carrier. Therefore, tubes do not have varying power
requirements and can be operated at maximum efficiency and the fm signal has a constant power output.
In fm a modulation of 100 percent simply means that the carrier is deviated in frequency by the full
permissible amount. For example, an 88.5-megahertz fm station operates at 100-percent modulation when
the modulating signal deviation frequency band is from 75 kilohertz above to 75 kilohertz below the
carrier (the maximum allowable limits). This maximum deviation frequency is set arbitrarily and will
vary according to the applications of a given fm transmitter. In the case given above, 50-percent
modulation would mean that the carrier was deviated 37.5 kilohertz above and below the resting
frequency (50 percent of the 150-kilohertz band divided by 2). Other assignments for fm service may
limit the allowable deviation to 50 kilohertz, or even 10 kilohertz. Since there is no fixed value for
comparison, the term "percent of modulation" has little meaning for fm. The term MODULATION
INDEX is more useful in fm modulation discussions. Modulation index is frequency deviation divided by
the frequency of the modulating signal.
2-12
MODULATION INDEX.—This ratio of frequency deviation to frequency of the modulating signal
is useful because it also describes the ratio of amplitude to tone for the audio signal. These factors
determine the number and spacing of the side frequencies of the transmitted signal. The modulation index
formula is shown below:
Views (A) and (B) of figure 2-9 show the frequency spectrum for various fm signals. In the four
examples of view (A), the modulating frequency is constant; the deviation frequency is changed to show
the effects of modulation indexes of 0.5, 1.0, 5.0, and 10.0. In view (B) the deviation frequency is held
constant and the modulating frequency is varied to give the same modulation indexes.
Figure 2-9.—Frequency spectra of fm waves under various conditions.
You can determine several facts about fm signals by studying the frequency spectrum. For example,
table 2-1 was developed from the information in figure 2-9. Notice in the top spectrums of both views (A)
and (B) that the modulation index is 0.5. Also notice as you look at the next lower spectrums that the
modulation index is 1.0. Next down is 5.0, and finally, the bottom spectrums have modulation indexes of
2-13
10.0. This information was used to develop table 2-1 by listing the modulation indexes in the left column
and the number of significant sidebands in the right. SIGNIFICANT SIDEBANDS (those with
significantly large amplitudes) are shown in both views of figure 2-9 as vertical lines on each side of the
carrier frequency. Actually, an infinite number of sidebands are produced, but only a small portion of
them are of sufficient amplitude to be important. For example, for a modulation index of 0.5 [top
spectrums of both views (A) and (B)], the number of significant sidebands counted is 4. For the next
spectrums down, the modulation index is 1.0 and the number of sidebands is 6, and so forth. This holds
true for any combination of deviating and modulating frequencies that yield identical modulating indexes.
Table 2-1.—Modulation index table
MODULATION
INDEX
.01
.4
.5
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
13.0
14.0
15.0
SIGNIFICANT
SIDEBANDS
2
2
4
6
8
12
14
16
18
22
24
26
28
32
32
36
38
38
You should be able to see by studying figure 2-9, views (A) and (B), that the modulating frequency
determines the spacing of the sideband frequencies. By using a significant sidebands table (such as table
2-1), you can determine the bandwidth of a given fm signal. Figure 2-10 illustrates the use of this table.
The carrier frequency shown is 500 kilohertz. The modulating frequency is 15 kilohertz and the deviation
frequency is 75 kilohertz.
2-14
77
Figure 2-10.—Frequency deviation versus bandwidth.
From table 2-1 we see that there are 16 significant sidebands for a modulation index of 5. To
determine total bandwidth for this case, we use:
The use of this math is to illustrate that the actual bandwidth of an fm transmitter (240 kHz) is
greater than that suggested by its maximum deviation bandwidth (±75 kHz or 150 kHz). This is
important to know when choosing operating frequencies or designing equipment.
Q-4.
What characteristic of a carrier wave is varied in frequency modulation?
Q-5.
How is the degree of modulation expressed in an fm system?
Q-6.
What two values may be used to determine the bandwidth of an fm wave?
2-15
METHODS OF FREQUENCY MODULATION.—The circuit shown earlier in figure 2-6 and the
discussion in previous paragraphs were for illustrative purposes only. In reality, such a circuit would not
be practical. However, the basic principle involved (the change in reactance of an oscillator circuit in
accordance with the modulating voltage) constitutes one of the methods of developing a frequencymodulated wave.
Reactance-Tube Modulation.—In direct modulation, an oscillator is frequency modulated by a
REACTANCE TUBE that is in parallel (SHUNT) with the oscillator tank circuit. (The terms "shunt" or
"shunting" will be used in this module to mean the same as "parallel" or "to place in parallel with"
components.) This is illustrated in figure 2-11. The oscillator is a conventional Hartley circuit with the
reactance-tube circuit in parallel with the tank circuit of the oscillator tube. The reactance tube is an
ordinary pentode. It is made to act either capacitively or inductively; that is, its grid is excited with a
voltage which either leads or lags the oscillator voltage by 90 degrees.
Figure 2-11.—Reactance-tube fm modulator.
When the reactance tube is connected across the tank circuit with no modulating voltage applied, it
will affect the frequency of the oscillator. The voltage across the oscillator tank circuit (L1 and C1) is also
in parallel with the series network of R1 and C7. This voltage causes a current flow through R1 and C7. If
R1 is at least five times larger than the capacitive reactance of C7, this branch of the circuit will be
essentially resistive. Voltage E1, which is across C7, will lag current by 90 degrees. E1 is applied to the
control grid of reactance tube V1. This changes plate current (Ip), which essentially flows only through
the LC tank circuit. This is because the value of R1 is high compared to the impedance of the tank circuit.
Since current is inversely proportional to impedance, most of the plate current coupled through C3 flows
through the tank circuit.
At resonance, the voltage and current in the tank circuit are in phase. Because E1 lags E by 90
degrees and I is in phase with grid voltage E1, the superimposed current through the tank circuit lags the
original tank current by 90 degrees. Both the resultant current (caused by Ip) and the tank current lag tank
voltage and current by some angle depending on the relative amplitudes of the two currents. Because this
resultant current is a lagging current, the impedance across the tank circuit cannot be at its maximum
unless something happens within the tank to bring current and voltage into phase. Therefore, this situation
continues until the frequency of oscillations in the tank circuit changes sufficiently so that the voltages
across the tank and the current flowing into it are again in phase. This action is the same as would be
produced by adding a reactance in parallel with the L1C1 tank. Because the superimposed current lags
voltage E by 90 degrees, the introduced reactance is inductive. In NEETS, Module 2, Introduction to
2-16
Alternating Current and Transformers, you learned that total inductance decreases as additional inductors
are added in parallel. Because this introduced reactance effectively reduces inductance, the frequency of
the oscillator increases to a new fixed value.
Now let’s see what happens when a modulating signal is applied. The magnitude of the introduced
reactance is determined by the magnitude of the superimposed current through the tank. The magnitude of
Ip for a given E1 is determined by the transconductance of V1. (Transconductance was covered in NEETS,
Module 6, Introduction to Electronic Emission, Tubes, and Power Supplies.) Therefore, the value of
reactance introduced into the tuned circuit varies directly with the transconductance of the reactance tube.
When a modulating signal is applied to the grid of V1, both E1 and I change, causing transconductance to
vary with the modulating signal. This causes a variable reactance to be introduced into the tuned circuit.
This variable reactance either adds to or subtracts from the fixed value of reactance that is introduced in
the absence of the modulating signal. This action varies the reactance across the oscillator which, in turn,
varies the instantaneous frequency of the oscillator. These variations in the oscillator frequency are
proportional to the instantaneous amplitude of the modulating voltage. Reactance-tube modulators are
usually operated at low power levels. The required output power is developed in power amplifier stages
that follow the modulators.
The output of a reactance-tube modulated oscillator also contains some unwanted amplitude
modulation. This unwanted modulation is caused by stray capacitance and the resistive component of the
RC phase splitter. The resistance is much less significant than the desired XC, but the resistance does
allow some plate current to flow which is not of the proper phase relationship for good tube operation.
The small amplitude modulation that this produces is easily removed by passing the oscillator output
through a limiter-amplifier circuit.
Semiconductor Reactance Modulator.—The SEMICONDUCTOR-REACTANCE MODULATOR
is used to frequency modulate low-power semiconductor transmitters. Figure 2-12 shows a typical
frequency-modulated oscillator stage operated as a reactance modulator. Q1, along with its associated
circuitry, is the oscillator. Q2 is the modulator and is connected to the circuit so that its collector-toemitter capacitance (CCE) is in parallel with a portion of the rf oscillator coil, L1. As the modulator
operates, the output capacitance of Q2 is varied. Thus, the frequency of the oscillator is shifted in
accordance with the modulation the same as if C1 were varied.
2-17
Figure 2-12.—Reactance-semiconductor fm modulator.
When the modulating signal is applied to the base of Q2, the emitter-to-base bias varies at the
modulation rate. This causes the collector voltage of Q2 to vary at the same modulating rate. When the
collector voltage increases, output capacitance CCE decreases; when the collector voltage decreases, CCE
increases. An increase in collector voltage has the effect of spreading the plates of CCE farther apart by
increasing the width of the barrier. A decrease of collector voltage reduces the width of the pn junction
and has the same effect as pushing the capacitor plates together to provide more capacitance.
When the output capacitance decreases, the instantaneous frequency of the oscillator tank circuit
increases (acts the same as if C1 were decreased). When the output capacitance increases, the
instantaneous frequency of the oscillator tank circuit decreases. This decrease in frequency produces a
lower frequency in the output because of the shunting effect of CCE. Thus, the frequency of the oscillator
tank circuit increases and decreases at an audio frequency (af) modulating rate. The output of the
oscillator, therefore, is a frequency modulated rf signal.
Since the audio modulation causes the collector voltage to increase and decrease, an AM component
is induced into the output. This produces both an fm and AM output. The amplitude variations are then
removed by placing a limiter stage after the reactance modulator and only the frequency modulation
remains.
Frequency multipliers or mixers (discussed in chapter 1) are used to increase the oscillator frequency
to the desired output frequency. For high-power applications, linear rf amplifiers are used to increase the
steady-amplitude signal to a higher power output. With the initial modulation occurring at low levels, fm
represents a savings of power when compared to conventional AM. This is because fm noise-reducing
properties provide a better signal-to-noise ratio than is possible with AM.
2-18
Multivibrator Modulator.—Another type of frequency modulator is the astable multivibrator
illustrated in figure 2-13. Inserting the modulating af voltage in series with the base-return of the
multivibrator transistors causes the gate length, and thus the fundamental frequency of the multivibrator,
to vary. The amount of variation will be in accordance with the amplitude of the modulating voltage. One
requirement of this method is that the fundamental frequency of the multivibrator be high in relation to
the highest modulating frequencies. A factor of at least 100 provides the best results.
Figure 2-13.—Astable multivibrator and filter circuit for generating an fm carrier.
Recall that a multivibrator output consists of the fundamental frequency and all of its harmonics.
Unwanted even harmonics are eliminated by using a SYMMETRICAL MULTIVIBRATOR circuit, as
shown in figure 2-13. The desired fundamental frequency, or desired odd harmonics, can be amplified
after all other odd harmonics are eliminated in the LCR filter section of figure 2-13. A single frequencymodulated carrier is then made available for further amplification and transmission.
Proper design of the multivibrator will cause the frequency deviation of the carrier to faithfully
follow (referred to as a "linear" function) the modulating voltage. This is true up to frequency deviations
which are considerable fractions of the fundamental frequency of the multivibrator. The principal design
consideration is that the RC coupling from one multivibrator transistor base to the collector of the other
has a time constant which is greater than the actual gate length by a factor of 10 or more. Under these
conditions, a rise in base voltage in each transistor is essentially linear from cutoff to the bias at which the
transistor is switched on. Since this rise in base voltage is a linear function of time, the gate length will
change as an inverse function of the modulating voltage. This action will cause the frequency to change as
a linear function of the modulating voltage.
The multivibrator frequency modulator has the advantage over the reactance-type modulator of a
greater linear frequency deviation from a given carrier frequency. However, multivibrators are limited to
frequencies below about 1 megahertz. Both systems are subject to drift of the carrier frequency and must,
therefore, be stabilized. Stabilization may be accomplished by modulating at a relatively low frequency
and translating by heterodyne action to the desired output frequency, as shown in figure 2-14. A
1-megahertz signal is heterodyned with 49 megahertz from the crystal-controlled oscillator to provide a
stable 50-megahertz output from the mixer. If a suitably stable heterodyning oscillator is used, the
frequency stability can be greatly improved. For instance, at the frequencies shown in figure 2-14, the
2-19
stability of the unmodulated 50-megahertz carrier would be 50 times better than that which harmonic
multiplication could provide.
Figure 2-14.—Method for improving frequency stability of fm system.
Varactor FM Modulator.—Another fm modulator which is widely used in transistorized circuitry
uses a voltage-variable capacitor (VARACTOR). The varactor is simply a diode, or pn junction, that is
designed to have a certain amount of capacitance between junctions. View (A) of figure 2-15 shows the
varactor schematic symbol. A diagram of a varactor in a simple oscillator circuit is shown in view (B).
This is not a working circuit, but merely a simplified illustration. The capacitance of a varactor, as with
regular capacitors, is determined by the area of the capacitor plates and the distance between the plates.
The depletion region in the varactor is the dielectric and is located between the p and n elements, which
serve as the plates. Capacitance is varied in the varactor by varying the reverse bias which controls the
thickness of the depletion region. The varactor is so designed that the change in capacitance is linear with
the change in the applied voltage. This is a special design characteristic of the varactor diode. The
varactor must not be forward biased because it cannot tolerate much current flow. Proper circuit design
prevents the application of forward bias.
Figure 2-15A.—Varactor symbol and schematic. SCHEMATIC SYMBOL.
2-20
Figure 2-15B.—Varactor symbol and schematic. SIMPLIFIED CIRCUIT.
Notice the simplicity of operation of the circuit in figure 2-16. An af signal that is applied to the
input results in the following actions: (1) On the positive alternation, reverse bias increases and the
dielectric (depletion region) width increases. This decreases capacitance which increases the frequency of
the oscillator. (2) On the negative alternation, the reverse bias decreases, which results in a decrease in
oscillator frequency.
Figure 2-16.—Varactor fm modulator.
Many different fm modulators are available, but they all use the basic principles you have just
studied. The main point to remember is that an oscillator must be used to establish the reference (carrier)
frequency. Secondly, some method is needed to cause the oscillator to change frequency in accordance
with an af signal. Anytime this can be accomplished, we have a frequency modulator.
Q-7.
How does the reactance-tube modulator impress intelligence onto an rf carrier?
Q-8.
What characteristic of a transistor is varied in a semiconductor-reactance modulator?
Q-9.
What circuit section is required in the output of a multivibrator modulator to eliminate
unwanted output frequencies?
Q-10.
What characteristic of a varactor is used in an fm modulator?
2-21
PHASE MODULATION
Frequency modulation requires the oscillator frequency to deviate both above and below the carrier
frequency. During the process of frequency modulation, the peaks of each successive cycle in the
modulated waveform occur at times other than they would if the carrier were unmodulated. This is
actually an incidental phase shift that takes place along with the frequency shift in fm. Just the opposite
action takes place in phase modulation. The af signal is applied to a PHASE MODULATOR in pm. The
resultant wave from the phase modulator shifts in phase, as illustrated in figure 2-17. Notice that the time
period of each successive cycle varies in the modulated wave according to the audio-wave variation.
Since frequency is a function of time period per cycle, we can see that such a phase shift in the carrier will
cause its frequency to change. The frequency change in fm is vital, but in pm it is merely incidental. The
amount of frequency change has nothing to do with the resultant modulated wave shape in pm. At this
point the comparison of fm to pm may seem a little hazy, but it will clear up as we progress.
Figure 2-17.—Phase modulation.
Let’s review some voltage phase relationships. Look at figure 2-18 and compare the three voltages
(A, B, and C). Since voltage A begins its cycle and reaches its peak before voltage B, it is said to lead
voltage B. Voltage C, on the other hand, lags voltage B by 30 degrees. In phase modulation the phase of
the carrier is caused to shift at the rate of the af modulating signal. In figure 2-19, note that the
unmodulated carrier has constant phase, amplitude, and frequency. The dotted wave shape represents the
modulated carrier. Notice that the phase on the second peak leads the phase of the unmodulated carrier.
On the third peak the shift is even greater; however, on-the fourth peak, the peaks begin to realign phase
with each other. These relationships represent the effect of 1/2 cycle of an af modulating signal. On the
negative alternation of the af intelligence, the phase of the carrier would lag and the peaks would occur at
times later than they would in the unmodulated carrier.
Figure 2-18.—Phase relationships.
2-22
Figure 2-19.—Carrier with and without modulation.
The presentation of these two waves together does not mean that we transmit a modulated wave
together with an unmodulated carrier. The two waveforms were drawn together only to show how a
modulated wave looks when compared to an unmodulated wave.
Now that you have seen the phase and frequency shifts in both fm and pm, let’s find out exactly how
they differ. First, only the phase shift is important in pm. It is proportional to the af modulating signal. To
visualize this relationship, refer to the wave shapes shown in figure 2-20. Study the composition of the fm
and pm waves carefully as they are modulated with the modulating wave shape. Notice that in fm, the
carrier frequency deviates when the modulating wave changes polarity. With each alternation of the
modulating wave, the carrier advances or retards in frequency and remains at the new frequency for the
duration of that cycle. In pm you can see that between one alternation and the next, the carrier phase must
change, and the frequency shift that occurs does so only during the transition time; the frequency then
returns to its normal rate. Note in the pm wave that the frequency shift occurs only when the modulating
wave is changing polarity. The frequency during the constant amplitude portion of each alternation is the
REST FREQUENCY.
Figure 2-20.—Pm versus fm.
The relationship, in pm, of the modulating af to the change in the phase shift is easy to see once you
understand AM and fm principles. Again, we can establish two clear-cut rules of phase modulation:
2-23
•
AMOUNT OF PHASE SHIFT IS PROPORTIONAL TO THE AMPLITUDE OF THE
MODULATING SIGNAL.
(If a 10-volt signal causes a phase shift of 20 degrees, then a 20-volt signal causes a phase shift of 40
degrees.)
•
RATE OF PHASE SHIFT IS PROPORTIONAL TO THE FREQUENCY OF THE
MODULATING SIGNAL.
(If the carrier were modulated with a 1-kilohertz tone, the carrier would advance and retard in phase
1,000 times each second.)
Phase modulation is also similar to frequency modulation in the number of sidebands that exist
within the modulated wave and the spacing between sidebands. Phase modulation will also produce an
infinite number of sideband frequencies. The spacing between these sidebands will be equal to the
frequency of the modulating signal. However, one factor is very different in phase modulation; that is, the
distribution of power in pm sidebands is not similar to that in fm sidebands, as will be explained in the
next section.
Modulation Index
Recall from frequency modulation that modulation index is used to calculate the number of
significant sidebands existing in the waveform. The higher the modulation index, the greater the number
of sideband pairs. The modulation index is the ratio between the amount of oscillator deviation and the
frequency of the modulating signal:
In frequency modulation, we saw that as the frequency of the modulating signal increased (assuming
the deviation remained constant) the number of significant sideband pairs decreased. This is shown in
views (A) and (B) of figure 2-21. Notice that although the total number of significant sidebands decreases
with a higher frequency-modulating signal, the sidebands spread out relative to each other; the total
bandwidth increases.
2-24
Figure 2-21.—Fm versus pm spectrum distribution.
In phase modulation the oscillator does not deviate, and the power in the sidebands is a function of
the amplitude of the modulating signal. Therefore, two signals, one at 5 kilohertz and the other at 10
kilohertz, used to modulate a carrier would have the same sideband power distribution. However, the
10-kilohertz sidebands would be farther apart, as shown in views (C) and (D) of figure 2-21. When
compared to fm, the bandwidth of the pm transmitted signal is greatly increased as the frequency of the
modulating signal is increased.
As we pointed out earlier, phase modulation cannot occur without an incidental change in frequency,
nor can frequency modulation occur without an incidental change in phase. The term fm is loosely used
when referring to any type of angle modulation, and phase modulation is sometimes incorrectly referred
to as "indirect fm." This is a definition that you should disregard to avoid confusion. Phase modulation is
just what the words imply — phase modulation of a carrier by an af modulating signal. You will develop
a better understanding of these points as you advance in your study of modulation.
Basic Modulator
In phase modulation you learned that varying the phase of a carrier at an intelligence rate caused that
carrier to contain variations which could be converted back into intelligence. One circuit that can cause
this phase variation is shown in figure 2-22.
2-25
Figure 2-22.—Phase shifting a sine wave.
The capacitor in series with the resistor forms a phase-shift circuit. With a constant frequency rf
carrier applied at the input, the output across the resistor would be 45 degrees out of phase with the input
if XC = R.
Now, let’s vary the resistance and observe how the output is affected in figure 2-23. As the resistance
reaches a value greater than 10 times XC, the phase difference between input and output is nearly 0
degrees. For all practical purposes, the circuit is resistive. As the resistance is decreased to 1/10 the value
of XC, the phase difference approaches 90 degrees. The circuit is now almost completely capacitive. By
replacing the resistor with a vacuum tube, as shown in view (A) of figure 2-24, we can vary the resistance
(vacuum-tube impedance) by varying the voltage applied to the grid of the tube. The frequency applied to
the circuit (from a crystal-controlled master oscillator) will be shifted in phase by 45 degrees with no
audio input [view (B)]. With the application of an audio signal, the phase will shift as the impedance of
the tube is varied.
Figure 2-23.—Control over the amount of phase shift.
2-26
Figure 2-24A.—Phase modulator.
Figure 2-24B.—Phase modulator.
In practice, a circuit like this could not provide enough phase shift to produce the desired results in
the output. Several of these circuits are arranged in cascade to provide the desired amount of phase shift.
Also, since the output of this circuit will vary in amplitude, the signal is fed to a limiter to remove
amplitude variations.
The major advantage of this type modulation circuit over frequency modulation is that this circuit
uses a crystal-controlled oscillator to maintain a stable carrier frequency. In fm the oscillator cannot be
crystal controlled because it is actually required to vary in frequency. That means that an fm oscillator
will require a complex automatic frequency control (afc) system. An afc system ensures that the oscillator
stays on the same carrier frequency and achieves a high degree of stability. The afc circuit will be covered
in a later module.
Phase-Shift Keying
Phase-shift keying (psk) is similar to ON-OFF cw keying in AM systems and frequency-shift keying
in fm systems. Psk is most useful when the code elements are all of equal length; that is, all marks and
spaces, whether message elements or synchronizing signals, occupy identical elements of time. It is not
fully suitable for use on start-stop teletypewriter circuits where the stop pulse is 1.42 times longer than the
2-27
other pulses. Neither is it applicable to those pulsed systems in which the duration or position of the
pulses are varied by the modulation frequency. In its simplest form, psk operates on the principle of phase
reversal of the carrier. Each time a mark is received, the phase is reversed. No phase reversal takes place
when a space is received. In binary systems, marks and spaces are called ONES and ZEROS,
respectively, so that a ONE causes a 180-degree phase shift, and a ZERO has no effect on the incoming
signal. Figure 2-25 shows the application of phase-shift keying to an unmodulated carrier [view (A)] in
the af range. For transmission over other than a conductive path, the wave shown in view (D) must be
used as the modulating signal for some other system of modulating an rf carrier.
Figure 2-25A.—Phase-shift keying. UNMODULATED CARRIER.
Figure 2-25B.—Phase-shift keying. MODULATION SIGNAL - DATA ELEMENTS.
Figure 2-25C.—Phase-shift keying. MODULATED CARRIER.
Figure 2-25D.—Phase-shift keying. MODULATED CARRIER AFTER FILTERING.
The modulating signal in view (B) consists of a bit stream of ZEROS and ONES. A ZERO does not
affect the carrier frequency which is usually set to equal the bit rate. For example, a data stream of 1,200
bits per second would have a carrier of 1,200 hertz. When a data bit ONE occurs, the phase of the carrier
frequency is shifted 180 degrees. In view (C) we find that the third, fifth, and sixth cycles (all ONE) have
been reversed in phase. This phase reversal produces CUSPS (sharp phase reversals) which are usually
2-28
removed by filtering before transmission or further modulation. This filtering action limits the bandwidth
of the output signal frequencies. The resulting wave is shown in view (D).
The exact waveform of figure 2-25, view (D), can be obtained by logic operations of timing and
data. This is illustrated in figure 2-26, where a timing signal [view (A)] is used rather than a carrier
frequency. The data (intelligence) is shown in view (B) and is combined with the timing signal to produce
a combination digital modulation signal, as shown in view (C). The square-wave pattern of the digital
modulation is filtered to limit the bandwidth of the signal frequencies, as shown in view (D). This system
has been used in some high-speed data equipment, but it offers no particular advantage over other systems
of modulation, particularly the pulse-modulated systems for high-speed data transmission.
Q-11.
What type of modulation depends on the carrier-wave phase shift?
Q-12.
What components may be used to build a basic phase modulator?
Q-13.
Phase-shift keying is similar to what other two types of modulation?
Figure 2-26A.—Simulated phase-shift keying. TIMING.
Figure 2-26B.—Simulated phase-shift keying. DATA.
Figure 2-26C.—Simulated phase-shift keying. DIGITAL MODULATION.
Figure 2-26D.—Simulated phase-shift keying. DIGITAL MODULATION AFTER FILTERING.
2-29
PULSE MODULATION
Another type of modulation is PULSE MODULATION. Pulse modulation has many uses, including
telegraphy, radar, telemetry, and multiplexing. Far too many applications of pulse modulation exist to
elaborate on any one of them, but in this section we will cover the basic principles of pulse modulation.
CHARACTERISTICS
Amplitude modulating a simple rf carrier to a point where it becomes drastically overmodulated
could produce a waveform similar to that required in pulse modulation. A modulating signal [view (A) of
figure 2-27] that is much larger than the carrier results in the modulation envelope shown in view (B).
The modulation envelope would be the same if the modulating wave shape were not sinusoidal; that is,
like the one shown in view (C).
Figure 2-27A.—Overmodulation of a carrier. MODULATING WAVE.
Figure 2-27B.—Overmodulation of a carrier. MODULATION ENVELOPE.
2-30
Figure 2-27C.—Overmodulation of a carrier. NONSINUSOIDAL MODULATING WAVE.
Observe the modulating square wave in figure 2-28. Remember that it contains an infinite number of
odd harmonics in addition to its fundamental frequency. Assume that a carrier has a frequency of 1
megahertz. The fundamental frequency of the modulating square wave is 1 kilohertz. When these signals
heterodyne, two new frequencies will be produced: a sum frequency of 1.001 megahertz and a difference
frequency of 0.999 megahertz. The fundamental frequency heterodynes with the carrier. This is also true
of all harmonics contained in the square wave. Side frequencies associated with those harmonics will be
produced as a result of this process. For example, the third harmonic of the square wave heterodynes with
the carrier and produces sideband frequencies at 1.003 and 0.997 megahertz. Another set will be produced
by the fifth, seventh, ninth, eleventh, thirteenth, fifteenth, seventeenth, and nineteenth harmonics of the
square wave, and so on to infinity.
Figure 2-28.—Spectrum distribution when modulating with a square wave.
Look at figure 2-28 and observe the relative amplitudes of the sidebands as they relate to the
amplitudes of the harmonics contained in the square wave. Note that the first set of sidebands is directly
related to the amplitude of the square wave. The second set of sidebands is related to the third harmonic
content of the square wave and is 1/3 the amplitude of the first set. The third set is related to the amplitude
of the first set of sidebands and is 115 the amplitude of the first set. This relationship will apply to each
additional set of sidebands.
2-31
View (A) of figure 2-29 shows the carrier modulated with a square wave. In view (B) the modulating
square wave is increased in amplitude; note that the rf peaks increase in amplitude during the positive
alternation of the square wave and decrease during the negative half of the square wave. In view (C) the
amplitude of the square wave is further increased and the amplitude of the rf wave is almost 0 during the
negative alternation of the square wave.
Figure 2-29A.—Various square-wave modulation levels with frequency-spectrum carrier and sidebands.
Figure 2-29B.—Various square-wave modulation levels with frequency-spectrum carrier and sidebands.
Figure 2-29C.—Various square-wave modulation levels with frequency-spectrum carrier and sidebands.
Figure 2-29D.—Various square-wave modulation levels with frequency-spectrum carrier and sidebands.
2-32
Note the frequency spectrum associated with each of these conditions. The carrier amplitude remains
constant, but the sidebands increase in amplitude in accordance with the amplitude of the modulating
square wave.
So far in pulse modulation, the same general rules apply as in AM. In view (C) the amplitude of the
square wave of voltage is equal to the peak voltage of the unmodulated carrier wave. This is 100-percent
modulation, just as in conventional AM. Note in the frequency spectrum that the sideband distribution is
also the same as in AM. Keep in mind that the total sideband power is 1/2 of the total power when the
modulator signal is a square wave. This is in contrast to 1/3 the total power with sine-wave modulation.
Now refer to view (D). The increase of the square-wave modulating voltage is greater in amplitude
than the unmodulated carrier. Notice that the sideband distribution does not change; but, as the sidebands
take on more of the transmitted power, so will the carrier.
Pulse Timing
Thus far, we have established a carrier and have caused its peaks to increase and decrease as a
modulating square wave is applied. Some pulse-modulation systems modulate a carrier in this manner.
Others produce no rf until pulsed; that is, rf occurs only during the actual pulse as shown in view (A) of
figure 2-30. For example, let’s start with an rf carrier frequency of 1 megahertz. Each cycle of the rf
requires a certain amount of time to complete. If we allow oscillations to occur for a given period of time
only during selected intervals, as in view (B), we are PULSING the system. Note that the pulse
transmitter does not produce an rf signal until one of the positive-going modulating pulses is applied. The
transmitter then produces the rf carrier until the positive input pulse ends and the input waveform again
becomes a negative potential.
Figure 2-30A.—Pulse transmission.
Figure 2-30B.—Pulse transmission.
Refer back to figure 1-41 and the over-modulation discussion in chapter 1. You will notice that the
overmodulation wave shape of view (D) in figure 2-29 and the pulse-modulation wave shape of figure
2-30, view (B), are very similar to figure 1-41.
Actually, both figure 1-41 and view (D) of figure 2-29 result from overmodulation. Even though the
output of the pulse transmitter in figure 2-30 looks like overmodulation, it is not; rather, it is pulsed.
2-33
However, the frequency spectrums are similar. Sideband distributions are similar, but not identical, since
the pulse transmitter in figure 2-30 is gated on and off instead of being modulated by a square wave as
was the case in view (D) of figure 2-29.
Remember, in pulse modulation the sidebands produced to accompany the carrier during
transmission are directly related to the harmonic content of the modulating wave shape. In figure 2-31,
(view A, view B and view C), observe the square and rectangular wave shapes used to pulse modulate the
same carrier frequency in each of the three views.
Figure 2-31A.—Varying pulse-modulating waves.
Figure 2-31B.—Varying pulse-modulating waves.
Figure 2-31C.—Varying pulse-modulating waves.
Let’s take note of some timing relationships in the three modulating sequences in figure 2-31:
•
the time for the rf cycle is the same in each case
•
the number of cycles occurring in each group is different
•
the ratio between transmitting and non-transmitting time varies
•
•
the transmitter produces an rf wave four times in view (A), three transmission groups in view (B),
and only two in view (C)
rf is generated only during the positive pulses
2-34
In figure 2-32, observe the relative time for individual rf cycles. The time for each cycle is the same
in views (A) and (B). Since this time is the same, we can assume that the carrier frequency is the same.
But in view (C) the time for each cycle is about half that in views (A) and (B). Therefore, the frequency
of the carrier in view (C) is nearly twice that of the other two. This illustration shows that carrier
frequencies in pulse systems can vary.
Figure 2-32A.—Carrier frequency.
Figure 2-32B.—Carrier frequency.
Figure 2-32C.—Carrier frequency.
The carrier frequency is not the only frequency we must concern ourselves with in pulse systems.
We must also be concerned with the frequency that is associated with the repetition rate of groups of
pulses. Figure 2-33 shows that a specific time period exists between each group of rf pulses. This time is
the same for each repetition of the pulse and is called the PULSE-REPETITION TIME (prt). To find out
how often these groups of pulses occur, compute PULSE-REPETITION FREQUENCY (prf) using the
formula:
2-35
Figure 2-33.—Pulse-repetition time (prt).
Just remember that the pulse-repetition time is the time it takes for a pulse to recur, as shown in
figure 2-34. The duration of time of the pulse (a) plus the time when no pulse occurs (b) equals the total
pulse-repetition time.
Figure 2-34.—Pulse cycles.
The time during which the pulse is occurring is called PULSE DURATION (pd) or PULSE WIDTH
(pw), as shown in figure 2-35. As you will soon see, pulse width is important in pulse modulation.
Figure 2-35.—Pulse width (pw).
2-36
The time we have been referring to as the time of no pulse, or nonpulse time, is referred to as REST
TIME (rt). The duration of this rest time will determine certain capabilities of the pulse-modulation
system. The pulse width is the time that the transmitter produces rf oscillations and is the actual pulse
transmission time. During the nonpulse time, shown in figure 2-36, the transmitter produces no
oscillations and the oscillator is cut off.
Figure 2-36.—Rest time (rt).
Some pulse transmitter-receiver systems transmit the pulse and then rest, awaiting the return of an
echo. Rest time provides the system time for the receive cycle of operation.
Power in a Pulse System
When discussing power in a pulse-modulation system, we have to consider PEAK POWER and
AVERAGE POWER. Peak power is the maximum value of the transmitted pulse; average power is the
peak power value averaged over the pulse-repetition time. Peak power is very easy to see in a pulse
system. In figure 2-37, all pulsed wave shapes have a peak power of 100 watts. Also note that in views
(A), (B), and (C) the pulse width is the same, even though the carrier frequency is different. In these three
cases average power would be the same. This is because average power is actually equal to the peak
power of a pulse averaged over 1 operating cycle. However, the pulse width is increased in view (D) and
we have a greater average power with the same prt. In view (E) the decreased pulse width has decreased
average power over the same prt.
Figure 2-37.—Peak and average power.
2-37
Use these simple rules to determine power in a pulsed-wave shape:
•
Peak power is the maximum power reached by the transmitter during the pulse.
•
Average power equals the peak power averaged over one cycle.
Duty Cycle
In pulse modulation you will need to know the percentage of time the system is actually producing
rf. For example, let’s say that a pulse system is transmitting 25 percent of the time. This would mean that
the pw is 1/4 the prt. For every 60 minutes we operate the pulse system, we actually transmit a total of
only 15 minutes.
The DUTY CYCLE is the ratio of working time to total time for intermittently operated devices.
Thus, duty cycle represents a ratio of actual transmitting time to transmitting time plus rest time. To
establish the duty cycle, divide the pw by the prt of the system. This yields the duty cycle and is
expressed as a decimal figure. With this information, we can figure percentage of transmitting time by
multiplying the duty cycle by 100.
Applications of Pulse Modulation
Pulse modulation has many applications in the transmission of intelligence information. In telemetry,
for example, the width of successive pulses may tell us humidity; the changing of the rest time may tell us
pressure. In other applications, as you will see later in this text, the changing of the average power can
provide us with intelligence information.
In radar a pulse is transmitted and travels some distance to a target where it is then reflected back to
the system. The amount of time it takes provides us with information that can be converted to distance.
Telemetry and radar systems use the principles of pulse modulation described in this section. Let’s
quickly review what has been presented:
•
Pulse width (pw) — the duration of time rf frequency is transmitted
•
Rest time (rt) — the time the transmitter is resting (not transmitting)
•
Carrier frequency — the frequency of the rf wave generated in the oscillator of the transmitter
•
Pulse-repetition time (prt) — the total time of 1 complete pulse cycle of operation (rest time
plus pulse width)
•
Pulse-repetition frequency (prf) — the rate, in pulses per second, that the pulse occurs
•
Power peak — the maximum power contained in the pulse
•
Average power — the peak power averaged over 1 complete operating cycle
•
Duty cycle — a decimal number that expresses a ratio in a pulse modulation system of transmit
time to total time
2-38
Pulse modulation will play a major part in your electronics career. In one way or another, you will
encounter it in some form. The function of the particular system may involve many variations of the
characteristics presented here. We will now look at some specific applications of pulse modulation in
radar and communications systems.
Q-14.
Overmodulating an rf carrier in amplitude modulation produces a waveform which is similar to
what modulated waveform?
Q-15.
What is prt?
Q-16.
What is nonpulse time?
Q-17.
What is average power in a pulsed system?
RADAR MODULATION
Radio frequency energy in radar is transmitted in short pulses with time durations that may vary
from 1 to 50 microseconds or more. If the transmitter is cut off before any reflected energy returns from a
target, the receiver can distinguish between the transmitted pulse and the reflected pulse. After all
reflections have returned, the transmitter can again be cut on and the process repeated. The receiver
output is applied to an indicator which measures the time interval between the transmission of energy and
its return as a reflection. Since the energy travels at a constant velocity, the time interval becomes a
measure of the distance traveled (RANGE). Since this method does not depend on the relative frequency
of the returned signal, or on the motion of the target, difficulties experienced in cw or fm methods are not
encountered. The pulse modulation method is used in many military radar applications.
Most radar oscillators operate at pulse voltages between 5 and 20 kilovolts. They require currents of
several amperes during the actual pulse which places severe requirements on the modulator. The function
of the high-vacuum tube modulator is to act as a switch to turn a pulse ON and OFF at the transmitter in
response to a control signal. The best device for this purpose is one which requires the least signal power
for control and allows the transfer of power from the transmitter power source to the oscillator with the
least loss. The pulse modulator circuits discussed in this section are typical pulse modulators used in radar
equipment.
Spark-Gap Modulator
The SPARK-GAP MODULATOR consists of a circuit for storing energy, a circuit for rapidly
discharging the storage circuit (spark gap), a pulse transformer, and an ac power source. The circuit for
storing energy is essentially a short section of artificial transmission line which is known as the
PULSE-FORMING NETWORK (pfn). The pulse-forming network is discharged by a spark gap. Two
types of spark gaps are used: FIXED GAPS and ROTARY GAPS. The fixed gap, discussed in this
section, uses a trigger pulse to ionize the air between the contacts of the spark gap and to initiate the
discharge of the pulse-forming network. The rotary gap is similar to a mechanically driven switch.
A typical fixed, spark-gap modulator circuit is shown in figure 2-38. Between trigger pulses the
spark gap is an open circuit. Current flows through the pulse transformer (T1), the pulse-forming network
(C1, C2, C3, C4, and L2), the diode (V1), and the inductor (L1) to the plate supply voltage (Eb). These
components form the charging circuit for the pulse-forming network.
2-39
Figure 2-38.—Fixed spark-gap modulator.
The spark gap is actually triggered (ionized) by the combined action of the charging voltage across
the pulse-forming network and the trigger pulse. (Ionization was discussed in NEETS, Module 6,
Introduction to Electronic Emission, Tubes and Power Supplies.) The air between the trigger pulse
injection point and ground is ionized by the trigger voltage. This, in turn, initiates the ionization of the
complete gap by the charging voltage. This ionization allows conduction from the charged pulse-forming
network through pulse transformer T1. The output pulse is then applied to an oscillating device, such as a
magnetron.
Thyratron Modulator
The hydrogen THYRATRON MODULATOR is an electronic switch which requires a positive
trigger of only 150 volts. The trigger potential must rise at the rate of 100 volts per microsecond to cause
the modulator to conduct. In contrast to spark gap devices, the hydrogen thyratron (figure 2-39) operates
over a wide range of anode voltages and pulse-repetition rates. The grid has complete control over the
initiation of cathode emission for a wide range of voltages. The anode is completely shielded from the
cathode by the grid. Thus, effective grid action results in very smooth firing over a wide range of anode
voltages and repetition frequencies. Unlike most other thyratrons, the positive grid-control characteristic
ensures stable operation. In addition, deionization time is reduced by using the hydrogen-filled tube.
Figure 2-39.—Typical thyratron gas-tube modulator.
The hydrogen thyratron modulator provides improved timing because the synchronized trigger pulse
is applied to the control grid of the thyratron (V2) and instantaneous firing is obtained. In addition, only
2-40
one gas tube is required to discharge the pulse-forming network, and a low amplitude trigger pulse is
sufficient to initiate discharge. A damping diode is used to prevent breakdown of the thyratron by
reverse-voltage transients. The thyratron requires a sharp leading edge for a trigger pulse and depends on
a sudden drop in anode voltage (controlled by the pulse-forming network) to terminate the pulse and cut
off the tube.
As shown in figure 2-39, the typical thyratron modulator is very similar to the spark-gap modulator.
It consists of a power source (Eb), a circuit for storing energy (L2, C2, C3, C4, and C5), a circuit for
discharging the storage circuit (V2), and a pulse transformer (T1). In addition this circuit has a damping
diode (V1) to prevent reverse-polarity signals from being applied to the plate of V2 which could cause V2
to breakdown.
With no trigger pulse applied, the pfn charges through T1, the pfn, and the charging coil L1 to the
potential of Eb. When a trigger pulse is applied to the grid of V2, the tube ionizes causing the pulseforming network to discharge through V2 and the primary of T1. As the voltage across the pfn falls below
the ionization point of V2, the tube shuts off. Because of the inductive properties of the pfn, the positive
discharge voltage has a tendency to swing negative. This negative overshoot is prevented from damaging
the thyratron and affecting the output of the circuit by V1, R1, R2, and C1. This is a damping circuit and
provides a path for the overshoot transient through V1. It is dissipated by R1 and R2 with C1 acting as a
high-frequency bypass to ground, preserving the sharp leading and trailing edges of the pulse. The
hydrogen thyratron modulator is the most common radar modulator.
Pulse modulation is also useful in communications systems. The intelligence-carrying capability and
power requirements for communications systems differ from those of radar. Therefore, other methods of
achieving pulse modulation that are more suitable for communications systems will now be studied.
Q-18.
What is the primary component for a spark-gap modulator?
Q-19.
What are the basic components of a thyratron modulator?
COMMUNICATIONS PULSE MODULATORS
To transmit intelligence using pulse modulation, you must provide a method to vary some
characteristic of the pulse train in accordance with the modulating signal. Figure 2-40 illustrates a simple
pulse train. The characteristics of these pulses that can be varied are amplitude, pulse width,
pulse-repetition time, and the pulse position as compared to a reference. In addition to these three
characteristics, pulses may be transmitted according to a code to represent the different levels of the
modulating signal. To ensure maximum fidelity (accuracy in reproducing a modulating wave), the
modulating signal has to be represented by enough pulses to restore the original wave shape. Logically,
the higher the sampling rate (the more often sampled) of the pulse modulator, the more accurately the
original modulating wave can be reproduced. Figure 2-41 illustrates the effectiveness of three
pulse-sampling rates. View (A) shows a sampling rate of more than two times the modulating frequency.
As you can see, this reproduces the modulating signal very accurately. However, the high sampling rate
requires a wide bandwidth and increases the average power required of the transmitter. If less than two
samples per cycle are made, you are not able to reproduce the original modulating signal, as shown in
view (B). View (C) shows a sampling rate that is two times the highest modulating frequency. This is the
minimum sampling rate that will give a sufficiently accurate reproduction of the modulating wave. The
standard sampling rate is 2.5 times the highest frequency that is to be transmitted. This ensures the ability
to accurately reproduce the modulating waveform. In military voice systems the bandwidth for voice
signals is limited to 300 to 3,000 hertz, requiring a sampling frequency of 8 kilohertz. Although the pulse
characteristic that is changed may vary for each type of pulse modulation, the sampling frequency will
remain constant. We will now briefly discuss common types of pulse modulation.
2-41
Figure 2-40.—Pulse train.
Figure 2-41A.—Pulse sampling rates. MORE THAN TWO PULSE SAMPLES FOR EVERY WAVE.
Figure 2-41B.—Pulse sampling rates. LESS THAN TWO PULSE SAMPLES FOR EVERY WAVE.
Figure 2-41C.—Pulse sampling rates. TWO PULSE SAMPLES FOR EVERY WAVE.
2-42
Pulse-Amplitude Modulation
Some characteristic of the sampling pulses must be varied by the modulating signal for the
intelligence of the signal to be present in the pulsed wave. Figure 2-42 shows three typical waveforms in
which the pulse amplitude is varied by the amplitude of the modulating signal. View (A) represents a sine
wave of intelligence to be modulated on a transmitted carrier wave. View (B) shows the timing pulses
which determine the sampling interval. View (C) shows PULSE-AMPLITUDE MODULATION (pam) in
which the amplitude of each pulse is controlled by the instantaneous amplitude of the modulating signal at
the time of each pulse.
Figure 2-42A.—Pulse-amplitude modulation (pam). MODULATION.
Figure 2-42B.—Pulse-amplitude modulation (pam). TIMING.
Figure 2-42C.—Pulse-amplitude modulation (pam). PAM.
Pulse-amplitude modulation is the simplest form of pulse modulation. It is generated in much the
same manner as analog-amplitude modulation. The timing pulses are applied to a pulse amplifier in which
the gain is controlled by the modulating waveform. Since these variations in amplitude actually represent
the signal, this type of modulation is basically a form of AM. The only difference is that the signal is now
in the form of pulses. This means that pam has the same built-in weaknesses as any other AM signal high susceptibility to noise and interference. The reason for susceptibility to noise is that any interference
in the transmission path will either add to or subtract from any voltage already in the circuit (signal
voltage). Thus, the amplitude of the signal will be changed. Since the amplitude of the voltage represents
the signal, any unwanted change to the signal is considered a SIGNAL DISTORTION. For this reason,
pam is not often used. When pam is used, the pulse train is used to frequency modulate a carrier for
transmission. Techniques of pulse modulation other than pam have been developed to overcome problems
of noise interference. The following sections will discuss other types of pulse modulation.
Q-20.
What action is necessary to impress intelligence on the pulse train in pulse modulation?
2-43
Q-21.
To ensure the accuracy of a transmission, what is the minimum number of times a modulating
wave should be sampled in pulse modulation?
Q-22.
What, if any, noise susceptibility advantage exists for pulse-amplitude modulation over analogamplitude modulation?
Pulse-Time Modulation
In pulse-modulated systems, as in an analog system, the intelligence may be impressed on the carrier
by varying any of its characteristics. In the preceding paragraphs the method of modulating a pulse train
by varying its amplitude was discussed. Time characteristics of pulses may also be modulated with
intelligence information. Two time characteristics may be affected: (1) the time duration of the pulses,
referred to as PULSE-DURATION MODULATION (pdm) or PULSE-WIDTH MODULATION (pwm);
and (2) the time of occurrence of the pulses, referred to as PULSE-POSITION MODULATION (ppm),
and a special type of PULSE-TIME MODULATION (ptm) referred to as PULSE-FREQUENCY
MODULATION (pfm). Figure 2-43 shows these types of ptm in views (C), (D), and (E). Views (A) and
(B) show the modulating signal and timing, respectively.
Figure 2-43A.—Pulse-time modulation (ptm). MODULATION.
Figure 2-43B.—Pulse-time modulation (ptm). TIMING.
Figure 2-43C.—Pulse-time modulation (ptm). PDM.
Figure 2-43D.—Pulse-time modulation (ptm). PPM.
2-44
Figure 2-43E.—Pulse-time modulation (ptm). PFM
PULSE-DURATION MODULATION.—Pdm and pwm are designations for a single type of
modulation. The width of each pulse in a train is made proportional to the instantaneous value of the
modulating signal at the instant of the pulse. Either the leading edges, the trailing edges, or both edges of
the pulses may be modulated to produce the variation in pulse width. Pdm can be obtained in a number of
ways, one of which is illustrated in views (A) through (D) in figure 2-44. A circuit to produce pdm is
shown in figure 2-45. Adding the modulating signal [figure 2-44, view (A)] to a repetitive sawtooth [view
(B)] will result in the waveform shown in view (C). This waveform is then applied to a circuit which
changes state when the input signal exceeds a specific threshold level. This action produces pulses with
widths that are determined by the length of time that the input waveform exceeds the threshold level. The
resulting waveform is shown in view (D).
Figure 2-44A.—Pulse-duration modulation (pdm). MODULATING SIGNAL.
Figure 2-44B.—Pulse-duration modulation (pdm). REPETITIVE SAWTOOTH PULSES.
Figure 2-44C.—Pulse-duration modulation (pdm). MODULATING SIGNAL AND SAWTOOTH ADDED.
2-45
Figure 2-44D.—Pulse-duration modulation (pdm). WIDTH MODULATED PULSES FROM CIRCUIT OF
FIGURE 2-45.
Figure 2-45.—Circuit for producing pdm.
In the circuit of figure 2-45, a series of sawtooth pulses, occurring at the sampling rate, is applied to
a one-shot multivibrator. The multivibrator has the signal voltage Essuperimposed on the bias voltage Ein.
Each pulse triggers a cycle of multivibrator operation which terminates after a time interval and varies
linearly with the voltage Es. The pulse of plate voltage produced by the multivibrator will have a leading
edge at T1. The leading edge will vary in position with the signal voltage, while the trailing edge at T2 is
fixed by the termination of the sawtooth pulse. The length of the output pulse is thus duration or width
modulated. If the sawtooth has an instantaneous buildup and a sloping trailing edge, then the leading edge
(T1) is fixed and the trailing edge (T2) varies. If the sawtooth generator produces a slope on both leading
and trailing edges, both T1 and T2 are variable in position, but the result is still pdm. Pdm is often used
because it is of a constant amplitude and is, therefore, less susceptible to noise. When compared with
ppm, pdm has the disadvantage of a varying pulse, width and, therefore, of varying power content. This
means that the transmitter must be powerful enough to handle the maximum-width pulses, although the
average power transmitted is much less than peak power. On the other hand, pdm will still work if the
synchronization between the transmitter and receiver fails; in ppm it will not, as will be seen in the next
section.
PULSE-POSITION MODULATION.—The amplitude and width of the pulse is kept constant in
the system. The position of each pulse, in relation to the position of a recurrent reference pulse, is varied
by each instantaneous sampled value of the modulating wave. Ppm has the advantage of requiring
constant transmitter power since the pulses are of constant amplitude and duration. It is widely used but
has the disadvantage of depending on transmitter-receiver synchronization.
2-46
Ppm can be generated in several ways, but we will discuss one of the simplest. Figure 2-46 shows
three waveforms associated with developing ppm from pdm. The pdm pulse train is applied to a
differentiating circuit. (Differentiation was presented in NEETS, Module 9, Introduction to
Wave-Generation and Wave-Shaping Circuits.) This provides positive- and negative-polarity pulses that
correspond to the leading and trailing edges of the pdm pulses. Considering pdm and its generation, you
can see that each pulse has a leading and trailing edge. In this case the position of the leading edge is
fixed, whereas the trailing edge is not, as shown in view (A) of figure 2-46. The resultant pulses after the
differentiation are shown in view (B). The negative pulses are position-modulated in accordance with the
modulating waveform. Both the negative and positive pulse are then applied to a rectification circuit. This
application eliminates the positive, non-modulated pulses and develops a ppm pulse train, as shown in
view (C).
Figure 2-46.—Pulse-position modulation (ppm).
PULSE-FREQUENCY MODULATION.—Pfm is a method of pulse modulation in which the
modulating wave is used to frequency modulate a pulse-generating circuit. For example, the pulse rate
may be 8,000 pulses per second (pps) when the signal voltage is 0. The pulse rate may step up to 9,000
pps for maximum positive signal voltage, and down to 7,000 pps for maximum negative signal voltage.
Figure 2-47, views (A), (B), and (C) show three typical waveforms for pfm. This method of modulation is
not used extensively because of complicated pfm generation methods. It requires a stable oscillator that is
frequency modulated to drive a pulse generator. Since the other forms of ptm are easier to achieve, they
are commonly used.
Figure 2-47A.—Pulse-frequency modulation (pfm). MODULATION.
2-47
Figure 2-47B.—Pulse-frequency modulation (pfm). TIMING.
Figure 2-47C.—Pulse-frequency modulation (pfm). PFM.
Q-23.
What characteristics of a pulse can be changed in pulse-time modulation?
Q-24.
Which edges of the pulse can be modulated in pulse-duration modulation?
Q-25.
What is the main disadvantage of pulse-position modulation?
Q-26.
What is pulse-frequency modulation?
Pulse-Code Modulation
PULSE-CODE MODULATION (pcm) refers to a system in which the standard values of a
QUANTIZED WAVE (explained in the following paragraphs) are indicated by a series of coded pulses.
When these pulses are decoded, they indicate the standard values of the original quantized wave. These
codes may be binary, in which the symbol for each quantized element will consist of pulses and spaces:
ternary, where the code for each element consists of any one of three distinct kinds of values (such as
positive pulses, negative pulses, and spaces); or n-ary, in which the code for each element consists of nay
number (n) of distinct values. This discussion will be based on the binary pcm system.
All of the pulse-modulation systems discussed previously provide methods of converting analog
wave shapes to digital wave shapes (pulses occurring at discrete intervals, some characteristic of which is
varied as a continuous function of the analog wave). The entire range of amplitude (frequency or phase)
values of the analog wave can be arbitrarily divided into a series of standard values. Each pulse of a pulse
train [figure 2-48, view (B)] takes the standard value nearest its actual value when modulated. The
modulating wave can be faithfully reproduced, as shown in views (C) and (D). The amplitude range has
been divided into 5 standard values in view (C). Each pulse is given whatever standard value is nearest its
actual instantaneous value. In view (D), the same amplitude range has been divided into 10 standard
levels. The curve of view (D) is a much closer approximation of the modulating wave, view (A), than is
the 5-level quantized curve in view (C). From this you should see that the greater the number of standard
levels used, the more closely the quantized wave approximates the original. This is also made evident by
the fact that an infinite number of standard levels exactly duplicates the conditions of nonquantization
(the original analog waveform).
2-48
Figure 2-48A.—Quantization levels. MODULATION.
Figure 2-48B.—Quantization levels. TIMING.
Figure 2-48C.—Quantization levels. QUANTIZED 5-LEVEL.
Figure 2-48D.—Quantization levels. QUANTIZED 10-LEVEL.
Although the quantization curves of figure 2-48 are based on 5- and 10-level quantization, in actual
practice the levels are usually established at some exponential value of 2, such as 4(22), 8(23), 16(24),
32(25) . . . N(2n). The reason for selecting levels at exponential values of 2 will become evident in the
discussion of pcm. Quantized fm is similar in every way to quantized AM. That is, the range of frequency
deviation is divided into a finite number of standard values of deviation. Each sampling pulse results in a
deviation equal to the standard value nearest the actual deviation at the sampling instant. Similarly, for
phase modulation, quantization establishes a set of standard values. Quantization is used mostly in
amplitude- and frequency-modulated pulse systems.
Figure 2-49 shows the relationship between decimal numbers, binary numbers, and a pulse-code
waveform that represents the numbers. The table is for a 16-level code; that is, 16 standard values of a
quantized wave could be represented by these pulse groups. Only the presence or absence of the pulses
are important. The next step up would be a 32-level code, with each decimal number represented by a
series of five binary digits, rather than the four digits of figure 2-49. Six-digit groups would provide a
64-level code, seven digits a 128-level code, and so forth. Figure 2-50 shows the application of
pulse-coded groups to the standard values of a quantized wave.
2-49
Figure 2-49.—Binary numbers and pulse-code equivalents.
Figure 2-50.—Pulse-code modulation of a quantized wave (128 bits).
In figure 2-50 the solid curve represents the unquantized values of a modulating sinusoid. The
dashed curve is reconstructed from the quantized values taken at the sampling interval and shows a very
close agreement with the original curve. Figure 2-51 is identical to figure 2-50 except that the sampling
interval is four times as great and the reconstructed curve is not faithful to the original. As previously
stated, the sampling rate of a pulsed system must be at least twice the highest modulating frequency to get
2-50
a usable reconstructed modulation curve. At the sampling rate of figure 2-50 and with a 4-element binary
code, 128 bits (presence or absence of pulses) must be transmitted for each cycle of the modulating
frequency. At the sampling rate of figure 2-51, only 32 bits are required; at the minimum sampling rate,
only 8 bits are required.
Figure 2-51.—Pulse-code modulation of a quantized wave (32 bits).
As a matter of convenience, especially to simplify the demodulation of pcm, the pulse trains actually
transmitted are reversed from those shown in figures 2-49, 2-50, and 2-51; that is, the pulse with the
lowest binary value (least significant digit) is transmitted first and the succeeding pulses have increasing
binary values up to the code limit (most significant digit). Pulse coding can be performed in a number of
ways using conventional circuitry or by means of special cathode ray coding tubes. One form of coding
circuit is shown in figure 2-52. In this case, the pulse samples are applied to a holding circuit (a capacitor
which stores pulse amplitude information) and the modulator converts pam to pdm. The pdm pulses are
then used to gate the output of a precision pulse generator that controls the number of pulses applied to a
binary counter. The duration of the gate pulse is not necessarily an integral number of the repetition
pulses from the precisely timed clock-pulse generator. Therefore, the clock pulses gated into the binary
counter by the pdm pulse may be a number of pulses plus the leading edge of an additional pulse. This
"partial" pulse may have sufficient duration to trigger the counter, or it may not. The counter thus
responds only to integral numbers, effectively quantizing the signal while, at the same time, encoding it.
Each bistable stage of the counter stores ZERO or a ONE for each binary digit it represents (binary 1110
or decimal 14 is shown in figure 2-52). An electronic commutator samples the 2 0, 21, 22, and 23 digit
positions in sequence and transmits a mark or space bit (pulse or no pulse) in accordance with the state of
each counter stage. The holding circuit is always discharged and reset to zero before initiation of the
sequence for the next pulse sample.
2-51
Figure 2-52.—Block diagram of quantizer and pcm coder.
The pcm demodulator will reproduce the correct standard amplitude represented by the pulse-code
group. However, it will reproduce the correct standard only if it is able to recognize correctly the presence
or absence of pulses in each position. For this reason, noise introduces no error at all if the signal-to-noise
ration is such that the largest peaks of noise are not mistaken for pulses. When the noise is random (circuit
and tube noise), the probability of the appearance of a noise peak comparable in amplitude to the pulses
can be determined. This probability can be determined mathematically for any ration of signal-to-averagenoise power. When this is done for 10 5 pulses per second, the approximate error rate for three values of
signal power to average noise power is:
17 dB — 10 errors per second
20 dB — 1 error every 20 minutes
22 dB — 1 error every 2,000 hours
Above a threshold of signal-to-noise ration of approximately 20 dB, virtually no errors occur. In all
other systems of modulation, even with signal-to-noise ratios as high as 60 dB, the noise will have some
effect. Moreover, the pcm signal can be retransmitted, as in a multiple relay link system, as many times as
desired, without the introduction of additional noise effects; that is, noise is not cumulative at relay
stations as it is with other modulation systems.
The system does, of course, have some distortion introduced by quantizing the signal. Both the
standard values selected and the sampling interval tend to make the reconstructed wave depart from the
original. This distortion, called QUANTIZING NOISE, is initially introduced at the quantizing and
coding modulator and remains fixed throughout the transmission and retransmission processes. Its
magnitude can be reduced by making the standard quantizing levels closer together. The relationship of
the quantizing noise to the number of digits in the binary code is given by the following standard
relationship:
Where:
n is the number of digits in the binary code
2-52
Thus, with the 4-digit code of figure 2-50 and 2-51, the quantizing noise will be about 35 dB weaker
than the peak signal which the channel will accommodate.
The advantages of pcm are two-fold. First, noise interference is almost completely eliminated when
the pulse signals exceed noise levels by a value of 20 dB or more. Second, the signal may be received and
retransmitted as many times as may be desired without introducing distortion into the signal.
Q-27.
Pulse-code modulation requires the use of approximations of value that are obtained by what
process?
Q-28.
If a modulating wave is sampled 10 times per cycle with a 5-element binary code, how many
bits of information are required to transmit the signal?
Q-29.
What is the primary advantage of pulse-modulation systems?
SUMMARY
Now that you have completed this chapter, a short review of what you have learned is in order. The
following review will refresh your memory of angle modulation and pulse modulation.
FREQUENCY-SHIFT KEYING (fsk) is similar to cw keying in amplitude modulation and is a
form of angle modulation. The carrier frequency is changed between two discrete values by the opening
and closing of a key.
2-53
In FREQUENCY MODULATION (fm) the instantaneous frequency of the radio-frequency wave
is varied in accordance with the modulating signal; the amplitude of the radio-frequency wave is kept
constant.
2-54
MODULATION INDEX is the ratio of the maximum frequency difference between the modulated
and the unmodulated carrier, or between the deviation frequency and the modulation frequency.
The number of SIGNIFICANT SIDEBANDS and the modulating frequency will determine the
bandwidth of the fm wave. The number of significant sidebands can be determined from the modulation
index.
MODULATION
INDEX
.01
.4
.5
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
13.0
14.0
15.0
SIGNIFICANT
SIDEBANDS
2
2
4
6
8
12
14
16
18
22
24
26
28
32
32
36
38
38
The REACTANCE-TUBE MODULATOR is frequency modulated by using a reactance tube in
shunt with the tank circuit.
2-55
The SEMICONDUCTOR-REACTANCE MODULATOR is used to frequency modulate
low-power semiconductor transmitters.
2-56
The MULTIVIBRATOR MODULATOR uses an astable multivibrator with a modulating voltage
inserted in series with the base return of the multivibrator transistors.
The VARACTOR FM MODULATOR uses a VARACTOR. This is a specially designed diode that
has a certain amount of capacitance between the junction that can be controlled by reverse biasing.
In PHASE MODULATION the carrier’s phase is caused to shift at the rate of the modulating audio.
The amount of phase shift is controlled by the amplitude of the modulating wave.
2-57
A BASIC PHASE MODULATOR may be a single tube in series with a capacitor to form a
phase-shift network. As the impedance of the tube changes, the phase of the output shifts.
PHASE-SHIFT KEYING (psk) is similar to cw and fsk. It consists of phase reversals of the carrier
frequency as modulating signal data elements open and close the modulator key.
2-58
PULSE MODULATION is modulation in which we allow oscillations to occur for a given period
of time only during selected intervals.
PULSE-REPETITION TIME (prt) is the specific time period between each group of rf pulses.
2-59
PULSE-REPETITION FREQUENCY (prf) is found by dividing the pulse repetition time into 1.
This defines how often the groups of pulses occur.
PULSE WIDTH (pw) or PULSE DURATION (pd) is the time that a pulse is occurring.
REST TIME (rt) is the time referred to as nonpulse time.
PEAK POWER is the maximum power during a pulse.
AVERAGE POWER equals the peak power averaged over one complete cycle.
DUTY CYCLE is the ratio of working time to total time, or the ratio of actual transmit time to
transmit time plus rest time, for intermittently operated devices.
The SPARK-GAP MODULATOR consists of a circuit for storing energy, a circuit for rapidly
discharging the storage circuit, a pulse transformer, and a power source.
The THYRATRON MODULATOR is an electronic switch which requires a positive trigger of
only 150 volts. The trigger must rise at the rate of 100 volts per microsecond to fire or cause the
modulator to conduct.
2-60
In communications PULSE-MODULATION SYSTEMS, the modulating wave must be
SAMPLED at 2.5 times the highest modulating frequency to ensure accuracy.
PULSE-AMPLITUDE MODULATION (pam) is modulation in which the amplitude of each pulse
is controlled by the instantaneous amplitude of the modulation signal at the time of each pulse.
PULSE-DURATION MODULATION (pdm) or PULSE-WIDTH MODULATION (pwm) are
both designations for a type of modulation. The width of each pulse in a train is made proportional to the
instantaneous value of the modulating signal at the instant of the pulse.
PULSE-POSITION MODULATION (ppm) has the advantage of requiring constant transmitter
power. The amplitude and width of the pulses are kept constant. At the same time, the position of each
pulse, in relation to the position of a recurrent reference pulse, is varied by each instantaneous sampled
value of the modulating wave.
2-61
PULSE-FREQUENCY MODULATION (pfm) is a method of pulse modulation in which the
modulating wave is used to frequency modulate a pulse-generating circuit.
PULSE-CODE MODULATION (pcm) refers to a system in which the standard value of a
quantized wave is indicated by a series of coded pulses that give the modulating wave’s value at the
instant of the sample.
2-62
ANSWERS TO QUESTIONS Q1. THROUGH Q29.
A-1.
Frequency and phase.
A-2.
Frequency-shift keying.
A-3.
Resistance to noise interference.
A-4.
Instantaneous frequency.
A-5.
As the ratio of the frequency deviation to the maximum frequency deviation allowable.
A-6.
The number of significant sidebands and the modulating frequency.
A-7.
By changing the reactance of an oscillator circuit in consonance with the modulating voltage.
A-8.
Collector-to-emitter capacitance.
A-9.
An LCR filter.
A-10.
Capacitance.
A-11.
Phase.
A-12.
A phase-shift network such as a variable resistor and capacitor in series.
A-13.
Cw and frequency-shift keying.
A-14.
Pulse modulation.
A-15.
Pulse-repetition time.
A-16.
Rest time.
A-17.
Peak power during a pulse averaged over pulse time plus rest time.
A-18.
Either a fixed spark gap that uses a trigger pulse to ionize the air between the contacts, or a
rotary gap that is similar to a mechanical switch.
A-19.
Power source, a circuit for storing energy, a circuit for discharging the storage circuit, and a
pulse transformer.
A-20.
Some characteristic of the pulses has to be varied.
A-21.
2.5 times the highest modulating frequency.
A-22.
Both are susceptible to noise and interference.
A-23.
The time duration of the pulses or the time of occurrence of the pulses.
A-24.
Either, or both at the same time.
A-25.
It requires synchronization between the transmitter and receiver.
A-26.
A method of pulse modulation in which a modulating wave is used to frequency modulate a
pulse-generating circuit.
2-63
A-27.
Quantization.
A-28.
50.
A-29.
Low susceptibility to noise.
2-64
CHAPTER 3
DEMODULATION
LEARNING OBJECTIVES
Upon completion of this chapter you will be able to:
1. Describe cw detector circuit operations for the heterodyne and regenerative detectors.
2. Discuss the requirements for recovery of intelligence from an AM signal and describe the theory
of operation of the following AM demodulators: series-diode, shunt-diode, common-emitter, and
common-base.
3. Describe fm demodulation circuit operation for the phase-shift and gated-beam discriminators
and the ratio-detector demodulator.
4. Describe phase demodulation circuit operation for the peak, low-pass filter, and conversion
detectors.
INTRODUCTION
In chapters 1 and 2 you studied how to apply intelligence (modulation) to an rf-carrier wave. Carrier
modulation allows the transmission of modulating frequencies without the use of transmission wire as a
medium. However, for the communication process to be completed or to be useful, the intelligence must
be recovered in its original form at the receiving site. The process of re-creating original modulating
frequencies (intelligence) from the rf carrier is referred to as DEMODULATION or DETECTION. Each
type of modulation is different and requires different techniques to recover (demodulate) the intelligence.
In this chapter we will discuss ways of demodulating AM, cw, fm, phase, and pulse modulation.
The circuit in which restoration is achieved is called the DETECTOR or DEMODULATOR (both of
these terms are used in NEETS). The term demodulator is used because the demodulation process is
considered to be the opposite of modulation. The output of an ideal detector must be an exact
reproduction of the modulation existing on the rf wave. Failure to accurately recover this intelligence will
result in distortion and degradation of the demodulated signal and intelligence will be lost. The distortion
may be in amplitude, frequency, or phase, depending on the nature of the demodulator. A nonlinear
device is required for demodulation. This nonlinear device is required to recover the modulating
frequencies from the rf envelope. Solid-state detector circuits may be either a pn junction diode or the
input junction of a transistor. In electron-tube circuits, either a diode or the grid or plate circuits of a
triode electron tube may be used as the nonlinear device.
Q-1.
What is demodulation?
Q-2.
What is a demodulator?
3-1
CONTINUOUS-WAVE DEMODULATION
Continuous-wave (cw) modulation consists of on-off keying of a carrier wave. To recover on-off
keyed information, we need a method of detecting the presence or absence of rf oscillations. The CW
DEMODULATOR detects the presence of rf oscillations and converts them into a recognizable form.
Figure 3-1 illustrates the received cw in view (A), the rectified cw from a diode detector in view (B), and
the dc output from a filter that can be used to control a relay or light indicator in view (C).
Figure 3-1A.—Cw demodulation. RECEIVED CW.
Figure 3-1B.—Cw demodulation. RECTIFIED CW FROM DETECTOR.
Figure 3-1C.—Cw demodulation. OUTPUT FROM FILTER.
Figure 3-2 is a, simplified circuit that could be used as a cw demodulator. The antenna receives the rf
oscillations from the transmitter. The tank circuit, L and C1, acts as a frequency-selective network that is
tuned to the desired rf carrier frequency. The diode rectifies the oscillations and C2 provides filtering to
provide a constant dc output to control the headset. This demodulator circuit is the equivalent of a wire
telegraphy circuit but it has certain disadvantages. For example, if two transmitters are very close in
frequency, distinguishing which transmitting station you are receiving is often impossible without a
method of fine tuning the desired frequency. Also, if the stations are within the frequency bandpass of the
input tank circuit, the tank output will contain a mixture of both signals. Therefore, a method, such as
HETERODYNE DETECTION, must be used which provides more than just the information on the
presence or absence of a signal.
3-2
Figure 3-2.—Cw demodulator.
HETERODYNE DETECTION
The use of an af voltage in the detector aids the operator in distinguishing between various signals.
Since the carrier is unmodulated, the af voltage can be developed by using the heterodyne procedure
discussed in chapter 1. The procedure is to mix the incoming cw signal with locally generated
oscillations. This provides a convenient difference frequency in the af range, such as 1,000 hertz. The af
difference frequency then is rectified and smoothed by a detector. The af voltage is reproduced by a
telephone headset or a loudspeaker.
Consider the heterodyne reception of the code letter A, as shown in figure 3-3, view (A). The code
consists of a short burst of cw energy (dot) followed by a longer burst (dash). Assume that the frequency
of the received cw signal is 500 kilohertz. The locally generated oscillations are adjusted to a frequency
which is higher or lower than the incoming rf signal (501 kilohertz in this case), as shown in view (B).
The voltage resulting from the heterodyning action between the cw signal [view (A)] and the local
oscillator signal [view (B)] is shown in view (C) as the mixed-frequency signal. ENVELOPE
(intelligence) amplitude varies at the BEAT (difference) frequency of 1,000 hertz (501,000 − 500,000).
The negative half cycles of the mixed frequency are rectified, as shown in view (D). The peaks of the
positive half cycles follow the 1,000-hertz beat frequency.
Figure 3-3A.—Heterodyne detection. RECEIVED CW SIGNAL.
Figure 3-3B.—Heterodyne detection. LOCAL OSCILLATOR SIGNAL.
3-3
Figure 3-3C.—Heterodyne detection. MIXED-FREQUENCY SIGNAL.
Figure 3-3D.—Heterodyne detection. RECTIFIED MIXED-FREQUENCY SIGNAL.
Figure 3-3E.—Heterodyne detection. AUDIO-BEAT NOTE FROM FILTER.
The cw signal pulsations are removed by the rf filter in the detector output and only the envelope of
the rectified pulses remains. The envelope, shown in view (E), is a 1,000-hertz audio-beat note. This
1,000 hertz, dot-dash tone may be heard in a speaker or headphone and identified as the letter A by the
operator.
The heterodyne method of reception is highly selective and allows little interference from adjacent
cw stations. If a cw signal from a radiotelegraph station is operating at 10,000,000 hertz and at the same
time an adjacent station is operating at 10,000,300 hertz, a simple detector cannot clearly discriminate
between the two stations because the signals are just 300 hertz apart. This is because the bandpass of the
tuning circuits is too wide and allows some of the other signal to interfere. The two carrier frequencies
differ by only 0.003 percent and a tuned tank circuit cannot easily discriminate between them. However,
if a heterodyne detector with a local-oscillator frequency of 10,001,000 hertz is used, then beat notes of
1,000 and 700 hertz are produced by the two signals. These are audio frequencies, which can be
3-4
distinguished easily by a selective circuit because they differ by 30 percent (compared to the 0.003
percent above).
Even if two stations produce identical beat frequencies, they can be separated by adjusting the
local-oscillator or BEAT-FREQUENCY OSCILLATOR (bfo) frequency. For example, if the second
station in the previous example had been operating at 10,002,000 hertz, then both stations would have
produced a 1,000-hertz beat frequency and interference would have occurred. Adjusting the
local-oscillator frequency to 9,999,000 hertz would have caused the desired station at 10,000,000 hertz to
produce a 1,000-hertz beat frequency. The other station, at 10,002,000 hertz, would have produced a beat
frequency of 3,000 hertz. Either selective circuits or the operator can easily distinguish between these
widely differing tones. A trained operator can use the variable local oscillator to distinguish between
stations that vary in frequency by only a few hundred hertz.
Q-3.
What is the simplest form of cw detector?
Q-4.
What are the essential components of a cw receiver system?
Q-5.
What principle is used to help distinguish between two cw signals that are close in frequency?
Q-6.
How does heterodyning distinguish between cw signals?
REGENERATIVE DETECTOR
A simple, one-transistor REGENERATIVE DETECTOR circuit that uses the heterodyning principle
for cw operation is shown in figure 3-4. The circuit can be made to oscillate by increasing the amount of
energy fed back to the tank circuit from the collector-output circuit (by physically moving tickler coil L2
closer to L1 using the regeneration control). This feedback overcomes losses in the base-input circuit and
causes self-oscillations which are controlled by tuning capacitor C1. The received signal from the antenna
and the oscillating frequency are both present at the base of transistor Q1. These two frequencies are
heterodyned by the nonlinearity of the transistor. The resulting beat frequencies are then rectified by the
emitter-base junction and produce a beat note which is amplified in the collector-output circuit. The af
currents in the collector circuit actuate the phones. The REGENERATIVE DETECTOR (figure 3-4)
produces its own oscillations, heterodynes them with an incoming signal, and rectifies or detects them.
Figure 3-4.—Regenerative detector.
3-5
The regenerative detector is used to receive short-wave code signals because it is easy to adjust and
has high sensitivity and good selectivity. At high frequencies, the amount of signal detuning necessary to
produce an audio-beat note is a small percentage of the signal frequency and causes no trouble. The use of
the regenerative detector for low-frequency code reception, however, is usually avoided. At low
frequencies the detuning required to produce the proper audio-beat frequency is a considerable percentage
of the signal frequency. Although this type detector may be used for AM signals, it has high distortion
and is not often used.
Q-7.
What simple, one-transistor detector circuit uses the heterodyne principle?
Q-8.
What three functions does the transistor in a regenerative detector serve?
AM DEMODULATION
Amplitude modulation refers to any method of modulating an electromagnetic carrier frequency by
varying its amplitude in accordance with the message intelligence that is to be transmitted. This is
accomplished by heterodyning the intelligence frequency with the carrier frequency. The vector
summation of the carrier, sum, and difference frequencies causes the modulation envelope to vary in
amplitude at the intelligence frequency, as discussed in chapter 1. In this section we will discuss several
circuits that can be used to recover this intelligence from the variations in the modulation envelope.
DIODE DETECTORS
The detection of AM signals ordinarily is accomplished by means of a diode rectifier, which may be
either a vacuum tube or a semiconductor diode. The basic detector circuit is shown in its simplest form in
view (A) of figure 3-5. Views (B), (C), and (D) show the circuit waveforms. The demodulator must meet
three requirements: (1) It must be sensitive to the type of modulation applied at the input, (2) it must be
nonlinear, and (3) it must provide filtering. Remember that the AM waveform appears like the diagram of
view (B) and the amplitude variations of the peaks represent the original audio signal, but no modulating
signal frequencies exist in this waveform. The waveform contains only three rf frequencies: (1) the
carrier frequency, (2) the sum frequency, and (3) the difference frequency. The modulating intelligence is
contained in the difference between these frequencies. The vector addition of these frequencies provides
the modulation envelope which approximates the original modulating waveform. It is this modulation
envelope that the DIODE DETECTORS use to reproduce the original modulating frequencies.
Figure 3-5A.—Series-diode detector and wave shapes. CIRCUIT.
3-6
Figure 3-5B.—Series-diode detector and wave shapes. RF INPUT SIGNAL.
Figure 3-5C.—Series-diode detector and wave shapes. RECTIFIED SIGNAL.
Figure 3-5D.—Series-diode detector and wave shapes. AUDIO SIGNAL.
Series-Diode Detector
Let’s analyze the operation of the circuit shown in view (A) of figure 3-5. This circuit is the basic
type of diode receiver and is known as a SERIES-DIODE DETECTOR. The circuit consists of an
antenna, a tuned LC tank circuit, a semiconductor diode detector, and a headset which is bypassed by
capacitor C2. The antenna receives the transmitted rf energy and feeds it to the tuned tank circuit. This
tank circuit (L1 and C1) selects which rf signal will be detected. As the tank resonates at the selected
frequency, the wave shape in view (B) is developed across the tank circuit. Because the semiconductor is
a nonlinear device, it conducts in only one direction. This eliminates the negative portion of the rf carrier
and produces the signal shown in view (C). The current in the circuit must be smoothed before the
headphones can reproduce the af intelligence. This action is achieved by C2 which acts as a filter to
3-7
provide an output that is proportional to the peak rf pulses. The filter offers a low impedance to rf and a
relatively high impedance to af. (Filters were discussed in NEETS, Module 9, Introduction to WaveGeneration and Wave-Shaping Circuits.) This action causes C2 to develop the waveform in view (D).
This varying af voltage is applied to the headset which then reproduces the original modulating
frequency. This circuit is called a series-diode detector (sometimes referred to as a VOLTAGE-DIODE
DETECTOR) because the semiconductor diode is in series with both the input voltage and the load
impedance. Voltages in the circuit cause an output voltage to develop across the load impedance that is
proportional to the input voltage peaks of the modulation envelope.
Q-9.
What are the three requirements for an AM demodulator?
Q-10.
What does the simplest diode detector use to reproduce the modulating frequency?
Q-11.
What is the function of the diode in a series-diode detector?
Q-12.
In figure 3-5, what is the function of C2?
Shunt-Diode Detector
The SHUNT-DIODE DETECTOR (figure 3-6) is similar to the series-diode detector except that the
output variations are current pulses rather than voltage pulses. Passing this current through a shunt
resistor develops the voltage output. The input is an rf modulated envelope. On the negative half cycles of
the rf, diode CR1 is forward biased and shunts the signal to ground. On the positive half cycles, current
flows from the output through L1 to the input. A field is built up around L1 that tends to keep the current
flowing. This action integrates the rf current pulses and causes the output to follow the modulation
envelope (intelligence) closely. (Integration was discussed in NEETS, Module 9, Introduction to
Wave-Generation and Wave-Shaping Circuits.) Shunt resistor R1 develops the output voltage from this
current flow. Although the shunt detector operates on the principle of current flow, it is the output voltage
across the shunt resistor that is used to reproduce the original modulation signal. The shunt-diode detector
is easily identified by noting that the detector diode is in parallel with both the input and load impedance.
The waveforms associated with this detector are identical to those shown in views (B), (C), and (D) of
figure 3-5.
Figure 3-6.—Shunt-diode detector.
The series-diode detector is normally used where large input signals are supplied and a linear output
is required. The shunt-diode detector is used where the voltage variations are too small to produce a full
output from audio amplifier stages. Additional current amplifiers are required to bring the output to a
usable level. Other methods of detection and amplification have been developed which will detect low-
3-8
level signals. The next sections will discuss two of these circuits, the common-emitter and common-base
detectors.
Q-13.
How does the current-diode detector differ from the voltage-diode detector?
Q-14.
Under what circuit conditions would the shunt detector be used?
COMMON-EMITTER DETECTOR
The COMMON-EMITTER DETECTOR is often used in receivers to supply an amplified detected
output. The schematic for a typical transistor common-emitter detector is shown in figure 3-7. Input
transformer T1 has a tuned primary that acts as a frequency-selective device. L2 inductively couples the
input modulation envelope to the base of transistor Q1. Resistors R1 and R2 are fixed-bias voltage
dividers that set the bias levels for Q1. Resistor R1 is bypassed by C2 to eliminate rf. This RC
combination also acts as the load for the diode detector (emitter-base junction of Q1). The detected audio
is in series with the biasing voltage and controls collector current. The output is developed across R4
which is also bypassed to remove rf by C4. R3 is a temperature stabilization resistor and C3 bypasses it
for both rf and af.
Figure 3-7.—Common-emitter detector.
Q1 is biased for slight conduction with no input signal applied. When an input signal appears on the
base of Q1, it is rectified by the emitter-base junction (operating as a diode) and is developed across R1 as
a dc bias voltage with a varying af component. This voltage controls bias and collector current for Q1.
The output is developed by collector current flow through R4. Any rf ripple in the output is bypassed
across the collector load resistor by capacitor C4. The af variations are not bypassed. After the modulation
envelope is detected in the base circuit, it is amplified in the output circuit to provide suitable af output.
The output of this circuit is higher than is possible with a simple detector. Because of the amplification in
this circuit, weaker signals can be detected than with a simple detector. A higher, more usable output is
thus developed.
Q-15.
Which junction of the transistor in the common-emitter detector detects the modulation
envelope?
Q-16.
Which component in figure 3-7 develops the af signal at the input?
Q-17.
How is the output signal developed in the common-emitter detector?
3-9
COMMON-BASE DETECTOR
Another amplifying detector that is used in portable receivers is the COMMON-BASE DETECTOR.
In this circuit detection occurs in the emitter-base junction and amplification occurs at the output of the
collector junction. The output developed is the equivalent of a diode detector which is followed by a stage
of audio amplification, but with more distortion. Figure 3-8 is a schematic of a typical common-base
detector. Transformer T1 is tuned by capacitor C3 to the frequency of the incoming modulated envelope.
Resistor R1 and capacitor C1 form a self-biasing network which sets the dc operating point of the emitter
junction. The af output is taken from the collector circuit through audio transformer T2. The primary of
T2 forms the detector output load and is bypassed for rf by capacitor C2.
Figure 3-8.—Common-base detector.
The input signal is coupled through T1. When capacitor C3 is tuned to the proper frequency, the
signal is passed to the emitter of Q1. When no input signal is present, bias is determined by resistor R1.
When the input signal becomes positive, current flows through the emitter-base junction causing it to be
forward biased. C1 and R1 establish the dc operating point by acting as a filter network. This action
provides a varying dc voltage that follows the peaks of the rf modulated envelope. This action is identical
to the diode detector with the emitter-base junction doing the detecting. The varying dc voltage on the
emitter changes the bias on Q1 and causes collector current to vary in accordance with the detected
voltage. Transformer T2 couples these af current changes to the output. Thus, Q1 detects the AM wave
and then provides amplification for the detected waveform.
The four AM detectors just discussed are not the only types that you will encounter. However, they
are representative of most AM detectors and the same characteristics will be found in all AM detectors.
Now let’s study some ways of demodulating frequency-modulated (fm) signals.
Q-18.
Which junction acts as the detector in a common-base detector?
Q-19.
To what circuit arrangement is a common-base detector equivalent?
Q-20.
In figure 3-8, which components act as the filter network in the diode detector?
FM DEMODULATION
In fm demodulators, the intelligence to be recovered is not in amplitude variations; it is in the
variation of the instantaneous frequency of the carrier, either above or below the center frequency. The
3-10
detecting device must be constructed so that its output amplitude will vary linearly according to the
instantaneous frequency of the incoming signal.
Several types of fm detectors have been developed and are in use, but in this section you will study
three of the most common: (1) the phase-shift detector, (2) the ratio detector, and (3) the gated-beam
detector.
SLOPE DETECTION
To be able to understand the principles of operation for fm detectors, you need to first study the
simplest form of frequency-modulation detector, the SLOPE DETECTOR. The slope detector is
essentially a tank circuit which is tuned to a frequency either slightly above or below the fm carrier
frequency. View (A) of figure 3-9 is a plot of voltage versus frequency for a tank circuit. The resonant
frequency of the tank is the frequency at point 4. Components are selected so that the resonant frequency
is higher than the frequency of the fm carrier signal at point 2. The entire frequency deviation for the fm
signal falls on the lower slope of the bandpass curve between points 1 and 3. As the fm signal is applied
to the tank circuit in view (B), the output amplitude of the signal varies as its frequency swings closer to,
or further from, the resonant frequency of the tank. Frequency variations will still be present in this
waveform, but it will also develop amplitude variations, as shown in view (B). This is because of the
response of the tank circuit as it varies with the input frequency. This signal is then applied to the diode
detector in view (C) and the detected waveform is the output. This circuit has the major disadvantage that
any amplitude variations in the rf waveform will pass through the tank circuit and be detected. This
disadvantage can be eliminated by placing a limiter circuit before the tank input. (Limiter circuits were
discussed in NEETS, Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits.) This
circuit is basically the same as an AM detector with the tank tuned to a higher or lower frequency than the
received carrier.
Figure 3-9A.—Slope detector. VOLTAGE VERSUS FREQUENCY PLOT.
Figure 3-9B.—Slope detector. TANK CIRCUIT.
3-11
Figure 3-9C.—Slope detector. DIODE DETECTOR.
Q-21.
What is the simplest form of fm detector?
Q-22.
What is the function of an fm detector?
FOSTER-SEELEY DISCRIMINATOR
The FOSTER-SEELEY DISCRIMINATOR is also known as the PHASE-SHIFT
DISCRIMINATOR. It uses a double-tuned rf transformer to convert frequency variations in the received
fm signal to amplitude variations. These amplitude variations are then rectified and filtered to provide a
dc output voltage. This voltage varies in both amplitude and polarity as the input signal varies in
frequency. A typical discriminator response curve is shown in figure 3-10. The output voltage is 0 when
the input frequency is equal to the carrier frequency (fr). When the input frequency rises above the center
frequency, the output increases in the positive direction. When the input frequency drops below the center
frequency, the output increases in the negative direction.
Figure 3-10.—Discriminator response curve.
The output of the Foster-Seeley discriminator is affected not only by the input frequency, but also to
a certain extent by the input amplitude. Therefore, using limiter stages before the detector is necessary.
Circuit Operation of a Foster-Seeley Discriminator
View (A) of figure 3-11 shows a typical Foster-Seeley discriminator. The collector circuit of the
preceding limiter/amplifier circuit (Q1) is shown. The limiter/amplifier circuit is a special amplifier
circuit which limits the amplitude of the signal. This limiting keeps interfering noise low by removing
3-12
excessive amplitude variations from signals. The collector circuit tank consists of C1 and L1. C2 and L2
form the secondary tank circuit. Both tank circuits are tuned to the center frequency of the incoming fm
signal. Choke L3 is the dc return path for diode rectifiers CR1 and CR2. R1 and R2 are not always
necessary but are usually used when the back (reverse bias) resistance of the two diodes is different.
Resistors R3 and R4 are the load resistors and are bypassed by C3 and C4 to remove rf. C5 is the output
coupling capacitor.
Figure 3-11.—Foster-Seeley discriminator. FOSTER-SEELEY DISCRIMINATOR.
CIRCUIT OPERATION AT RESONANCE.—The operation of the Foster-Seeley discriminator
can best be explained using vector diagrams [figure 3-11, view (B)] that show phase relationships
between the voltages and currents in the circuit. Let's look at the phase relationships when the input
frequency is equal to the center frequency of the resonant tank circuit.
The input signal applied to the primary tank circuit is shown as vector ep. Since coupling capacitor
C8 has negligible reactance at the input frequency, rf choke L3 is effectively in parallel with the primary
tank circuit. Also, because L3 is effectively in parallel with the primary tank circuit, input voltage ep also
appears across L3. With voltage ep applied to the primary of T1, a voltage is induced in the secondary
which causes current to flow in the secondary tank circuit. When the input frequency is equal to the center
frequency, the tank is at resonance and acts resistive. Current and voltage are in phase in a resistance
circuit, as shown by is and ep. The current flowing in the tank causes voltage drops across each half of the
balanced secondary winding of transformer T1. These voltage drops are of equal amplitude and opposite
3-13
polarity with respect to the center tap of the winding. Because the winding is inductive, the voltage across
it is 90 degrees out of phase with the current through it. Because of the center-tap arrangement, the
voltages at each end of the secondary winding of T1 are 180 degrees out of phase and are shown as e1 and
e2 on the vector diagram.
The voltage applied to the anode of CR1 is the vector sum of voltages ep and e1, shown as e 3 on the
diagram. Likewise, the voltage applied to the anode of CR2 is the vector sum of voltages e p and e 2,
shown as e4 on the diagram. At resonance e3 and e4 are equal, as shown by vectors of the same length.
Equal anode voltages on diodes CR1 and CR2 produce equal currents and, with equal load resistors, equal
and opposite voltages will be developed across R3 and R4. The output is taken across R3 and R4 and will
be 0 at resonance since these voltages are equal and of appositive polarity.
The diodes conduct on opposite half cycles of the input waveform and produce a series of dc pulses
at the rf rate. This rf ripple is filtered out by capacitors C3 and C4.
OPERATION ABOVE RESONANCE.—A phase shift occurs when an input frequency higher
than the center frequency is applied to the discriminator circuit and the current and voltage phase
relationships change. When a series-tuned circuit operates at a frequency above resonance, the inductive
reactance of the coil increases and the capacitive reactance of the capacitor decreases. Above resonance
the tank circuit acts like an inductor. Secondary current lags the primary tank voltage, ep. Notice that
secondary voltages e 1 and e2 are still 180 degrees out of phase with the current (iS) that produces them.
The change to a lagging secondary current rotates the vectors in a clockwise direction. This causes el to
become more in phase with ep while e2 is shifted further out of phase with ep. The vector sum of ep and e2
is less than that of ep and e1. Above the center frequency, diode CR1 conducts more than diode CR2.
Because of this heavier conduction, the voltage developed across R3 is greater than the voltage developed
across R4; the output voltage is positive.
OPERATION BELOW RESONANCE.—When the input frequency is lower than the center
frequency, the current and voltage phase relationships change. When the tuned circuit is operated at a
frequency lower than resonance, the capacitive reactance increases and the inductive reactance decreases.
Below resonance the tank acts like a capacitor and the secondary current leads primary tank voltage ep.
This change to a leading secondary current rotates the vectors in a counterclockwise direction. From the
vector diagram you should see that e2 is brought nearer in phase with ep, while el is shifted further out of
phase with ep. The vector sum of ep and e2 is larger than that of e and e1. Diode CR2 conducts more than
diode CR1 below the center frequency. The voltage drop across R4 is larger than that across R3 and the
output across both is negative.
Disadvantages
These voltage outputs can be plotted to show the response curve of the discriminator discussed
earlier (figure 3-10). When weak AM signals (too small in amplitude to reach the circuit limiting level)
pass through the limiter stages, they can appear in the output. These unwanted amplitude variations will
cause primary voltage ep [view (A) of figure 3-11] to fluctuate with the modulation and to induce a
similar voltage in the secondary of T1. Since the diodes are connected as half-wave rectifiers, these small
AM signals will be detected as they would be in a diode detector and will appear in the output. This
unwanted AM interference is cancelled out in the ratio detector (to be studied next in this chapter) and is
the main disadvantage of the Foster-Seeley circuit.
Q-23.
What type of tank circuit is used in the Foster-Seeley discriminator?
Q-24.
What is the purpose of CR1 and CR2 in the Foster-Seeley discriminator?
Q-25.
What type of impedance does the tank circuit have above resonance?
3-14
RATIO DETECTOR
The RATIO DETECTOR uses a double-tuned transformer to convert the instantaneous frequency
variations of the fm input signal to instantaneous amplitude variations. These amplitude variations are
then rectified to provide a dc output voltage which varies in amplitude and polarity with the input signal
frequency. This detector demodulates fm signals and suppresses amplitude noise without the need of
limiter stages.
Circuit Operation
Figure 3-12 shows a typical ratio detector. The input tank capacitor (C1) and the primary of
transformer T1 (L1) are tuned to the center frequency of the fm signal to be demodulated. The secondary
winding of T1 (L2) and capacitor C2 also form a tank circuit tuned to the center frequency. Tertiary
(third) winding L3 provides additional inductive coupling which reduces the loading effect of the
secondary on the primary circuit. Diodes CR1 and CR2 rectify the signal from the secondary tank.
Capacitor C5 and resistors R1 and R2 set the operating level of the detector. Capacitors C3 and C4
determine the amplitude and polarity of the output. Resistor R3 limits the peak diode current and
furnishes a dc return path for the rectified signal. The output of the detector is taken from the common
connection between C3 and C4. Resistor RL is the load resistor. R5, C6, and C7 form a low-pass filter to
the output.
Figure 3-12.—Ratio detector.
This circuit operates on the same principles of phase shifting as did the Foster-Seeley discriminator.
In that discussion, vector diagrams were used to illustrate the voltage amplitudes and polarities for
conditions at resonance, above resonance, and below resonance. The same vector diagrams apply to the
ratio detector but will not be discussed here. Instead, you will study the resulting current flows and
polarities on simplified schematic diagrams of the detector circuit.
OPERATION AT RESONANCE.—When the input voltage ep is applied to the primary in figure
3-12 it also appears across L3 because, by inductive coupling, it is effectively connected in parallel with
the primary tank circuit. At the same time, a voltage is induced in the secondary winding and causes
current to flow around the secondary tank circuit. At resonance the tank acts like a resistive circuit; that is,
3-15
the tank current is in phase with the primary voltage ep. The current flowing in the tank circuit causes
voltages e1 and e2 to be developed in the secondary winding of T1. These voltages are of equal magnitude
and of opposite polarity with respect to the center tap of the winding. Since the winding is inductive, the
voltage drop across it is 90 degrees out of phase with the current through it.
Figure 3-13 is a simplified schematic diagram of a ratio detector at resonance. The voltage applied to
the cathode of CR1 is the vector sum of e1 and ep. Likewise, the voltage applied to the anode of CR2 is
the vector sum of e2 and ep. No phase shift occurs at resonance and both voltages are equal. Both diodes
conduct equally. This equal current flow causes the same voltage drop across both R1 and R2. C3 and C4
will charge to equal voltages with opposite polarities. Let’s assume that the voltages across C3 and C4 are
equal in amplitude (5 volts) and of opposite polarity and the total charge across C5 is 10 volts. R1 and R2
will each have 5 volts dropped across them because they are of equal values. The output is taken between
points A and B. To find the output voltage, you algebraically add the voltages between points A and B
(loop ACB or ADB). Point A to point D is −5 volts. Point D to point B is + 5 volts. Their algebraic sum is
0 volts and the output voltage is 0 at resonance. If the voltages on branch ACB were figured, the same
output would be found because the circuit branches are in parallel.
Figure 3-13.—Current flow and polarities at resonance.
When the input signal reverses polarity, the secondary voltage across L2 also reverses. The diodes
will be reverse biased and no current will flow. Meanwhile, C5 retains most of its charge because of the
long time constant offered in combination with R1 and R2. This slow discharge helps to maintain the
output.
OPERATION ABOVE RESONANCE.—When a tuned circuit (figure 3-14) operates at a
frequency higher than resonance, the tank is inductive. The secondary current i lags the primary voltage
ep. Secondary voltage e1 is nearer in phase with primary voltage e, while e2 is shifted further out of phase
with ep. The vector sum of e1 and ep is larger than that of e2 and ep. Therefore, the voltage applied to the
cathode of CR1 is greater than the voltage applied to the anode of CR2 above resonance.
Figure 3-14.—Current flow and polarities above resonance.
3-16
Assume that the voltages developed above resonance are such that the higher voltage on the cathode
of CR1 causes C3 to charge to 8 volts. The lower voltage on the anode of CR2 causes C4 to charge to 2
volts. Capacitor C5 remains charged to the sum of these two voltages, 10 volts. Again, by adding the
voltages in loop ACB or ADB between points A and B, you can find the output voltage. Point A to point
D equals -2 volts. Point D to point B equals +5 volts. Their algebraic sum, and the output, equals +3 volts
when tuned above resonance. During the negative half cycle of the input signal, the diodes are reverse
biased and C5 helps maintain a constant output.
OPERATION BELOW RESONANCE.—When a tuned circuit operates below resonance (figure
3-15), it is capacitive. Secondary current is leads the primary voltage ep and secondary voltage e2 is nearer
in phase with primary voltage ep. The vector sum of e2 and ep is larger than the sum of e1 and ep. The
voltage applied to the anode of CR2 becomes greater than the voltage applied to the cathode of CR1
below resonance.
Figure 3-15.—Current flow and polarities below resonance.
Assume that the voltages developed below resonance are such that the higher voltage on the anode of
CR2 causes C4 to charge to 8 volts. The lower voltage on the cathode of CR1 causes C3 to charge to 2
volts. Capacitor C5 remains charged to the sum of these two voltages, 10 volts. The output voltage equals
−8 volts plus +5 volts, or −3 volts, when tuned below resonance. During the negative half cycle of the
input signal, the diodes are reverse biased and C5 helps maintain a constant output.
Advantage of a Ratio Detector
The ratio detector is not affected by amplitude variations on the fm wave. The output of the detector
adjusts itself automatically to the average amplitude of the input signal. C5 charges to the sum of the
voltages across R1 and R2 and, because of its time constant, tends to filter out any noise impulses. Before
C5 can charge or discharge to the higher or lower potential, the noise disappears. The difference in charge
across C5 is so slight that it is not discernible in the output. Ratio detectors can operate with as little as
100 millivolts of input. This is much lower than that required for limiter saturation and less gain is
required from preceding stages.
Q-26.
What is the primary advantage of a ratio detector?
Q-27.
What is the purpose of C5 in figure 3-12?
GATED-BEAM DETECTOR
An fm demodulator employing a completely different detection principle is the GATED-BEAM
DETECTOR (sometimes referred to as the QUADRATURE DETECTOR). A simplified diagram of a
3-17
gated-beam detector is shown in figure 3-16. It uses a gated-beam tube to limit, detect, and amplify the
received fm signal. The output voltage is 0 when the input frequency is equal to the center frequency.
When the input frequency rises above the center frequency, the output voltage goes positive. When the
input frequency drops below the center frequency, the output voltage goes negative.
Figure 3-16.—Gated-beam detector.
Circuit Operation
The gated-beam detector employs a specially designed gated-beam tube. The elements of this tube
are shown in figure 3-17. The focus electrode forms a shield around the tube cathode except for a narrow
slot through which the electron beam flows. The beam of electrons flows toward the limiter grid which
acts like a gate. When the gate is open, the electron beam flows through to the next grid. When closed, the
gate completely stops the beam.
Figure 3-17.—Gated-beam tube physical layout.
After the electron beam passes the limiter grid, the screen grid refocuses the beam toward the
quadrature grid. The quadrature grid acts much the same as the limiter grid; it either opens or closes the
passage for electrons. These two grids act similar to an AND gate in digital devices; both gates must be
open for the passage of electrons to the plate. Either grid can cut off plate current. AND gates were
presented in NEETS, Module 13, Introduction to Number Systems, Boolean Algebra, and Logic Circuits.
Look again at the circuit in figure 3-16. With no signal applied to the limiter grid (3), the tube
conducts. The electron beam moving near the quadrature grid (5) induces a current into the grid which
develops a voltage across the high-Q tank circuit (L3 and C3). C3 charges until it becomes sufficiently
3-18
negative to cut off the current flow. L3 tends to keep the current moving and, as its field collapses,
discharges C3. When C3 discharges sufficiently, the quadrature grid becomes positive, grid current flows,
and the cycle repeats itself. This tank circuit (L3 and C3) is tuned to the center frequency of the received
fm signal so that it will oscillate at that frequency.
The waveforms for the circuit are shown in figure 3-18. View (A) is the fm input signal. The
limiter-grid gate action creates a wave shape like view (B) because the tube is either cut off or saturated
very quickly by the input wave. Note that this is a square wave and is the current waveform passing the
limiter grid.
Figure 3-18A.—Gated-beam detector waveforms.
Figure 3-18B.—Gated-beam detector waveforms.
At the quadrature grid the voltage across C3 lags the current which produces it [view (C)]. The result
is a series of pulses, shown in view (D), appearing on the quadrature grid at the center frequency, but
lagging the limiter-grid voltage by 90 degrees. Because the quadrature grid has the same conduction and
cutoff levels as the limiter grid, the resultant current waveform will be transformed into a square wave.
Figure 3-18C.—Gated-beam detector waveforms.
Figure 3-18D.—Gated-beam detector waveforms.
Both the limiter and quadrature grids must be positive at the same time to have plate current. You
can see how much conduction time occurs for each cycle of the input by overlaying the current
waveforms in views (B) and (D), as shown in view (E). The times when both grids are positive are
shown by the shaded area of view (E). These plate current pulses are shown for operation at resonance in
view (F).
3-19
Figure 3-18E.—Gated-beam detector waveforms.
Figure 3-18F.—Gated-beam detector waveforms.
Now consider what happens with a deviation in frequency at the input. If the frequency increases, the
frequency across the quadrature tank also increases. Above resonance, the tank appears capacitive to the
induced current; voltage then lags the applied voltage by more than 90 degrees, as shown in view (G).
Note in view (H) that the two grid signals have moved more out of phase and the average plate current
level has decreased.
Figure 3-18G.—Gated-beam detector waveforms.
Figure 3-18H.—Gated-beam detector waveforms.
As the input frequency decreases, the opposite action takes place. The two grid signals move more in
phase, as shown in view (I), and the average plate current increases, as shown in view (J).
Figure 3-18I.—Gated-beam detector waveforms.
Figure 3-18J.—Gated-beam detector waveforms.
View (K) shows the resultant plate-current pulses when an fm signal is applied to a gated-beam
detector. Plate load resistor R4 and capacitor C6 form an integrating network which filters these pulses to
form the sine-wave output.
3-20
Figure 3-18K.—Gated-beam detector waveforms.
Advantages of the Gated-Beam Detector
The primary advantage of the gated-beam detector lies in its extreme simplicity. It employs only one
tube, yet provides a very effective limiter with linear detection. It requires relatively few components and
is very easily adjusted.
There are more than the three types of fm demodulators presented in this chapter. However, these are
representative of the types with which you will be working. The principles involved in their operation are
similar to the other types. You will now briefly study PHASE DEMODULATION which uses the same
basic circuitry as fm demodulators.
Q-28.
What circuit functions does the tube in a gated-beam detector serve?
Q-29.
What condition must exist on both the limiter and quadrature grids for current to flow in a
gated-beam detector?
Q-30.
Name two advantages of the gated-beam detector.
PHASE DEMODULATION
In phase modulation (pm) the intelligence is contained in the amount and rate of phase shift in a
carrier wave. You should recall from your study of pm that there is an incidental shift in frequency as the
phase of the carrier is shifted. Because of this incidental frequency shift, fm demodulators, such as the
Foster-Seeley discriminator and the ratio detector, can also be used to demodulate phase-shift signals.
Another circuit that may be used is the gated-beam (quadrature) detector. Remember that the fm
phase detector output was determined by the phase of the signals present at the grids. A QUADRATURE
DETECTOR FOR PHASE DEMODULATION works in the same manner.
A basic schematic is shown in figure 3-19. The quadrature-grid signal is excited by a reference from
the transmitter. This may be a sample of the unmodulated master oscillator providing a phase reference
for the detector.
3-21
Figure 3-19.—Phase detector.
The modulated waveform is applied to the limiter grid. Gating action in the tube will occur as the
phase shifts between the input waveform and the reference. The combined output current from the gatedbeam tube will be a series of current pulses. These pulses will vary in width as shown in figure 3-20. The
width of these pulses will vary in accordance with the phase difference between the carrier and the
modulated wave.
Figure 3-20.—Phase-detector waveforms.
Q-31.
Where is the intelligence contained in a phase-modulated signal?
Q-32.
Why can phase-modulated signals be detected by fm detectors?
Q-33.
How is a quadrature detector changed when used for phase demodulation?
3-22
PULSE DEMODULATION
Pulse modulation is used in radar circuits as well as communications circuits, as discussed in chapter
2. A pulse-modulated signal in radar may be detected by a simple circuit that detects the presence of rf
energy. Circuits that are capable of this were covered in this chapter in the cw detection discussion;
therefore, the information will not be repeated here. A RADAR DETECTOR, in its simplest form, must
be capable of producing an output when rf energy (reflected from a target) is present at its input.
In COMMUNICATIONS PULSE DETECTORS the modulated waveform must be restored to its
original form. In this chapter you will study three basic methods of pulse demodulation: PEAK,
LOW-PASS FILTER, and CONVERSION.
PEAK DETECTION
Peak detection uses the amplitude of a pulse-amplitude modulated (pam) signal or the duration of a
pulse-duration modulated (pdm) signal to charge a holding capacitor and restore the original waveform.
This demodulated waveform will contain some distortion because the output wave is not a pure sine
wave. However, this distortion is not serious enough to prevent the use of peak detection.
Pulse-Amplitude Demodulation
Peak detection is used to detect pam. Figure 3-21 includes a simplified circuit [view (A)] for this
demodulator and its waveforms [views (B) and (C)]. CR1 is the input diode which allows capacitor C1 to
charge to the peak value of the pam input pulse. Pam input pulses are shown in view (B). CR1 is reverse
biased between input pulses to isolate the detector circuit from the input. CR2 and CR3 are biased so that
they are normally nonconducting. The discharge path for the capacitor is through the resistor (R1). These
components are chosen so that their time constant is at least 10 times the interpulse period (time between
pulses). This maintains the charge on C1 between pulses by allowing only a small discharge before the
next pulse is applied. The capacitor is discharged just prior to each input pulse to allow the output voltage
to follow the peak value of the input pulses. This discharge is through CR2 and CR3. These diodes are
turned on by a negative pulse from a source that is time-synchronous with the timing-pulse train at the
transmitter. Diode CR3 ensures that the output voltage is near 0 during this discharge period. View (C)
shows the output wave shape from this circuit. The peaks of the output signal follow very closely the
original modulating wave, as shown by the dotted line. With additional filtering this stepped waveform
closely approximates its original shape.
Figure 3-21A.—Peak detector. CIRCUIT OF PEAK DETECTOR.
3-23
Figure 3-21B.—Peak detector. AMPLITUDE MODULATED PULSES.
Figure 3-21C.—Peak detector. PEAK DETECTION.
Pulse-Duration Modulation
The peak detector circuit may also be used for pdm. To detect pdm, you must modify view (A) of
figure 3-21 so that the time constant for charging C1 through CR1 is at least 10 times the maximum
received pulse width. This may be done by adding a resistor in series with the cathode or anode circuit of
CR1. The amplitude of the voltage to which C1 charges, before being discharged by the negative pulse,
will be directly proportional to the input pulse width. A longer pulse width allows C1 to charge to a
higher potential than a short pulse. This charge is held, because of the long time constant of R1 and C1,
until the discharge pulse is applied to diodes CR2 and CR3 just prior to the next incoming pulse. These
charges across C1 result in a wave shape similar to the output shown for pam detection in view (C) of
figure 3-21.
Q-34.
In its simplest form, what functions must a radar detector be capable of performing?
Q-35.
What characteristic of a pulse does a peak detector sample?
Q-36.
What is the time constant of the resistor and capacitor in a peak detector for pam?
Q-37.
How can a peak detector for pam be modified to detect pdm?
LOW-PASS FILTER
Another method of demodulating pdm is by the use of a low-pass filter. If the voltage of a pulse
waveform is averaged over both the pulse and no-pulse time, average voltage is the result. Since the
amplitude of pdm pulses is constant, average voltage is directly proportional to pulse width. The pulse
width varies with the modulation (intelligence) in pdm. Because the average value of the pulse train
varies in accordance with the modulation, the intelligence may be extracted by passing the widthmodulated pulses through a low-pass filter. The components of such a filter must be selected so that the
filter passes only the desired modulation frequencies. As the varying-width pulses are applied to the low-
3-24
pass filter, the average voltage across the filter will vary in the same way as the original modulating
voltage. This varying voltage will closely approximate the original modulating voltage.
CONVERSION
Pulse-position modulation (ppm), pulse-frequency modulation (pfm), and pulse-code modulation
(pcm) are most easily demodulated by first converting them to either pdm or pam. After conversion these
pulses are demodulated using either peak detection or a low-pass filter. This conversion may be done in
many ways, but your study will be limited to the simpler methods.
Pulse-Position Modulation
Ppm can be converted to pdm by using a flip-flop circuit. (Flip flops were discussed in NEETS,
Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits.) Figure 3-22 shows the
waveforms for conversion of ppm to pdm. View (A) is the pulse-modulated pulse train and view (B) is a
series of reset trigger pulses. The trigger pulses must be synchronized with the unmodulated position of
the ppm pulses, but with a fixed time delay from these pulses. As the position-modulated pulse is applied
to the flip-flop, the output is driven positive, as shown in view (C). After a period of time, the trigger
pulse is again generated and drives the flip-flop output negative and the pulse ends. Because the ppm
pulses are constantly varying in position with reference to the unmodulated pulses, the output of the flipflop also varies in duration or width. This pdm signal can now be applied to one of the circuits that has
already been discussed for demodulation.
Figure 3-22.—Conversion of ppm to pdm.
Pulse-frequency modulation is a variation of ppm and may be converted by the same method.
3-25
Pulse-Code Modulation
Pulse-code modulation can easily be decoded, provided the pulse-code groups have been transmitted
in reverse order; that is, if the pulse with the lowest value is transmitted first, the pulse with the highest
value is transmitted last. A circuit that will provide a constant value of current without regard to its load is
known as a current source. A current source is used to apply the pcm pulses to an RC circuit, such as that
shown in figure 3-23, view (A). The current source must be capable of supplying a linear charge to C1
that will increase each time a pulse is applied if C1 is not allowed to discharge between pulses. In other
words, if C1 charges to 16 volts during the period of one pulse, then each additional pulse increases the
charge by 16 volts. Thus, the cumulative value increases by 16 volts for each received pulse. This does
not provide a usable output unless a resistor is chosen that allows C1 to discharge to one-half its value
between pulses. If only one pulse is received at T1, C1 charges to 16 volts and then begins to discharge.
At T2 the charge has decayed to 8 volts and continues to decay unless another pulse is received. At T3 it
has a 4-volt charge and at T4 it only has a 2-volt charge. At the sampling time, a 1-volt charge remains;
this charge corresponds to the binary-weighted pulse train of 0001. Now we will apply a pcm signal
which corresponds to the binary-coded equivalent of 7 volts (0111) in figure 3-23, view (A). View (B) is
the pulse code that is received. Remember that the pulses are transmitted in reverse order. View (C) is the
response curve of the circuit. At T1 the pulse corresponding to the least significant digit is applied and C1
charges to 16 volts. C1 discharges between pulses until it reaches 8 volts at T2. At T2 another pulse
charges it to 24 volts. At T3, C1 has discharged through R1 to a value of 12 volts. The pulse at T3
increases the charge on C1 by 16 volts to a total charge of 28 volts. At T4, C1 has discharged to one-half
its value and is at 14 volts. No pulse is present at T4 so C1 will not receive an additional charge. C1
continues to discharge until T5 when it has reached 7 volts and is sampled to provide a pam pulse which
can be peak detected. This sampled output corresponds to the original sampling of the analog voltage in
the modulation.
3-26
Figure 3-23.—Pcm conversion.
When the pcm demodulator recognizes the presence or absence of pulses in each position, it
reproduces the correct standard amplitude represented by the pulse code group. For this reason, noise
introduces no error if the largest peaks of noise are not mistaken for pulses. The pcm signal can be
retransmitted as many times as desired without the introduction of additional noise effects so long as the
signal-to-noise ratio is maintained at a level where noise pulses are not mistaken for a signal pulse. This is
not the only method for demodulating pcm, but it is one of the simplest.
This completes your study of demodulation. You should remember that this module has been a basic
introduction to the principles of modulation and demodulation. With the advent of solid-state electronics,
integrated circuits have replaced discrete components. Although you cannot trace the signal flow through
3-27
these circuits, the end result of the electronic action within the integrated circuit is the same as it would be
with discrete components.
Q-38.
How does a low-pass filter detect pdm?
Q-39.
How is conversion used in pulse demodulation?
Q-40.
What is the discharge rate for the capacitor in a pcm converter?
SUMMARY
Now that you have completed this chapter, a short review of what you have learned is in order. The
following summary will refresh your memory of demodulation, its basic principles, and typical circuitry
required to accomplish this task.
DEMODULATION, also called DETECTION, is the process of re-creating original modulating
frequencies (intelligence) from radio frequencies.
The DEMODULATOR, or DETECTOR, is the circuit in which the original modulating frequencies
are restored.
A CW DEMODULATOR is a circuit that is capable of detecting the presence of rf energy.
HETERODYNE DETECTION uses a locally generated frequency to beat with the cw carrier
frequency to provide an audio output.
3-28
The REGENERATIVE DETECTOR produces its own oscillations, heterodynes them with an
incoming signal, and detects them.
The SERIES- (VOLTAGE-) DIODE DETECTOR has a rectifier diode that is in series with the
input voltage and the load impedance.
3-29
SHUNT- (CURRENT-) DIODE DETECTOR is characterized by a rectifier diode in parallel with
the input and load impedance.
The COMMON-EMITTER DETECTOR is usually used in receivers to supply a detected and
amplified output.
3-30
The COMMON-BASE DETECTOR is an amplifying detector that is used in portable receivers.
The SLOPE DETECTOR is the simplest form of frequency detector. It is essentially a tank circuit
tuned slightly away from the desired fm carrier.
The FOSTER-SEELEY DISCRIMINATOR uses a double tuned rf transformer to convert
frequency changes of the received fm signal into amplitude variations of the rf wave.
The RATIO DETECTOR uses a double-tuned transformer connected so that the instantaneous
frequency variations of the fm input signal are converted into instantaneous amplitude variations.
3-31
The GATED-BEAM DETECTOR uses a specially-designed tube to limit, detect, and amplify the
received fm signal.
PHASE DEMODULATION may be accomplished using a frequency discriminator or a quadrature
detector.
3-32
PEAK DETECTION uses the amplitude, or duration, of a pulse to charge a holding capacitor and
restore the modulating waveform.
A LOW-PASS FILTER is used to demodulate pdm by averaging the pulse amplitude over the
entire period between pulses.
PULSE CONVERSION is used to convert ppm, pdm, or pcm to pdm or pam for demodulation.
ANSWERS TO QUESTIONS Q1. THROUGH Q40.
A-1. Re-creating original modulating frequencies (intelligence) from radio frequencies.
A-2. Circuit in which intelligence restoration is achieved.
A-3. A circuit that can detect the presence or absence of rf energy.
A-4. An antenna, tank circuit for tuning, rectifier for detection, filter to give constant output, and an
indicator device.
A-5. Heterodyning.
A-6. By giving a different beat frequency for each signal.
A-7. Regenerative detector.
A-8. Oscillator, mixer, and detector.
A-9. (1) Sensitive to the type of modulation applied, (2) nonlinear, and (3) provide filtering.
A-10. The modulation envelope.
A-11. Rectifies the rf pulses in the received signal.
3-33
A-12. To filter the rf pulses and develop the modulating wave (intelligence) from the modulation
envelope.
A-13. The current-diode detector is in parallel with the input and load.
A-14. When the input voltage variations are too small to give a usable output from a series detector.
A-15. Emitter-base junction.
A-16. R1.
A-17. By the collector current flow through R4.
A-18. Emitter-base junction.
A-19. A diode detector followed by a stage of audio amplification.
A-20. C1 and R1.
A-21. Slope detector.
A-22. Converting frequency variations of received fm signals to amplitude variations.
A-23. A double-tuned tank circuit.
A-24. Rectify the rf voltage from the discriminator.
A-25. Inductive.
A-26. Suppresses amplitude noise without limiter stages.
A-27. It helps to maintain a constant circuit voltage to prevent noise fluctuations from interfering with
the output.
A-28. Limits, detects, and amplifies.
A-29. Both grids must be positively biased.
A-30. Extreme simplicity, few components, and ease of adjustment.
A-31. In the amount and rate of phase shift of the carrier wave.
A-32. Because of the incidental frequency shift that is caused while phase-shifting a carrier wave that is
similar to fm modulation.
A-33. The quadrature grid signal is excited by a reference from the transmitter.
A-34. Detecting the presence of rf energy.
A-35. Pulse amplitude or pulse duration.
A-36. At least 10 times the interpulse period.
A-37. By making the time constant for charging the capacitor at least 10 times the maximum received
pulse width.
3-34
A-38. By averaging the value of the pulses over the period of the pulse-repetition rate.
A-39. Ppm, pfm, and pcm are converted to either pdm or pam for demodulation.
A-40. It will discharge to one-half its value between pulses.
3-35
APPENDIX I
GLOSSARY
AMPLITUDE—Used to represent values of electrical current or voltage. The greater its height,
the greater the value it represents.
AMPLITUDE MODULATION—Any method of varying the amplitude of an electromagnetic
carrier frequency in accordance with the intelligence to be transmitted
ANGLE MODULATION—Modulation in which the angle of a sine-wave carrier is varied by a
modulating wave.
AVERAGE POWER—The peak power value averaged over the pulse-repetition time.
BANDWIDTH—The section of the frequency spectrum that specific signals occupy.
BASE-INJECTION MODULATOR—Similar to control-grid modulator. Gain of a transistor is
varied by changing the bias on its base.
BLOCKED-GRID KEYING—A method of keying in which the bias is varied to turn plate
current on and off.
BUFFER—A voltage amplifier used between the oscillator and power amplifier.
CARBON MICROPHONE—Microphone in which sound waves vary the resistance of a pile of
carbon granules. May be single-button or double-button.
CARRIER FREQUENCY—The assigned transmitter frequency.
CARRIER—Radio-frequency sine wave.
CATHODE KEYING—A system in which the cathode circuit is interrupted so that neither grid
current nor plate current can flow.
CATHODE MODULATOR—Voltage on the cathode is varied to produce the modulation
envelope.
CHANNEL—Carrier frequency assignment usually with a fixed bandwidth.
COLLECTOR-INJECTION MODULATOR—Transistor equivalent of plate modulator.
Modulating voltage is applied to collector circuit.
COMMON-BASE DETECTOR—An amplifying detector where detection occurs in
emitter-base junction and amplification occurs at the output of the collector junction.
COMMON-EMITTER DETECTOR—Often used in receivers to supply detected and
amplified output. The emitter-base junction acts as the detector.
COMPLEX WAVE—A wave composed of two or more parts.
CONTINUOUS-WAVE KEYING—The "on-off" keying of a carrier.
AI-1
CONTROL-GRID MODULATOR—Uses a variation of grid bias to vary the instantaneous
plate voltage and current. The modulating signal is applied to the control grid.
CONVERSION—The process of changing ppm or pcm to pdm or pam to make them easier to
demodulate.
CRYSTAL MICROPHONE—Uses the piezo-electric effect of crystalline materials to generate
a voltage from sound waves.
CUSPS—Sharp phase reversals.
CW DEMODULATOR—A circuit that detects the presence of rf oscillations and converts them
into a useful form.
CYCLE—360 degree rotation of a vector generating a sine wave.
DEMODULATION—The removal of intelligence from a transmission medium.
DEMODULATION or DETECTION—The process of re-creating original modulatingfrequency intelligence from the rf carrier.
DEMODULATOR or DETECTOR—A circuit in which demodulation or restoration of the
original intelligence is achieved.
DIODE DETECTOR—A simple type of crystal receiver.
DUTY CYCLE—The ratio of working time to total time for intermittently operated devices.
DYNAMIC MICROPHONE—A device in which sound waves move a coil of fine wire that is
mounted on the back of a diaphragm and located in the magnetic field of a permanent
magnet.
EMITTER-INJECTION MODULATOR—The transistor equivalent of the cathode modulator.
The gain is varied by changing the voltage on the emitter.
FIDELITY—The ability to faithfully re-produce the input in the output.
FINAL POWER AMPLIFIER (fpa)—The final stage of amplification in a transmitter.
FIXED SPARK GAP—A device used to discharge the pulse-forming network. A trigger pulse
ionizes the air between two contacts to initiate the discharge.
FOSTER-SEELEY DISCRIMINATOR—A circuit that uses a double-tuned rf transformer to
convert frequency variations in the received fm signal to amplitude variations. Also known
as a phase-shift discriminator.
FREQUENCY DEVIATION—The amount the frequency departs from the carrier frequency.
FREQUENCY MODULATION (fm)—Angle modulation in which the modulating signal
causes the carrier frequency to vary. The amplitude of the modulating signal determines how
far the frequency changes and the frequency of the modulating signal determines how fast
the frequency changes.
FREQUENCY MULTIPLIERS—Special rf power amplifiers that multiply the input frequency.
AI-2
FREQUENCY—The rate at which the vector that generates a sine wave rotates.
FREQUENCY-SHIFT KEYING (fsk)—Frequency modulation somewhat similar to
continuous-wave (cw) keying in AM transmitters. The carrier is shifted between two
differing frequencies by opening and closing a key.
GATED-BEAM DETECTOR—An fm demodulator that uses a special gated-beam tube to
limit, detect, and amplify the received fm signal. Also known as a quadrature detector.
HARMONIC FREQUENCIES—Integral multiples of a fundamental frequency
HETERODYNE DETECTION—The use of an af voltage to distinguish between available
signals. The incoming cw signal is mixed with locally generated oscillations to give an af
output.
HETERODYNING—Mixing two frequencies across a nonlinear impedance.
HIGH-LEVEL MODULATION—Modulation produced in the plate circuit of the last radio
stage of the system.
INSTANTANEOUS AMPLITUDE—The amplitude at any given point along a sine wave at a
specific instant in time.
INTERMEDIATE POWER AMPLIFIER (ipa)—The amplifier between the oscillator and
final power amplifier.
KEY CLICKS—Interference in the form of "clicks" or "thumps" caused by the sudden
application or removal of power.
KEY-CLICK FILTERS—Filters used in keying systems to prevent key-click interference.
KEYING RELAYS—Relays used in high- power transmitters where the ordinary hand key
cannot accommodate the plate current without excessive arcing.
LINEAR IMPEDANCE—An impedance in which a change in current through a device changes
in direct proportion to the voltage applied to the device.
LOW-LEVEL MODULATION—Modulation produced in an earlier stage than the final.
LOW-PASS FILTER—A method of demodulating pdm by averaging the voltage over pulse and
no-pulse time.
LOWER SIDEBAND—All difference frequencies below that of the carrier.
MACHINE KEYING—A method of cw keying using punched tape or other mechanical means
to key a transmitter.
MAGNETIC MICROPHONE—A microphone in which the sound waves vibrate a moving
armature. The armature consists of a coil wound on the armature and located between the
pole pieces of a permanent magnet. The armature is mechanically linked to the diaphragm.
MARK—An interval during which a signal is present. Also the presence of an rf signal in cw
keying. The key-closed condition (presence of data) in communications systems.
AI-3
MASTER OSCILLATOR POWER AMPLIFIER (MOPA)—A transmitter in which the
oscillator is isolated from the antenna by a power amplifier.
MICROPHONE—An energy converter that changes sound energy into electrical energy.
MODULATED WAVE—A complex wave consisting of a carrier and a modulating wave that is
transmitted through space.
MODULATING WAVE—An information wave representing intelligence.
MODULATION FACTOR (M)—An indication of relative magnitudes of the rf carrier and the
audio-modulating signal.
MODULATION INDEX—The ratio of frequency deviation to the frequency of the modulating
signal.
MODULATION—The ability to impress intelligence upon a transmission medium, such as radio
waves.
MODULATOR—The last audio stage in which intelligence is applied to the rf stage to modulate
the carrier.
MULTIPLICATION FACTOR—The number of times an input frequency is multiplied.
MULTIVIBRATOR MODULATOR—An astable multivibrator used to provide frequency
modulation by inserting the modulating af voltage in series with the base-return of the
multivibrator transistors.
NEGATIVE ALTERNATION—That part of a sine wave that is below the reference level.
NONLINEAR DEVICE—A device in which the output does not rise and fall directly with the
input.
NONLINEAR IMPEDANCE—An impedance in which the resulting current through the device
is not proportional to the applied voltage.
OVERMODULATION—A condition that exists when the peaks of the modulating signal are
limited.
PEAK AMPLITUDE—The maximum value above or below the reference line.
PEAK DETECTION—Detection that uses the amplitude of pam or the duration of pdm to
charge a holding capacitor and restore the original waveform.
PEAK POWER—The maximum value of the transmitted pulse.
PERCENT OF MODULATION—The degree of modulation defined in terms of the maximum
permissible amount of modulation.
PERIOD—The duration of a waveform.
PHASE MODULATION (pm)—Angle modulation in which the phase of the carrier is
controlled by the modulating waveform. The amplitude of the modulating wave determines
the amount of phase shift and the frequency of the modulation determines how often the
phase shifts.
AI-4
PHASE or PHASE ANGLE—The angle that exists between the starting point of a vector and its
position at that instant. An indication of how much of a cycle has been completed at any
given instant in time.
PHASE-SHIFT DISCRIMINATOR—See Foster-Seeley discriminator.
PHASE-SHIFT KEYING—Similar to ON-OFF cw keying in AM systems and frequency-shift
keying in fm systems. Each time a mark is received, the phase is reversed. No phase reversal
takes place when a space is received.
PLATE KEYING—A keying system in which the plate supply is interrupted.
PLATE MODULATOR—An electron-tube modulator in which the modulating voltage is
applied to the plate circuit of the tube.
POSITIVE ALTERNATION—That part of a sine wave that is above the reference line.
PULSE DURATION (pd)—The period of time during which a pulse is present.
PULSE MODULATION—A form of modulation in which one of the characteristics of a pulse
train is varied.
PULSE WIDTH (pw)—The period of time during which a pulse occurs.
PULSE—A surge of plate current that occurs when a tube is momentarily saturated.
PULSE-AMPLITUDE MODULATION (pam)—Pulse modulation in which the amplitude of
the pulses is varied by the modulating signal.
PULSE-CODE MODULATION (pcm)—A modulation system in which the standard values of
a quantized wave are indicated by a series of coded pulses.
PULSE-DURATION MODULATION (pdm)—Pulse modulation in which the time duration
of the pulses is changed by the modulating signal.
PULSE-FORMING NETWORK (pfn)—A circuit used for storing energy. Essentially a short
section of artificial transmission line.
PULSE-FREQUENCY MODULATION (pfm)—Pulse modulation in which the modulating
voltage varies the repetition rate of a pulse train.
PULSE-POSITION MODULATION (ppm)—Pulse modulation in which the position of the
pulses is varied by the modulating voltage.
PULSE-REPETITION FREQUENCY—The rate, in pulses per second, at which the pulses
occur.
PULSE-REPETITION TIME (prt)—The total time for one complete pulse cycle of operation
(rest time plus pulse width).
PULSE-TIME MODULATION (ptm)—Pulse modulation that varies one of the time
characteristics of a pulse train (pwm, pdm, ppm, and pfm).
PULSE-WIDTH MODULATION (pwm)—Pulse modulation in which the duration of the
pulses is varied by the modulating voltage.
AI-5
PULSING—Allowing oscillations to occur for a specific period of time only during selected
intervals.
QUANTIZED WAVE—A wave created by arbitrarily dividing the entire range of amplitude (or
frequency, or phase) values of an analog wave into a series of standard values. Each sample
takes the standard value nearest its actual value when modulated.
QUANTIZING NOISE—A distortion introduced by quantizing the signal.
RADAR DETECTOR—A detector which, in its simplest form, only needs to be capable of
producing an output when rf energy (reflected from a target) is present at its input.
RATIO DETECTOR—A detector that uses a double-tuned transformer to convert the
instantaneous frequency variations of the fm input signal to instantaneous amplitude
variations.
RATIO OF TRANSMITTED POWERS—The power ratio (fsk verses AM) that expresses the
overall improvement of fsk transmission when compared to AM under rapid-fading and
high-noise conditions.
REACTANCE TUBE—A tube connected in parallel with the tank circuit of an oscillator.
Provides a signal that will either lag or lead the signal produced by the tank.
REACTANCE-TUBE MODULATOR—An fm modulator that uses a reactance tube in parallel
with the oscillator tank circuit.
REGENERATIVE DETECTOR—A detector circuit that produces its own oscillations,
heterodynes them with an incoming signal, and detects them.
REST FREQUENCY—The carrier frequency during the constant-amplitude portions of a phase
modulation signal.
REST TIME (rt)—The time when there is no pulse or nonpulse.
ROTARY GAP—Similar to a mechanically driven switch. Used to discharge a pulse-forming
network.
SENSITIVITY OF A MICROPHONE—Efficiency of a microphone. Describes micro- phone
power delivered to a matched-impedance load as compared to the sound level being
converted. Usually expressed in terms of the electrical power level.
SERIES-DIODE DETECTOR—The semiconductor diode in series with the input voltage and
the load impedance. Sometimes called a voltage-diode detector.
SHUNT—Means the same as parallel or to place in parallel with other components.
SHUNT-DIODE DETECTOR—A diode detector in which the diode is in parallel with the input
voltage and the load impedance. Also known as a current detector because it operates with
smaller input levels.
SIGNAL DISTORTION—Any unwanted change to the signal.
SIGNIFICANT SIDEBANDS—Those sidebands with significantly large amplitude.
AI-6
SINE WAVE—The basic synchronous alternating waveform for all complex waveforms.
SLOPE DETECTOR—A tank circuit tuned to a frequency, either slightly above or below an fm
carrier frequency, that is used to detect intelligence.
SPACE—Absence of an rf signal in cw keying. Key-open condition or lack of data in
communications systems. Also a period of no signal.
SPARK-GAP MODULATOR—Modulator consists of a circuit for storing energy, a circuit for
rapidly discharging the storage circuit (spark gap), a pulse transformer, and a power source.
SPECTRUM ANALYSIS—The display of electromagnetic energy arranged according to
wavelength or frequency.
SPLATTER—Unwanted sideband frequencies that are generated from overmodulation.
THYRATRON MODULATOR—An electronic switch that requires a low potential to turn it
on.
TRANSMISSION MEDIUM—A means of transferring intelligence from point to point. Can be
described as light, smoke, sound, wirelines, or radio-frequency waves.
UPPER SIDEBAND—All of the sum frequencies above the carrier.
VARACTOR—A diode, or pn junction, that is designed to have a certain amount of capacitance
between junctions.
VARACTOR FM MODULATOR—An fm modulator which uses avoltage-variable
capacitordiode (varactor).
VOLTAGE-DIODE DETECTOR—Series detector in which the crystal is in series with the
input voltage and the load impedance.
VECTOR—Mathematical method of showing both magnitude and direction.
WAVELENGTH—The physical dimension of a sine wave.
AI-7
MODULE 12 INDEX
A
Ac applied to linear and nonlinear impedances,
1-18
AM demodulation, 3-6 to 3-10
AM transmitter principles, 1-42
Amplitude, 1-7
Amplitude modulation, 1-1 to 1-62
amplitude-modulated systems, 1-26
AM transmitter principles, 1-42
amplitude modulation, 1-36
continuous wave (cw), 1-26
modulation systems, 1-54
heterodyning, 1-10
ac applied to linear and nonlinear
impedances, 1-18
combined linear and nonlinear
impedance, 1-17
linear impedance, 1-11
nonlinear impedance, 1-16
spectrum analysis, 1-25
two sine-wave generators and a
combination of linear and nonlinear
impedances, 1-22
two sine-wave generators in linear
circuits, 1-20
typical heterodyning circuit, 1-25
introduction to modulation principles, 1-1
sine-wave characteristics, 1-2
amplitude, 1-7
frequency, 1-7
generation of sine waves, 1-3
period, 1-7
phase, 1-7
wavelength, 1-8
summary, 1-62
Angle and pulse modulation, 2-1
angle modulation, 2-2
basic modulator, 2-25
frequency-modulation systems, 2-2
frequency-shift keying, 2-2
modulation index, 2-23
phase modulation, 2-21
phase-shift keying, 2-26
Angle and pulse modulation—Continued
introduction, 2-1
pulse modulation, 2-29
characteristics, 2-29
communications pulse modulators,
2-40
pulse timing, 2-32
radar modulation, 2-38
spark-gap modulator, 2-38
thyratron modulator, 2-39
summary, 2-52
B
Basic modulator, 2-25
C
Characteristics, pulse modulation, 2-29
Combined linear and nonlinear impedance,
1-17
Common-base detector, 3-10
Common-emitter detectors, 3-9
Communications pulse modulators, 2-40
Continuous wave (cw), 1-26
Continuous-wave demodulation, 3-2
Conversion, 3-25
D
Demodulation, 3-1
AM demodulation, 3-6
common-base detector, 3-10
common-emitter detectors, 3-9
diode detectors, 3-6
continuous-wave demodulation, 3-2
heterodyne detection, 3-3
regenerative detector, 3-5
fm demodulation, 3-10
Foster-Seeley discriminator, 3-12
gated-beam detector, 3-17
ratio detector, 3-15
slope detection, 3-11
introduction, 3-1
INDEX-1
Demodulation—Continued
phase demodulation, 3-21
pulse demodulation, 3-23
conversion, 3-25
low-pass filter, 3-24
peak detection, 3-23
summary, 3-28
Diode detectors, 3-6
F
Fm demodulation, 3-10
Foster-Seeley discriminator, 3-12
Frequency, 1-7
Frequency-modulation systems, 2-2
Frequency-shift keying, 2-2
G
Gated-beam detector, 3-17
Generation of sine waves, 1-3
Glossary, AI-1 to AI-7
H
Heterodyne detection, 3-3
Heterodyning, 1-10
L
Learning objectives, 1-1, 2-1, 3-1
Linear impedance, 1-11
Low-pass filter, 3-24
M
Modulation index, 2-23
Modulation principles, introduction to, 1-1
Modulation systems, 1-54
N
Nonlinear impedance, 1-16
P
Peak detection, 3-23
Period, 1-7
Phase, 1-7
Phase demodulation, 3-21
Phase modulation, 2-21
Phase-shift keying, 2-26
Pulse and angle modulation, 2-1
Pulse demodulation, 3-23
R
Radar modulation, 2-38
Ratio detector, 3-15
Regenerative detector, 3-5
S
Sine-wave characteristics, 1-2
Slope detection, 3-11
Spark-gap modulator, 2-38
Spectrum analysis, 1-25
T
Thyratron modulator, 2-39
Timing, pulse, 2-32
Two sine-wave generators and a combination
of linear and nonlinear impedances, 1-22
Two sine-wave generators, in linear circuits,
1-20
Typical heterodyning circuit, 1-25
W
Wavelength, 1-8
INDEX-2
Assignment Questions
Information: The text pages that you are to study are
provided at the beginning of the assignment questions.
ASSIGNMENT 1
Textbook assignment: Chapter 1, “Amplitude Modulation,” pages 1-1 through 1-75.
_________________________________________________________________________________
1-5. A rotating coil in the uniform magnetic
field between two magnets produces a
sine wave. It is called a sine wave
because the voltage depends on which of
the following factors?
1-1. The action of impressing intelligence
upon a transmission medium is referred
to as
1.
2.
3.
4.
modulating
demodulating
heterodyning
wave generating
1. The number of turns in the coil
2. The speed at which the coil is rotating
3. The angular position of the coil in the
magnetic field
4. Each of the above
1-2. You can communicate with others using
which of the following transmissions
mediums?
1.
2.
3.
4.
1-6. The trigonometric relationship for the
sine of an angle in a right triangle is
figured using which of the following
ratios?
Light
Wire lines
Radio waves
Each of the above
1.
2.
3.
4.
1-3. When you use a vector to indicate force
in a diagram, what do (a) length and
(b) arrowhead position indicate?
1.
2.
3.
4.
(a) Magnitude
(a) Magnitude
(a) Phase
(a) Phase
(b) direction
(b) frequency
(b) frequency
(b) direction
1-7. The part of a sine wave that is above the
voltage reference line is referred to as the
1.
2.
3.
4.
1-4. Vectors are used to show which of the
following characteristics of a sine wave?
1.
2.
3.
4.
Opposite side ÷ hypotenuse
Adjacent side ÷ hypotenuse
Hypotenuse ÷ opposite side
Hypotenuse ÷ adjacent side
Fidelity
Amplitude
Resonance
Distortion
peak amplitude
positive alternation
negative alternation
instantaneous amplitude
1-8. The degree to which a cycle has been
completed at any given instant is referred
to as the
1.
2.
3.
4.
1
phase
period
frequency
amplitude
1-14. What is the wavelength of a 1.5 MHz
frequency?
1-9. The frequency of the sine wave is
determined by which of the following
sine-wave factors?
1.
2.
3.
4.
1. The maximum voltage
2. The rate at which the vector rotates
3. The number of degrees of vector
rotation
4. Each of the above
1-15. As the frequency of a signal is increased,
what change can be noted about its
wavelength?
1-10. Which of the following mathematical
relationships do you use to figure the
period of a sine wave?
1.
1. It decreases
2. It increases
3. It remains the same
3.
2.
1-16. The ability of a circuit to faithfully
reproduce the input signal in the output is
known by what term?
4.
1.
2.
3.
4.
1-11. Which of the following Greek letters is
the symbol for wavelength?
1.
2.
3.
4.
θ
ϑ
λ
ω
Fidelity
Fluctuation
Directivity
Discrimination
1-17. In rf communications, modulation
impresses information on which of the
following types of waves?
1.
2.
3.
4.
1-12. Which of the following waveform
characteristics determines the wavelength
of a sine wave?
1.
2.
3.
4.
100 meters
200 meters
300 meters
400 meters
Phase
Period
Amplitude
Phase Angle
Carrier wave
Complex wave
Modulated wave
Modulating wave
1-18. Which of the following types of
modulation is a form of amplitude
modulation?
1.
2.
3.
4.
1-13. An electromagnetic wavefront moves
through free space at approximately what
speed in meters per second?
1.
3,000,000
2.
30,000,000
3. 300,000,000
4. 3,000,000,000
2
Angle
Phase
Frequency
Continuous-wave
1-24. What is the purpose of the key in a cw
transmitter?
1-19. With a sine-wave input, how will the
output compare to the input in (a) a linear
circuit and (b) a nonlinear circuit?
1.
2.
3.
4.
1. (a) Proportional
(b) proportional
2. (a) Proportional
(b) not proportional
3. (a) Not proportional
(b) not proportional
4. (a) Not proportional
(b) proportional
1-25. To ensure frequency stability in a cw
transmitter, you should NOT key what
circuit?
1.
2.
3.
4.
1-20. What effect, if any, does a nonlinear
device have on a sine wave?
1.
2.
3.
4.
It amplifies without distortion
It attenuates without distortion
It generates harmonic frequencies
None
(a) Two
(a) Two
(a) Three
(a) Three
1.
2.
3.
4.
1.
2.
3.
4.
Phase
Bandwidth
Modulating wave
Modulation envelope
Power filter
On-off filter
Key-click filter
Rf detector filter
1-28. Transmitter machine keying was
developed for which of the following
purposes?
1-23. The method of rf communication that
uses either the presence or absence of a
carrier in a prearranged code is what type
of modulation?
1.
2.
3.
4.
A coil
A relay
A resistor
A capacitor
1-27. Interference detected by a receiver is
often caused by the application and
removal of power in nearby transmitters.
This interference can be prevented by
using what type of circuit in such
transmitters?
(b) linear
(b) nonlinear
(b) nonlinear
(b) linear
1-22. Spectrum analysis is used to view which
of the following characteristics of an rf
signal?
1.
2.
3.
4.
The mixer
The detector
The oscillator
The rf amplifier
1-26. When keying a high-power transmitter,
what component should you use to
reduce the shock hazard?
1-21. For the heterodyning action to occur in a
circuit, (a) what number of frequencies
must be present and (b) to what type of
circuit must they be applied?
1.
2.
3.
4.
It generates the rf oscillations
It heterodynes the rf oscillations
It controls the rf output
It amplifies the rf signal
1. To increase the speed of
communications
2. To make communications more
intelligible
3. To reduce interference
4. Each of the above
Pulse modulation
Amplitude modulation
Continuous-wave modulation
Pulse-time modulation
3
1-34. Which of the following is the schematic
symbol for a microphone?
1-29. Which of the following advantages is a
benefit of cw communications?
1.
2.
3.
4.
Wide bandwidth
Fast transmission
Long-range operation
Each of the above
1-30. To prevent a transmitter from being
loaded unnecessarily, where should you
connect the antenna?
1.
2.
3.
4.
At the oscillator input
At the oscillator output
At the power-amplifier input
At the power-amplifier output
4.
Button
Diaphragm
Transformer
Carbon granules
1-36. The action of the double-button carbon
microphone is similar to which of the
following electronic circuits?
To increase power
To increase frequency
To increase stability
To increase selectivity
1.
2.
3.
4.
A limiter
An oscillator
A voltage doulber
A push-pull amlifier
1-37. A carbon microphone has which of the
following advantages over other types of
microphones?
36, 36
4, 3, 3, 2
4, 4, 3, 2
18, 18, 18, 18
1.
2.
3.
4.
1-33. To change sound energy into electrical
energy, which of the following devices
should you use?
1.
2.
3.
4.
2.
1.
2.
3.
4.
1-32. Which of the following combinations of
frequency multiplier stages will produce
a total multiplication factor of 72?
1.
2.
3.
4.
3.
1-35. What component in a carbon microphone
converts a dc voltage into a varying
current?
1-31. Amplifier tubes are added to the output
of a transmitter for which of the
following reasons?
1.
2.
3.
4.
1.
Ruggedness
Sensitivity
Low output voltage
Frequency response
1-38. The voltage produced by mechanical
stress placed on certain crystals is a result
of which of the following effects?
A speaker
A microphone
An amplifier
An oscillator
1.
2.
3.
4.
4
Hall
Acoustic
Electrostatic
Piezoelectric
1-43. The final audio stage in an AM
transmitter is the
1-39. If you require a microphone that is
lightweight, has high sensitivity, is
rugged, requires no external voltage, can
withstand temperature, vibration, and
moisture extremes, and has a uniform
frequency response of 40 to 15,000 hertz,
which of the following types of
microphones should you select?
1.
2.
3.
4.
1.
2.
3.
4.
1-44. The vertical axis on a frequencyspectrum graph represents which of the
following waveform characteristics?
Carbon
Crystal
Dynamic
Electrostatic
1.
2.
3.
4.
1-40. What component in a magnetic
microphone causes the lines of flux to
alternate?
1.
2.
3.
4.
mixer
modulator
multipler
multiplexer
Phase
Duration
Frequency
Amplitude
1-45. When a 500-hertz signal modulates a
1-megahertz carrier, the 1-megahertz
carrier and what two other frequencies
are transmitted?
The coil
The magnet
The diaphragm
The armature
1.
500 and 999,500 hertz
2.
500 and 1,000,500 hertz
3. 999,500 and 1,500,000 hertz
4. 999,500 and 1,000,500 hertz
1-41. What are the two major sections of an
AM transmitter?
1. Audio frequency unit and radio
frequency unit
2. Audio frequency unit and master
oscillator
3. Audio frequency unit and final power
amplifier
4. Audio frequency unit and
intermediate power amplifier
1-46. If 750 hertz modulates a 100-kilohertz
carrier, what would the upper-sideband
frequency be?
1. 99,250 hertz
2. 100,000 hertz
3. 100,500 hertz
4. 100,750 hertz
1-42. The intermediate power amplifier serves
what function in a transmitter?
1-47. In an AM wave, where is the audio
intelligence located?
1. It generates the carrier
2. It modulates the carrier
3. It increases the frequency of the
signal
4. It increases the power level of the
signal
1. In the carrier frequency
2. In the spacing between the sideband
frequencies
3. In the spacing between the carrier and
sideband frequencies
4. In the sideband frequencies
5
1-52. In an AM signal that is 100 percent
modulated, what maximum voltage value
is present in each sideband?
1-48. What determines the bandwidth of an
AM wave?
1.
2.
3.
4.
The carrier frequency
The number of sideband frequencies
The lowest modulating frequency
The highest modulating frequency
1.
2.
3.
4.
1-49. If an 860-kilohertz AM signal is
modulated by frequencies of 5 and 10
kilohertz, what is the bandwidth?
1-53. Overmodulation of an AM signal will
have which, if any, of the following
effects on the bandwidth?
1. 5 kilohertz
2. 10 kilohertz
3. 15 kilohertz
4. 20 kilohertz
1. It will increase
2. It will decrease
3. It will remain the same
1-54. In a carrier wave with a peak amplitude
of 400 volts and a peak modulating
voltage of 100 volts, what is the
modulation factor?
1-50. If a 1-megahertz signal is modulated by
frequencies of 50 and 75 kilohertz, what
is the resulting maximum frequency
range?
1.
2.
3.
4.
1. 925,000 to 1,000,000 hertz
2. 925,000 to 1,075,000 hertz
3. 975,000 to 1,025,000 hertz
4. 1,000,000 to 1,075,000 hertz
2.
3.
(a)
(b)
(a)
(b)
(a)
(b)
4.
(a)
(b)
0.15
0.25
0.45
0.55
1-55. The percent of modulation for a
modulated carrier wave is figured using
which of the following formulas?
1-51. If an rf carrier is 100 percent AMmodulated, what will be the rf output
when the modulating signal is (a) at its
negative peak and (b) at its positive
peak?
1.
1/4 the carrier voltage
1/2 the carrier voltage
3/4 the carrier voltage
Same as the carrier voltage
0
2 times the amplitude of the
unmodulated carrier
0
1/2 the amplitude of the
unmodulated carrier
1/2 the amplitude of the
unmodulated carrier
1/2 the amplitude of the
unmodulated carrier
1/2 the amplitude of the
unmodulated carrier
2 times the amplitude of the
unmodulated carrier
1.
3.
2.
4.
1-56. Modulation produced in the plate circuit
of the last radio stage of a system is
known by what term?
1.
2.
3.
4.
6
Low-level modulation
High-level modulation
Final-amplifier modulation
Radio frequency modulation
1-62. In a plate modulator, with no modulation,
how will the plate current of the final rf
amplifier appear on a scope?
1-57. Which, if any, of the following
advantages is a primary benefit of plate
modulation?
1.
2.
3.
4.
1. A series of pulses at the carrier
frequency
2. A series of pulses at twice the carrier
frequency
3. A series of pulses at 1/4 the carrier
frequency
4. A series of pulses at 1/2 the carrier
frequency
It operates at low efficiency
It operates at low power levels
It operates with high efficiency
None of the above
1-58. A final rf power amplifier biased for
plate modulation operates in what class
of operation?
1.
2.
3.
4.
1-63. In the collector-injection modulator, af
and rf are heterodyned by injecting the rf
into (a) what circuit and the af into
(b) what circuit?
A
B
AB
C
1.
2.
3.
4.
1-59. Heterodyning action in a plate modulator
takes place in what circuit?
1.
2.
3.
4.
Grid
Plate
Screen
Cathode
1. The rf amplifier stages can be
operated class C for linearity
2. The rf amplifier stages can be
operated class C for maximum
efficiency
3. They require small amounts of audio
power
4. They require large amounts of audio
power
Plate voltage
Cathode voltage
Grid-bias voltage
Grid-input voltage
1-61. To achieve 100-percent modulation in a
plate modulator, what maximum voltage
must the modulator tube be capable of
providing to the final power amplifier
(fpa)?
1.
2.
3.
4.
(b) collector
(b) emitter
(b) collector
(b) base
1-64. Plate- and collector-injection modulators
are the most commonly used modulators
for which of the following reasons?
1-60. A plate modulator produces a modulated
rf output by controlling which of the
following voltages?
1.
2.
3.
4.
(a) Base
(a) Base
(a) Emitter
(a) Emitter
1-65. A control-grid modulator would be used
in which of the following situations?
1. In extremely high-power, wideband
equipment where high-level
modulation is difficult to achieve
2. In cases where the use of a minimum
of audio power is desired
3. In portable and mobile equipment to
reduce size and power requirements
4. Each of the above
Twice the fpa plate voltage
The same as the fpa plate voltage
Three times the fpa plate voltage
Half the fpa plate voltage
7
1-66. Which of the following inputs is/are
applied to the grid of a control-grid
modulator?
1.
2.
3.
4.
Rf
Af
Dc bias
Each of the above
1-67. Excessive modulating signal levels have
which, if any, of the following effects on
a control-grid modulator?
1.
2.
3.
4.
Figure 1A.—Modulator circuit.
IN ANSWERING QUESTIONS 1-70
THROUGH 1-72, REFER TO FIGURE 1A.
They increase output. amplitude
They decrease output amplitude
They create distortion
None
1-70. What components in the circuit establish
the bias for Q1?
1. R1 and R2
2. R2 and R3
3. R1 and R3
1-68. Compared to a plate modulator, the
control-grid modulator has which of the
following advantages?
1-71. The rf voltage in the circuit is applied at
(a) what points and the af voltage is
applied at (b) what points?
1. It is more efficient
2. It has less distortion
3. It requires less power from the
modulator
4. It requires less power from the
amplifier
1.
2.
3.
4.
1-69. The control-grid modulator is similar to
which of the following modulator
circuits?
1.
2.
3.
4.
(a) A and B
(a) C and D
(a) C and D
(a) E and F
(b) C and D
(b) A and B
(b) E and F
(b) C and D
1-72. What components develop the rf
modulation envelope?
Plate
Cathode
Base-injection
Emitter-injection
1.
2.
3.
4.
8
C1 and R1
C2 and R1
C3 and R3
C4 and L1
1-73. A cathode modulator is used in which of
the following situations?
1. When rf power is unlimited and
distortion can be tolerated
2. When rf power is limited and
distortion cannot be tolerated
3. When af power is unlimited and
distortion can be tolerated
4. When af power is limited and
distortion cannot be tolerated
1-74. In a cathode modulator, the modulating
voltage is in series with which of the
following voltages?
1.
2.
3.
4.
Figure 1B.—Emitter-injection modulator.
IN ANSWERING QUESTION 1-75, REFER
TO FIGURE 1B.
The grid voltage only
The plate voltage only
Both the grid and plate voltages
The cathode voltage only
1-75. In the circuit, what components develop
the modulation envelope?
1.
2.
3.
4.
9
Q1
C2 and R1
C3 and R3
C4 and L1
ASSIGNMENT 2
Textbook assignment: Chapter 2, “Angle and Pulse Modulation,” pages 2-1 through 2-64.
___________________________________________________________________________________
2-5. Fsk is NOT affected by noise interference
for which of the following reasons?
2-1. Frequency-shift keying resembles what
type of AM modulation?
1.
2.
3.
4.
1. Noise is outside the bandwidth of an
fsk signal
2. Fsk does not rely on the amplitude of
the transmitted signal to carry
intelligence
3. The wide bandwidth of an fsk signal
prevents noise interference
4. Each of the above
CW modulation
Analog AM modulation
Plate modulation
Collector-injection modulation
2-2. Frequency-shift keying is generated using
which of the following methods?
1. By shifting the frequency of an
oscillator at an af rate
2. By shifting the frequency of an
oscillator at an rf rate
3. By keying an af oscillator at an rf rate
4. By keying an af oscillator at an af rate
2-6. In an fsk transmitter, what stage is keyed?
1.
2.
3.
4.
2-3. In a frequency-shift keyed signal, where is
the intelligence contained?
1.
2.
3.
4.
2-7. When the amount of oscillator frequency
shift in an fsk transmitter is determined,
which of the following factors must be
considered?
In the duration of the rf energy
In the frequency of the rf energy
In the amplitude of the rf energy
In the spacing between bursts of rf
energy
1. The number of buffer amplifiers
2. The transmitter power output
3. The frequency multiplication factor for
the transmitter amplifiers
4. The oscillator rest frequency
2-4. If an fsk transmitter has a MARK
frequency of 49.575 kilohertz and a
SPACE frequency of 50.425 kilohertz,
what is the assigned channel frequency?
1.
2.
3.
4.
The oscillator
The power supply
The power amplifier
The buffer amplifier
2-8. In an fsk transmitter with a doubler and a
tripler stage, the desired frequency shift is
1,200 hertz. To what maximum amount is
the oscillator frequency shift limited?
49 kilohertz
49.575 kilohertz
50 kilohertz
50.425 kilohertz
1. 60 hertz
2. 100 hertz
3. 120 hertz
4. 200 hertz
10
2-9. Fsk has which of the following advantages
over cw?
1.
2.
3.
4.
Fsk has a more stable oscillator
Fsk is easier to generate
Fsk rejects unwanted weak signals
Fsk does not have noise in its output
2-10. The "ratio of transmitted powers" provides
what information?
1. Transmitter power out in a cw system
2. Transmitter power out in an fsk system
3. Improvement shown using cw instead
of fsk transmission
4. Improvement shown using fsk instead
of cw transmission methods
Figure 2A.—Oscillator circuit.
IN ANSWERING QUESTIONS 2-12
THROUGH 2-14, REFER TO FIGURE 2A.
2-12. When a sound wave strikes the condenser
microphone (M), it has which, if any, of
the following effects on the oscillator
circuit?
2-11. In an fm signal, (a) the RATE of shift is
proportional to what characteristic of the
modulating signal, and (b) the AMOUNT
of shift is proportional to what
characteristic?
1.
2.
3.
4.
(a) Amplitude
(a) Amplitude
(a) Frequency
(a) Frequency
1.
2.
3.
4.
(b) amplitude
(b) frequency
(b) frequency
(b) amplitude
It changes output phase
It changes output voltage
It changes output frequency
It has no effect
2-13. What is the purpose of capacitor C in the
circuit?
1. It helps set the carrier frequency of the
oscillator
2. It prevents amplitude variations in the
oscillator output
3. It sets the maximum frequency
deviation of the oscillator
4. It varies the output frequency in
accordance with the modulating
voltage
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INTENTIONALLY.
11
2-14. A 1,000-hertz tone of a certain loudness
causes the frequency-modulated carrier for
the circuit to vary ±1,000 hertz at a rate of
1,000 times per second. If the
AMPLITUDE of the modulating tone is
doubled, what will be the maximum
carrier variation?
1.
2.
3.
4.
2-19. A 50-megahertz fm carrier varies between
49.925 megahertz and 50.075 megahertz
10,000 times per second. What is its
modulation index?
1. 5
2. 10
3. 15
4. 20
±1,000 hertz at 1,000 times per second
±1,000 hertz at 2,000 times per second
±2,000 hertz at 1,000 times per second
±2,000 hertz at 2,000 times per second
2-15. The maximum deviation for a 1.5 MHz
carrier is set at ±50 kHz. If the carrier
varies between 1.5125 MHz and 1.4875
MHz (±12.5 kHz), what is the percentage
of modulation?
1. 25 %
2. 50 %
3. 75 %
4. 100 %
2-16. An fm transmitter has a 50-watt carrier
with no modulation. What maximum
amount of output power will it have when
it is 50-percent modulated?
Figure 2B.—Modulation index table.
IN ANSWERING QUESTIONS 2-20 AND 2-21,
REFER TO FIGURE 2B.
1. 25 watts
2. 50 watts
3. 75 watts
4. 100 watts
2-20. An fm-modulated carrier varies between
925 kilohertz and 1,075 kilohertz 15,000
times per second. What is the bandwidth,
in kilohertz, of the transmitted signal?
(HINT: You will need to figure MI to be
able to find the sidebands.)
2-17. Frequencies that are located between
adjacent channels to prevent interference
are referred to as
1.
2.
3.
4.
1.
2.
3.
4.
sidebands
bandwidths
guard bands
blank channels
2-18. Modulation index may be figured by using
which of the following formulas?
1.
2.
3.
4.
2f/fm
fm/2f
fm/∆f
∆f/fm
12
340
420
480
560
2-21. The spectrum of a 500 kilohertz fmmodulated carrier has a 60-kilohertz
bandwidth and contains 12 significant
sidebands. How much, in kilohertz, is the
carrier deviated?
1.
2.
3.
4.
±5
±7.5
±10
±15
2-22. In a reactance-tube modulator, the
reactance tube shunts what part of the
oscillator circuitry?
1.
2.
3.
4.
The amplifier
The tank circuit
The biasing network
The feedback network
Figure 2C.—Semiconductor reactance modulator.
2-23. With no modulating signal applied, a
reactance tube has which, if any, of the
following effects on the output of an
oscillator?
1.
2.
3.
4.
IN ANSWERING QUESTIONS 2-25 AND 2-26,
REFER TO FIGURE 2C.
2-25. The semiconductor reactance modulator in
the circuit is in parallel with a portion of
the oscillator tank circuit coil. Modulation
results because of interaction with which
of the following transistor characteristics?
It will decrease amplitude
It will increase amplitude
It will change resonant frequency
It will have no effect
1.
2.
3.
4.
2-24. The reactance-tube frequency modulates
the oscillator by which of the following
actions?
1. By shunting the tank circuit with a
variable resistance
2. By shunting the tank circuit with a
variable reactance
3. By shunting the tank circuit with a
variable capacitance
4. By causing a resultant current flow in
the tank circuit which either leads or
lags resonant current
Collector-to-emitter resistance
Collector-to-emitter capacitance
Base-to-emitter resistance
Base-to-emitter capacitance
2-26. With a positive-going modulating signal
applied to the base of Q2, (a) what will
circuit capacitance do and (b) what will
the output frequency do?
1.
2.
3.
4.
13
(a) Decrease
(a) Decrease
(a) Increase
(a) Increase
(b) decrease
(b) increase
(b) increase
(a) decrease
2-30. A multivibrator frequency modulator is
limited to frequencies below what
maximum frequency?
2-27. What type of circuit is used to remove the
AM component in the output of a
semiconductor reactance modulator?
1.
2.
3.
4.
1. 1 megahertz
2. 2 megahertz
3. 5 megahertz
4. 10 megahertz
A mixer
A filter
A limiter
A buffer amplifier
2-31. To ensure the frequency stability of an fm
transmitter, which, if any, of the following
actions could be taken?
1. Modulate a crystal-controlled oscillator
at the desired frequency
2. Modulate a low-frequency oscillator,
and use frequency multipliers to
achieve the operating frequency
3. Modulate a low-frequency oscillator,
and heterodyne it with a higherfrequency oscillator to achieve the
desired frequency
4. None of the above
2-32. A varactor is a variable device that acts as
which of the following components?
Figure 2D.—Multivibrator modulator.
1.
2.
3.
4.
IN ANSWERING QUESTIONS 2-28 AND 2-29,
REFER TO FIGURE 2D.
2-28. The multivibrator modulator produces fm
modulation by which of the following
actions?
Resistor
Inductor
Capacitor
Transistor
2-33. As the positive potential is increased on
the cathode of a varactor, (a) what happens
to reverse bias and (b) how is dielectric
width affected?
1. By modulating the collector voltages
2. By modulating the base-return voltages
3. By modulating the value of the base
value of the base capacitors
4. By modulating the value of the base
resistors
1.
2.
3.
4.
2-29. What is the purpose of the filter on the
output of the multivibrator modulator?
1. To establish the fundamental operating
frequency
2. To eliminate unwanted frequency
variations
3. To eliminate unwanted odd harmonics
4. To eliminate unwanted even harmonics
14
(a) Increases
(a) Increases
(a) Decreases
(a) Decreases
(b) increases
(b) decreases
(b) decreases
(b) increases
2-37. In phase modulation, (a) the
AMPLITUDE of the modulating signal
determines what characteristic of the
phase shift, and (b) the FREQUENCY of
the modulating signal determines what
characteristic of the phase shift?
1.
2.
3.
4.
Figure 2E.—Phase relationships.
IN ANSWERING QUESTION 2-34, REFER TO
FIGURE 2E.
(a) Lags
(a) Lags
(a) Leads
(a) Leads
1.
2.
3.
4.
(b) leads
(b) lags
(b) lags
(b) leads
Amplitude modulated
Frequency modulated
Continuous-wave modulated
None of the above
2-39. Compared to fm, increasing the
modulating frequency in phase modulation
has what effect, if any, on the bandwidth
of the phase-modulated signal?
2-35. A 10 kilohertz, 10-volt square wave is
applied as the phase-modulating signal to
a transmitter with a carrier frequency of 60
megahertz. What is the output frequency
during the constant-amplitude portions of
the modulating signal?
1. It increases
2. It decreases
3. None
1.
10 kilohertz
2. 59,990 kilohertz
3. 60,000 kilohertz
4. 60,010 kilohertz
2-40. A simple phase modulator consists of a
capacitor in series with a variable
resistance. What total amount of carrier
shift will occur when XC is 10 times the
resistance?
2-36. In a phase modulator, the frequency
during the constant-amplitude portion of
the modulating wave is the
1.
2.
3.
4.
(b) rate
(b) amount
(b) amount
(b) rate
2-38. The frequency spectrums of a phasemodulated signal resemble the spectrum of
which, if any, of the following types of
modulation?
2-34. In the figure, (a) waveform X has what
phase relationship to waveform Y, and
(b) waveform Y has what relationship to
waveform Z?
1.
2.
3.
4.
(a) Rate
(a) Rate
(a) Amount
(a) Amount
1. 0 degrees
2. 45 degrees
3. 60 degrees
4. 90 degrees
peak frequency
rest frequency
deviation frequency
modulating frequency
2-41. The primary advantage of phase
modulation over frequency modulation is
that phase modulation has better carrier
1.
2.
3.
4.
15
power stability
amplitude stability
frequency stability
directional stability
2-46. As the square wave modulating voltage is
increased to the same amplitude as that of
the carrier, what will be the effect on
(a) the carrier amplitude and (b) amplitude
of the sidebands?
2-42. Phase-shift keying is most useful under
which of the following code element
conditions?
1. When mark elements are longer than
space elements
2. When mark elements are shorter than
space elements
3. When mark and space elements are the
same length
4. When mark and space elements are
longer than synchronizing elements
1. (a) Remains constant
(b) Increases
2. (a) Decreases
(b) Increases
3. (a) Increases
(b) Remains constant
4. (a) Increases
(b) Decreases
2-43. When a carrier is phase-shift keying
modulated, (a) a data bit ONE will
normally cause the carrier to shift its phase
what total number of degrees, and (b) a
data bit ZERO will cause the carrier to
shift its phase what total number of
degrees?
1.
2.
3.
4.
(a) 60
(a) 0
(a) 180
(a) 180
2-47. In a square-wave modulated signal, total
sideband power is what percentage of the
total power?
1. 0 percent
2. 25 percent
3. 33 percent
4. 50 percent
(b) 0
(b) 180
(b) 180
(b) 0
2-44. Which of the following circuits is used to
generate a phase-shift keyed signal?
1.
2.
3.
4.
Logic circuit
Phasor circuit
Phasitron circuit
Longitudinal circuit
Figure 2F.—Waveform.
2-45. When a carrier is modulated by a square
wave, what maximum number of sideband
pairs will be generated?
1.
2.
3.
4.
IN ANSWERING QUESTIONS 2-48
THROUGH 2-51, REFER TO FIGURE 2F.
SELECT THE FIGURE LETTER THAT
CORRESPONDS WITH THE WAVEFORM
LISTED IN THE QUESTIONS. LETTERS MAY
BE USED ONCE, MORE THAN ONCE, OR
NOT AT ALL.
1
9
3
An infinite number
2-48. Pulse width.
1.
2.
3.
4.
16
A
B
C
D
2-55. In a pulse-modulation system, which of
the following formulas is used to figure
the percentage of transmitting time?
2-49. Rest time.
1.
2.
3.
4.
B
C
D
E
2-50. Pulse duration.
1.
2.
3.
4.
A
B
D
E
2.
4.
1.
2.
3.
4.
B
C
D
E
1.
3.
2.
4.
Reflected pulse return interval
Reflected pulse duration
Reflected pulse amplitude
Reflected pulse frequency
2-57. In a spark-gap modulator, what is the
function of the pulse-forming network?
2-52. Which of the following ratios is used to
determine pulse-repetition frequency
(prf)?
1.
2.
3.
4.
To store energy
To increase the level of stored energy
To act as a power bleeder
To rapidly discharge stored energy
2-58. The damping diode in a thyratron
modulator serves which of the following
purposes?
2-53. Average power in a pulse-modulation
system is defined as the
1. It discharges the pulse-forming
network
2. It limits the input signal
3. It prevents the breakdown of the
thyratron by reverse-voltage transients
4. It rectifies the input signal
1. power during rest time
2. power during each pulse
3. power during each pulse averaged over
rest time
4. power during each pulse averaged over
one operating cycle
2-59. Compared to a spark-gap modulator, the
thyratron modulator exhibits which of the
following advantages?
2-54. In pulse modulation, what term is used to
indicate the ratio of time the system is
actually producing rf?
1.
2.
3.
4.
3.
2-56. When pulse modulation is used for range
finding in a radar application, which of the
following types of pulse information is
used?
2-51. Pulse-repetition time.
1.
2.
3.
4.
1.
1.
2.
3.
4.
Rest cycle
Duty cycle
Average cycle
Transmit cycle
17
Improved timing
Higher output pulses
Higher trigger voltage
Operates over a narrower range of
anode voltages and pulse-repetition
rates
2-60. To transmit intelligence using pulse
modulation, which of the following pulse
characteristics may be varied?
1.
2.
3.
4.
Pulse duration
Pulse amplitude
Pulse-repetition time
Each of the above
2-61. To accurately reproduce a modulating
signal in a pulse-modulated system, what
minimum number of samples must be
taken per cycle?
1.
2.
3.
4.
Figure 2G.—Waveform.
IN ANSWERING QUESTION 2-64, REFER TO
FIGURE 2G.
2-64. Which of the points shown in the
waveform may be varied in pulse-duration
modulation?
One
Two
Three
Four
1.
2.
3.
4.
2-62. What is the simplest form of pulse
modulation?
1.
2.
3.
4.
Pulse-code modulation
Pulse-duration modulation
Pulse-frequency modulation
Pulse-amplitude modulation
2-65. Which, if any, of the following is the
primary disadvantage of pulse-position
modulation?
1. It depends on transmitter-receiver
synchronization
2. It is susceptible to noise interference
3. Transmitter power varies
4. None of the above
2-63. The same pulse characteristic is varied in
which of the following types of pulse
modulations?
1.
2.
3.
4.
A only
B only
C only
A and/or C
Pam and pdm
Pdm and pwm
Pwm and ppm
Ppm and pam
2-66. A pfm transmitter transmits 10,000 pulses
per second without a modulating signal
applied. How, if at all, will a modulating
signal affect the transmitted pulse rate?
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1. It will decrease the transmitted pulse
rate
2. It will increase the transmitted pulse
rate
3. Both 1 and 2 above
4. It will not affect the transmitted pulse
rate
18
2-69. Which of the following is a characteristic
of a pcm system that makes it
advantageous for use in multiple-relay link
systems?
2-67. The process of arbitrarily dividing a wave
into a series of standard values is referred
to as
1.
2.
3.
4.
arbitration
quantization
interposition
approximation
1. Average power is constant
2. Average power decreases with each
relay
3. Noise is not cumulative at relay
stations
4. Quantization noise decreases with each
relay
2-68. A pcm system is capable of transmitting
32 standard levels that ate sampled 2.5
times per cycle of a 3-kilohertz
modulating signal. What maximum
number of bits per second are transmitted?
1. 18,750
2. 37,500
3. 75,000
4. 240,000
19
ASSIGNMENT 3
Textbook assignment: Chapter 3, “Demodulation,” pages 3-1 through 3-35.
_________________________________________________________________________________
3-1. The process of recreating the original
modulating frequencies (intelligence)
from an rf carrier is known by which of
the following terms?
1.
2.
3.
4.
Detection
Demodulation
Both 1 and 2 above
Distribution
3-2. When a demodulator fails to accurately
recover intelligence from a modulated
carrier, which of the following types of
distortion result?
1.
2.
3.
4.
Figure 3A.—Cw demodulation.
IN ANSWERING QUESTION 3-5, REFER TO
FIGURE 3A.
Phase
Frequency
Amplitude
Each of the above
3-5. In the figure, L and C1 form a frequencyselective network that serves what
purpose?
1. It removes the carrier
2. It rectifies the oscillations
3. It tunes the circuit to the desired rf
carrier
4. It provides filtering to maintain a
constant dc output
3-3. In a demodulator circuit, which of the
following components is required for
demodulation to occur?
1. A linear device
2. A nonlinear device
3. A variable resistor
3-6. To aid in distinguishing between two or
more cw signals that are close to the
same frequency, which of the following
detectors is used?
3-4. In cw demodulation, the first requirement
of the circuit is the ability to detect
1.
2.
3.
4.
the presence or absence of the carrier
amplitude variations in the carrier
frequency variations in the carrier
phase variations in the carrier
1.
2.
3.
4.
20
Diode
Crystal
Heterodyne
Transistor
3-10. What component provides the feedback
necessary for oscillations to occur?
3-7. Assume that two signals are received,
one at 500 kHz and the other at 501 kHz.
What frequency, in kHz, should be
mixed with them to distinguish the 501
kHz signal by producing a 1 kHz output?
1.
2.
3.
4.
1.
2.
3.
4.
499
500
501
502
R1
C2
C3
L2
3-11. Which of the following circuit functions
does Q1 perform?
1.
2.
3.
4.
Mixer
Detector
Oscillator
Each of the above
3-12. A circuit that is nonlinear, provides
filtering, and is sensitive to the type of
modulation applied to it fulfills the
requirements of which, if any, of the
following circuits?
1.
2.
3.
4.
Figure 3B.—Detector.
IN ANSWERING QUESTIONS 3-8
THROUGH 3-11, REFER TO FIGURE 3B.
3-13. A detector uses which of the following
signals to approximate the original
waveform?
3-8. The detector circuit in the figure uses the
heterodyning principle to detect the
incoming signal. What type of detector is
it?
1.
2.
3.
4.
1. The sum frequency
2. The carrier frequency
3. The modulation envelope
Hartley
Colpitts
Armstrong
Regenerative
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INTENTIONALLY.
3-9. What component controls the operating
frequency of the detector?
1.
2.
3.
4.
Mixer
Modulator
Demodulator
None of the above
C1
C2
L2
R1
21
Figure 3C.—Detector.
Figure 3D.—Detector.
IN ANSWERING QUESTIONS 3-14
THROUGH 3-16, REFER TO FIGURE 3C.
IN ANSWERING QUESTIONS 3-18
THROUGH 3-21, REFER TO FIGURE 3D.
3-14. What type of detector is shown in the
figure?
1.
2.
3.
4.
3-18. What type of detector is shown in the
figure?
Series-diode
Parallel-diode
Inductive-diode
Capacitive-diode
1.
2.
3.
4.
3-15. What is the purpose of C1 and L?
1.
2.
3.
4.
3-19. What circuit component acts as the load
for the detected audio?
To smooth the incoming rf
To select the desired af signal
To select the desired rf signal
To smooth the detected af signal
1.
2.
3.
4.
3-16. What is the purpose of C2?
1.
2.
3.
4.
Ratio
Common-base
Regenerative
Common-emitter
R1
R2
R3
R4
3-20. What is the purpose of C4?
To smooth the incoming rf signal
To select the desired af signal
To select the desired rf signal
To smooth the detected af signal
1. Tp bypass af
2. To bypass rf
3. To remove power supply voltage
variations
4. To determine the operating frequency
of the circuit
3-17. A shunt diode circuit is used as a detector
in which of the following instances?
1. When a large input signal is supplied
2. When a large output current is
required
3. When the input signal variations
overdrive the audio amplifier stages
4. When the input signal variations are
too small to produce a full output
from audio amplifier stages
3-21. This detector circuit is used under which
of the following circuit conditions?
1. When higher frequencies are used
2. When the best possible frequency
selection is required
3. When weak signals need to be
detected
4. When strong signals need to be
detected
22
3-24. What is the function of C1 and R1?
1. To act as an integrator
2. To act as a frequency-selective
network
3. To act as a filter network
4. To act as a differentiator
Figure 3E.—Circuit.
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INTENTIONALLY.
IN ANSWERING QUESTIONS 3-22
THROUGH 3-24, REFER TO FIGURE 3E.
3-22. What type of Circuits is/are shown in the
figure?
1.
2.
3.
4.
A detector
An amplifier
Both 1 and 2 above
An oscillator
3-23. What is the purpose of T2?
1.
2.
3.
4.
To filter rf
To filter af
To couple the af output
To couple the rf output
23
Figure 3F.—Foster-Seeley discriminator.
3-27. What is the function of L3?
IN ANSWERING QUESTIONS 3-25
THROUGH 3-30, REFER TO FIGURE 3F.
1. To couple af to the output
2. To couple rf from the tank circuits to
CR1 and CR2
3. To prevent af from being coupled to
the power supply
4. To provide the dc return path for CR1
and CR2
3-25. In the figure, what is the purpose of Q1?
1.
2.
3.
4.
To act as a limiter only
To act as an amplifier only
To act as a limiter and an amplifier
To act as an oscillator
3-28. At resonance, what is the amplitude of e3
compared to e4?
3-26. To what frequency are C1/L1 and C2/L2
tuned?
1.
2.
3.
4.
The af input
The center frequency of fm signal
The lowest fm deviation frequency
The highest fm deviation frequency
1. e3 is less than e4
2. e3 is equal to e4
3. e3 is greater than e4
24
3-29. When the circuit is operating ABOVE
resonance, (a) does inductive reactance
increase or decrease, and (b) does
capacitive reactance increase or
decrease?
1.
2.
3.
4.
(a) Increases
(a) Increases
(a) Decreases
(a) Decreases
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(b) increases
(b) decreases
(b) decreases
(b) increases
3-30. Circuit operation BELOW resonance is
represented by which of the following
vector diagrams?
1.
2.
3.
25
3-34. At resonance, what is the phase
relationship between tank current and
primary voltage?
1. Tank current leads primary voltage by
90 degrees
2. Tank current lags primary voltage by
90 degrees
3. Tank current and primary voltage are
in phase
4. Tank current and primary voltage are
out of phase
3-35. At resonance, what relative amount of
conduction takes place through CR1
compared to that for CR2?
Figure 3G.—Ratio detector.
1. CR1 conducts more than CR2
2. CR1 conducts less than CR2
3. CR1 and CR2 conduct the same
amount
IN ANSWERING QUESTIONS 3-31
THROUGH 3-40, REFER TO FIGURE 3G.
3-31. To what frequency(ies) are (a) L1 and C1
and (b) L2 and C2 tuned?
3-36. At resonance, (a) will the charges on C3
and C4 be equal or unequal, and (b) will
their polarities be the same or opposite?
1. (a) Center frequency
(b) lower frequency limit
2. (a) Center frequency
(b) center frequency
3. (a) Lower frequency limit
(b) center frequency
4. (a) Lower frequency limit
(b) lower frequency limit
1.
2.
3.
4.
Low-pass
High-pass
Band-pass
Band-reject
1. (a) e1 is nearer to ep
(b) e2 is nearer to ep
2. (a) e1 is nearer to ep
(b) e2 is farther from ep
3. (a) e1 is farther from ep
(b) e2 is nearer to ep
4. (a) e1 is farther from ep
(b) e2 is farther from ep
3-33. At resonance, what type of circuit does
the tank circuit appear to be?
1.
2.
3.
4.
(b) same
(b) opposite
(b) opposite
(b) same
3-37. ABOVE resonance, both voltages e1 and
e 2 have specific phase shift relationships
to voltage e in that they either shift nearer
to or farther from the phase of ep. What
are the phase relationships between
(a) e1 and ep and (b) e2 and ep?
3-32. What circuit filtering function do R5, C6,
and C7 provide?
1.
2.
3.
4.
(a) Equal
(a) Equal
(a) Unequal
(a) Unequal
Reactive
Resistive
Inductive
Capacitive
26
3-38. If C3 is charged to 6 volts and C4 is
charged to 4 volts, what is the output
voltage?
1.
2.
3.
4.
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INTENTIONALLY.
1 volt
2 volts
3 volts
4 volts
3-39. When operating BELOW resonance,
what is the relationship of the vector sum
of e 1 and ep to the vector sum of e2 and
ep?
1. The sum of e1 and ep is larger than the
sum of e2 and ep
2. The sum of e1 and ep is smaller than
the sum of e2 and ep
3. The slam of e1 and ep is equal to the
sum of e2 an ep
3-40. What components help to reduce the
effects of amplitude variations at the
input of the circuit?
1.
2.
3.
4.
R1, R2, and C5
R5, C6, and C7
R1, R2, C3, and C4
L1, L2, L3, and C2
3-41. What is the minimum input voltage, in
millivolts, required for a ratio detector?
1.
2.
3.
4.
100
200
300
400
3-42. Which of the following circuit functions
is performed by the gated-beam detector?
1.
2.
3.
4.
Limiter
Detector
Amplifier
Each of the above
27
3-47. ABOVE the center frequency of the
received fm signal, (a) will the tank
appear capacitive or inductive, and
(b) will the average plate current increase
or decrease?
1.
2.
3.
4.
Figure 3H.—Gated-beam detector.
1.
2.
3.
4.
3-43. What components in the circuit are used
to set the reference frequency for a gatedbeam detector?
C1 and L1
C2 and L2
C3 and L3
C4 and R1
1.
2.
3.
4.
Pins 1 and 3
Pins 3 and 4
Pins 3 and 5
Pins 4 and 5
Low-Q
High-Q
Nonresonant
Series-resonant
3-46. For plate current to flow, what must be
the polarities of (a) the quadrature grid
and (b) the limiter grid?
1.
2.
3.
4.
(a) Negative
(a) Negative
(a) Positive
(a) Positive
Conversion
Peak detector
Low-pass filter
Each of the above
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INTENTIONALLY.
3-45. What type of tank circuit is the
quadrature tank (L3 and C3)?
1.
2.
3.
4.
Peak
Quadrature
Series-diode
None of the above
3-49. Which of the following circuits can be
used as a communications pulse
demodulator?
3-44. What tube pins connect to elements that
perform in a manner similar to an AND
gate in a digital device?
1.
2.
3.
4.
(b) increase
(b) decrease
(b) decrease
(b) increase
3-48. To demodulate a phase-modulated signal,
which, if any, of the following types of
demodulators may be used?
IN ANSWERING QUESTIONS 3-43
THROUGH 3-47, REFER TO FIGURE 3H.
1.
2.
3.
4.
(a) Inductive
(a) Inductive
(a) Capacitive
(a) Capacitive
(b) negative
(b) positive
(b) positive
(b) negative
28
3-54. To detect pulse-duration modulation, the
low-pass filter components must be
selected so that they pass only the
1.
2.
3.
4.
3-55. What type(s) of modulation is/are
normally detected by first converting
it/them to another type of modulation?
Figure 3I.—Detector.
IN ANSWERING QUESTIONS 3-50
THROUGH 3-52, REFER TO FIGURE 3I.
1.
2.
3.
4.
3-50. To detect pulse-amplitude modulation,
what value must the RC time constant of
R1 and C1 in the circuit be?
1.
2.
3.
4.
1.
2.
3.
4.
1. To quickly discharge C1 between
received pulses
2. To rectify input pulses
3. To clamp the output to a positive
level
4. None of the above
An amplifier
A flip-flop
An oscillator
A transformer
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INTENTIONALLY.
3-52. What change must be made to the circuit
to detect pulse-duration modulation?
Remove R1
Increase the value of R1
Decrease the value of R1
Add a resistor in series with CR1
3-53. When a pulse-duration modulated signal
is determined by using a low-pass filter,
what characteristic of the signal is used?
1.
2.
3.
4.
Ppm only
Pfm only
Pcm only
Ppm, pfm, and pcm
3-56. What type of circuit can be used to
convert from ppm to pdm for
demodulation?
Five times the pulse width
Ten times the pulse width
Five times the interpulse period
Ten times the interpulse period
3-51. Which, if any, of the following functions
is the purpose of CR2?
1.
2.
3.
4.
carrier frequency
intermediate frequency
pulse-repetition frequency
desired modulating frequency
Width
Amplitude
Frequency
Pulse position
29
Figure 3J.—Pcm conversion.
IN ANSWERING QUESTIONS 3-57
THROUGH 3-59, REFER TO FIGURE 3J.
3-59. Between pulses, R1 must allow C1 to
discharge to what voltage?
3-57. To convert from pcm to pam, what type
of circuit is used to apply the pcm to the
input of the circuit shown?
1.
2.
3.
4.
1.
2.
3.
4.
A constant-current source
A constant-voltage source
A limiter-amplifier source
An oscillator-amplifier source
3-58. If C1 is allowed to charge 16 volts during
the period of one pulse, each additional
pulse increases the charge by 16 volts.
With the binary-coded equivalent of an
analog 12 applied to the input, what will
be the output of the circuit at sampling
time?
1.
2.
3.
4.
10 volts
12 volts
14 volts
16 volts
30
0 volts
One fourth of the charge on C1
One half of the charge on C1
Three fourths of the charge on C1
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