Chapter 7 Atomic Clock Atomic clock

Chapter 7 Atomic Clock Atomic clock
Revised Edition: 2016
ISBN 978-1-280-21347-2
© All rights reserved.
Published by:
College Publishing House
48 West 48 Street, Suite 1116,
New York, NY 10036, United States
Email: [email protected] Table of Contents
Chapter 1 - Electronic Test Equipment
Chapter 2 - Multimeter
Chapter 3 - Network Analyzer (electrical)
Chapter 4 - Signal Generator and Voltmeter
Chapter 5 - Wattmeter and Vectorscope
WT
Chapter 6 - Oscilloscope
Chapter 7 - Atomic Clock
Chapter 8 - Breadboard
Chapter 9 - Pliers
Chapter 10 - Semiconductor Curve Tracer
Chapter 11 - Soldering Iron
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Chapter 1
Electronic Test Equipment
WT
A Tektronix model 475A portable analogue oscilloscope
Electronic test equipment (sometimes called "testgear" or "bench top") is used to create
signals and capture responses from electronic Devices Under Test (DUTs). In this way,
the proper operation of the DUT can be proven or faults in the device can be traced and
repaired. Use of electronic test equipment is essential to any serious work on electronics
systems.
Practical electronics engineering and assembly requires the use of many different kinds of
electronic test equipment ranging from the very simple and inexpensive (such as a test
light consisting of just a light bulb and a test lead) to extremely complex and
sophisticated such as Automatic Test Equipment. ATE often includes many of these
instruments in real and simulated forms.
Generally, more advanced test gear is necessary when developing circuits and systems
than is needed when doing production testing or when troubleshooting existing
production units in the field.
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Test Equipment Switching
The addition of a high-speed switching system to a test system’s configuration allows for
faster, more cost-effective testing of multiple devices, and is designed to reduce both test
errors and costs. Designing a test system’s switching configuration requires an
understanding of the signals to be switched and the tests to be performed, as well as the
switching hardware form factors available.
Types of test equipment
Basic equipment
WT
Agilent commercial digital voltmeter checking a prototype
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The following items are used for basic measurement of voltages, currents, and
components in the circuit under test.
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Voltmeter (Measures voltage)
Ohmmeter (Measures resistance)
Ammeter, e.g. Galvanometer or Milliameter (Measures current)
Multimeter e.g., VOM (Volt-Ohm-Milliameter) or DMM (Digital Multimeter)
(Measures all of the above)
The following are used for stimulus of the circuit under test:
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Power supplies
Signal generator
Digital pattern generator
Pulse generator
WT
Howard piA digital multimeter
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The following analyze the response of the circuit under test:
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Oscilloscope (Measures all of the above as they change over time)
Frequency counter (Measures frequency)
And connecting it all together:
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Test probes
Advanced or less commonly used equipment
Meters
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Solenoid voltmeter (Wiggy)
Clamp meter (current transducer)
Wheatstone bridge (Precisely measures resistance)
Capacitance meter (Measures capacitance)
LCR meter (Measures inductance, capacitance, resistance and combinations
thereof)
EMF Meter (Measures Electric and Magnetic Fields)
Electrometer (Measures charge)
Probes
WT
A multimeter with a built in clampfacility. Pushing the large button at the bottom opens
the lower jaw of the clamp, allowing the clamp to be placed around a conductor (wire).
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RF probe
Signal tracer
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Analyzers
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Logic analyzer (Tests digital circuits)
Spectrum analyzer (SA) (Measures spectral energy of signals)
Protocol analyzer (Tests functionality, performance and conformance of
protocols)
Vector signal analyzer (VSA) (Like the SA but it can also perform many more
useful digital demodulation functions)
Time-domain reflectometer (Tests integrity of long cables)
Semiconductor curve tracer
Signal-generating devices
WT
Leader Instruments LSG-15 signal generator
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Signal generator
Frequency synthesiser
Function generator
Digital pattern generator
Pulse generator
Signal injector
Miscellaneous devices
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Continuity tester
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Cable tester
Hipot tester
Network analyzer (used to characterize components or complete computer
networks)
Test light
Transistor tester
Tube tester
Electrical tester pen
Receptacle tester
Test Equipment Platforms
WT
Keithley Instruments Series 4200 CVU
Several modular electronic instrumentation platforms are currently in common use for
configuring automated electronic test and measurement systems. These systems are
widely employed for incoming inspection, quality assurance, and production testing of
electronic devices and subassemblies. Industry-standard communication interfaces link
signal sources with measurement instruments in “rack-and-stack” or chassis-/mainframebased systems, often under the control of a custom software application running on an
external PC.
GPIB/IEEE-488
The General Purpose Interface Bus (GPIB) is an IEEE-488 (a standard created by the
Institute of Electrical and Electronics Engineers) standard parallel interface used for
attaching sensors and programmable instruments to a computer. GPIB is a digital 8-bit
parallel communications interface capable of achieving data transfers of more than 8
Mbytes/s. It allows daisy-chaining up to 14 instruments to a system controller using a 24pin connector. It is one of the most common I/O interfaces present in instruments and is
designed specifically for instrument control applications. The IEEE-488 specifications
standardized this bus and defined its electrical, mechanical, and functional specifications,
while also defining its basic software communication rules. GPIB works best for
applications in industrial settings that require a rugged connection for instrument control.
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The original GPIB standard was developed in the late 1960s by Hewlett-Packard to
connect and control the programmable instruments the company manufactured. The
introduction of digital controllers and programmable test equipment created a need for a
standard, high-speed interface for communication between instruments and controllers
from various vendors. In 1975, the IEEE published ANSI/IEEE Standard 488-1975, IEEE
Standard Digital Interface for Programmable Instrumentation, which contained the
electrical, mechanical, and functional specifications of an interfacing system. This
standard was subsequently revised in 1978 (IEEE-488.1) and 1990 (IEEE-488.2). The
IEEE 488.2 specification includes the Standard Commands for Programmable
Instrumentation (SCPI), which define specific commands that each instrument class must
obey. SCPI ensures compatibility and configurability among these instruments.
The IEEE-488 bus has long been popular because it is simple to use and takes advantage
of a large selection of programmable instruments and stimuli. Large systems, however,
have the following limitations:
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WT
Driver fanout capacity limits the system to 14 devices plus a controller.
Cable length limits the controller-device distance to two meters per device or 20
meters total, whichever is less. This imposes transmission problems on systems
spread out in a room or on systems that require remote measurements.
Primary addresses limit the system to 30 devices with primary addresses.
Modern instruments rarely use secondary addresses so this puts a 30-device limit
on system size.
LAN eXtensions for Instrumentation (LXI)
The LXI Standard defines the communication protocols for instrumentation and data
acquisition systems using Ethernet. These systems are based on small, modular
instruments, using low-cost, open-standard LAN (Ethernet). LXI-compliant instruments
offer the size and integration advantages of modular instruments without the cost and
form factor constraints of card-cage architectures. Through the use of Ethernet
communications, the LXI Standard allows for flexible packaging, high-speed I/O, and
standardized use of LAN connectivity in a broad range of commercial, industrial,
aerospace, and military applications. Every LXI-compliant instrument includes an
Interchangeable Virtual Instrument (IVI) driver to simplify communication with non-LXI
instruments, so LXI-compliant devices can communicate with devices that are not
themselves LXI compliant (i.e., instruments that employ GPIB, VXI, PXI, etc.). This
simplifies building and operating hybrid configurations of instruments.
LXI instruments sometimes employ scripting using embedded test script processors for
configuring test and measurement applications. Script-based instruments provide
architectural flexibility, improved performance, and lower cost for many applications.
Scripting enhances the benefits of LXI instruments, and LXI offers features that both
enable and enhance scripting. Although the current LXI standards for instrumentation do
not require that instruments be programmable or implement scripting, several features in
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the LXI specification anticipate programmable instruments and provide useful functionality that enhances scripting’s capabilities on LXI-compliant instruments.
VME eXtensions for Instrumentation (VXI)
The VXI bus architecture is an open standard platform for automated test based on the
VMEbus. Introduced in 1987, VXI uses all Eurocard form factors and adds trigger lines,
a local bus, and other functions suited for measurement applications. VXI systems are
based on a mainframe or chassis with up to 13 slots into which various VXI instrument
modules can be installed. The chassis also provides all the power supply and cooling
requirements for the chassis and the instruments it contains. VXI bus modules are
typically 6U in height.
PCI eXtensions for Instrumentation (PXI)
WT
PXI is a peripheral bus specialized for data acquisition and real-time control systems.
Introduced in 1997, PXI uses the CompactPCI 3U and 6U form factors and adds trigger
lines, a local bus, and other functions suited for measurement applications. PXI hardware
and software specifications are developed and maintained by the PXI Systems Alliance.
More than 50 manufacturers around the world produce PXI hardware.
Universal Serial Bus (USB)
The USB connects peripheral devices, such as keyboards and mice, to PCs. The USB is a
Plug and Play bus that can handle up to 127 devices on one port, and has a theoretical
maximum throughput of 480 Mb/s (high-speed USB defined by the USB 2.0
specification). Because USB ports are standard features of PCs, they are a natural
evolution of conventional serial port technology. However, it is not widely used in
building industrial test and measurement systems for a number of reasons; for example,
USB cables are not industrial grade, are noise sensitive, can accidentally become
detached, and the maximum distance between the controller and the device is 30 m. Like
RS-232, USB is useful for applications in a laboratory setting that do not require a rugged
bus connection.
RS-232
RS-232 is a specification for serial communication that is popular in analytical and
scientific instruments, as well for controlling peripherals such as printers. Unlike GPIB,
with the RS-232 interface, it is possible to connect and control only one device at a time.
RS-232 is also a relatively slow interface with typical data rates of less than 20 kbytes/s.
RS-232 is best suited for laboratory applications compatible with a slower, less rugged
connection.
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Test Script Processors and a Channel Expansion Bus
One of the most recently developed test system platforms employs instrumentation
equipped with onboard test script processors combined with a high-speed bus. In this
approach, one “master” instrument runs a test script (a small program) that controls the
operation of the various “slave” instruments in the test system, to which it is linked via a
high-speed LAN-based trigger synchronization and inter-unit communication bus.
Scripting is writing programs in a scripting language to coordinate a sequence of actions.
This approach is optimized for small message transfers that are characteristic of test and
measurement applications. With very little network overhead and a 100Mbit/sec data
rate, it is significantly faster than GPIB and 100BaseT Ethernet in real applications.
The advantage of this platform is that all connected instruments behave as one tightly
integrated multi-channel system, so users can scale their test system to fit their required
channel counts cost-effectively. A system configured on this type of platform can stand
alone as a complete measurement and automation solution, with the master unit
controlling sourcing, measuring, pass/fail decisions, test sequence flow control, binning,
and the component handler or prober. Support for dedicated trigger lines means that
synchronous operations between multiple instruments equipped with onboard Test Script
Processors that are linked by this high speed bus can be achieved without the need for
additional trigger connections.
WT
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Chapter 2
Multimeter
WT
A digital multimeter
A multimeter or a multitester, also known as a volt/ohm meter or VOM, is an
electronic measuring instrument that combines several measurement functions in one
unit. A typical multimeter may include features such as the ability to measure voltage,
current and resistance. Multimeters may use analog or digital circuits—analog
multimeters and digital multimeters (often abbreviated DMM or DVOM.) Analog
instruments are usually based on a microammeter whose pointer moves over a scale
calibration for all the different measurements that can be made; digital instruments
usually display digits, but may display a bar of a length proportional to the quantity
measured.
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A multimeter can be a hand-held device useful for basic fault finding and field service
work or a bench instrument which can measure to a very high degree of accuracy. They
can be used to troubleshoot electrical problems in a wide array of industrial and
household devices such as electronic equipment, motor controls, domestic appliances,
power supplies, and wiring systems.
Multimeters are available in a wide ranges of features and prices. Cheap multimeters can
cost less than US$10, while the top of the line multimeters can cost more than US$5,000.
History
WT
1920s Pocket Multimeter
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WT
Avometer Model 8
The first moving-pointer current-detecting device was the galvanometer in 1820. These
were used to measure resistance and voltage by using a Wheatstone bridge, and
comparing the unknown quantity to a reference voltage or resistance. While useful in the
lab, the devices were very slow and impractical in the field. These galvanometers were
bulky and delicate.
The D'Arsonval/Weston meter movement used a fine metal spring to give proportional
measurement rather than just detection, and built-in permanent field magnets made
deflection independent of the 3D orientation of the meter. These features enabled
dispensing with Wheatstone bridges, and made measurement quick and easy. By adding a
series or shunt resistor, more than one range of voltage or current could be measured with
one movement.
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Multimeters were invented in the early 1920s as radio receivers and other vacuum tube
electronic devices became more common. The invention of the first multimeter is
attributed to British Post Office engineer, Donald Macadie, who became dissatisfied with
having to carry many separate instruments required for the maintenance of the
telecommunications circuits. Macadie invented an instrument which could measure
amperes (aka amps), volts and ohms, so the multifunctional meter was then named
Avometer. The meter comprised a moving coil meter, voltage and precision resistors, and
switches and sockets to select the range.
Macadie took his idea to the Automatic Coil Winder and Electrical Equipment Company
(ACWEEC, founded in ~1923). The first AVO was put on sale in 1923, and although it
was initially a DC. Many of its features remained almost unaltered through to the last
Model 8.
Pocket watch style meters were in widespread use in the 1920s, at much lower cost than
Avometers. The metal case was normally connected to the negative connection, an
arrangement that caused numerous electric shocks. The technical specifications of these
devices were often crude, for example the one illustrated has a resistance of just 33 ohms
per volt, a non-linear scale and no zero adjustment.
WT
The usual analog multimeter when used for voltage measurements loads the circuit under
test to some extent (a microammeter with full-scale current of 50 μA, the highest
sensitivity commonly available, must draw at least 50 microamps from the circuit under
test to deflect fully). This may load a high-impedance circuit so much as to affect the
circuit, and to give a low reading.
Vacuum Tube Voltmeters or valve voltmeters (VTVM, VVM) were used for voltage
measurements in electronic circuits where high impedance was necessary. The VTVM
had a fixed input impedance of typically 1 megohm or more, usually through use of a
cathode follower input circuit, and thus did not significantly load the circuit being tested.
Before the introduction of digital electronic high-impedance analog transistor and field
effect transistor (FETs) voltmeters were used. Modern digital meters and some modern
analog meters use electronic input circuitry to achieve high-input impedance—their
voltage ranges are functionally equivalent to VTVMs.
Additional scales such as decibels, and functions such as capacitance, transistor gain,
frequency, duty cycle, display hold, and buzzers which sound when the measured
resistance is small have been included on many multimeters. While multimeters may be
supplemented by more specialized equipment in a technician's toolkit, some modern
multimeters include even more additional functions for specialized applications (e.g.,
temperature with a thermocouple probe, inductance, connectivity to a computer, speaking
measured value, etc.).
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Quantities measured
Contemporary multimeters can measure many quantities. The common ones are:
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Voltage, alternating and direct, in volts.
Current, alternating and direct, in amperes.
The frequency range for which AC measurements are accurate must be specified.
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Resistance in ohms.
Additionally, some multimeters measure:
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Capacitance in farads.
Conductance in siemens.
Decibels.
Duty cycle as a percentage.
Frequency in hertz.
Inductance in henrys.
Temperature in degrees Celsius or Fahrenheit, with an appropriate temperature
test probe, often a thermocouple.
WT
Digital multimeters may also include circuits for:
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Continuity; beeps when a circuit conducts.
Diodes (measuring forward drop of diode junctions, i.e., diodes and transistor
junctions) and transistors (measuring current gain and other parameters).
Battery checking for simple 1.5 volt and 9 volt batteries. This is a current loaded
voltage scale. Battery checking (ignoring internal resistance, which increases as
the battery is depleted), is less accurate when using a DC voltage scale.
Various sensors can be attached to multimeters to take measurements such as:
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Light level
Acidity/Alkalinity(pH)
Wind speed
Relative humidity
Resolution
Digital
The resolution of a multimeter is often specified in "digits" of resolution. For example,
the term 5½ digits refers to the number of digits displayed on the display of a multimeter.
By convention, a half digit can display either a zero or a one, while a three-quarters digit
can display a numeral higher than a one but not nine. Commonly, a three-quarters digit
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refers to a maximum value of 3 or 5. The fractional digit is always the most significant
digit in the displayed value. A 5½ digit multimeter would have five full digits that display
values from 0 to 9 and one half digit that could only display 0 or 1. Such a meter could
show positive or negative values from 0 to 199,999. A 3¾ digit meter can display a
quantity from 0 to 3,999 or 5,999, depending on the manufacturer.
While a digital display can easily be extended in precision, the extra digits are of no value
if not accompanied by care in the design and calibration of the analog portions of the
multimeter. Meaningful high-resolution measurements require a good understanding of
the instrument specifications, good control of the measurement conditions, and
traceability of the calibration of the instrument.
Specifying "display counts" is another way to specify the resolution. Display counts give
the largest number, or the largest number plus one (so the count number looks nicer) the
multimeter's display can show, ignoring a decimal separator. For example, a 5½ digit
multimeter can also be specified as a 199999 display count or 200000 display count
multimeter. Often the display count is just called the count in multimeter specifications.
Analog
WT
Display face of an analog multimeter
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Resolution of analog multimeters is limited by the width of the scale pointer, vibration of
the pointer, the accuracy of printing of scales, zero calibration, number of ranges, and
errors due to non-horizontal use of the mechanical display. Accuracy of readings
obtained is also often compromised by miscounting division markings, errors in mental
arithmetic, parallax observation errors, and less than perfect eyesight. Mirrored scales and
larger meter movements are used to improve resolution; two and a half to three digits
equivalent resolution is usual (and is usually adequate for the limited precision needed for
most measurements).
Resistance measurements, in particular, are of low precision due to the typical resistance
measurement circuit which compresses the scale heavily at the higher resistance values.
Inexpensive analog meters may have only a single resistance scale, seriously restricting
the range of precise measurements. Typically an analog meter will have a panel
adjustment to set the zero-ohms calibration of the meter, to compensate for the varying
voltage of the meter battery.
WT
Accuracy
Digital multimeters generally take measurements with accuracy superior to their analog
counterparts. Standard analog multimeters measure with typically three percent accuracy,
though instruments of higher accuracy are made. Standard portable digital multimeters
are specified to have an accuracy of typically 0.5% on the DC voltage ranges.
Mainstream bench-top multimeters are available with specified accuracy of better than
±0.01%. Laboratory grade instruments can have accuracies of a few parts per million.
Accuracy figures need to be interpreted with care. The accuracy of an analog instrument
usually refers to full-scale deflection; a measurement of 30V on the 100V scale of a 3%
meter is subject to an error of 3V, 10% of the reading. Digital meters usually specify
accuracy as a percentage of reading plus a percentage of full-scale value, sometimes
expressed in counts rather than percentage terms.
Quoted accuracy is specified as being that of the lower millivolt (mV) DC range, and is
known as the "basic DC volts accuracy" figure. Higher DC voltage ranges, current,
resistance, AC and other ranges will usually have a lower accuracy than the basic DC
volts figure. AC measurements only meet specified accuracy within a specified range of
frequencies.
Manufacturers can provide calibration services so that new meters may be purchased with
a certificate of calibration indicating the meter has been adjusted to standards traceable
to, for example, the US National Institute of Standards and Technology (NIST), or other
national standards laboratory.
Test equipment tends to drift out of calibration over time, and the specified accuracy
cannot be relied upon indefinitely. For more expensive equipment, manufacturers and
third parties provide calibration services so that older equipment may be recalibrated and
recertified. The cost of such services is disproportionate for inexpensive equipment;
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however extreme accuracy is not required for most routine testing. Multimeters used for
critical measurements may be part of a metrology program to assure calibration.
Some instrument assume sine waveform for measurements but for distorted wave forms a
true RMS converter (TrueRMS) may be needed for correct RMS calculation.
Sensitivity and input impedance
When used for measuring voltage, the input impedance of the multimeter must be very
high compared to the impedance of the circuit being measured; otherwise circuit
operation may be changed, and the reading will also be inaccurate.
Meters with electronic amplifiers (all digital multimeters and some analog meters) have a
fixed input impedance that is high enough not to disturb most circuits. This is often either
one or ten megohms; the standardization of the input resistance allows the use of external
high-resistance probes which form a voltage divider with the input resistance to extend
voltage range up to tens of thousands of volts.
WT
Most analog multimeters of the moving-pointer type are unbuffered, and draw current
from the circuit under test to deflect the meter pointer. The impedance of the meter varies
depending on the basic sensitivity of the meter movement and the range which is
selected. For example, a meter with a typical 20,000 ohms/volt sensitivity will have an
input resistance of two million ohms on the 100 volt range (100 V * 20,000 ohms/volt =
2,000,000 ohms). On every range, at full scale voltage of the range, the full current
required to deflect the meter movement is taken from the circuit under test. Lower
sensitivity meter movements are acceptable for testing in circuits where source
impedances are low compared to the meter impedance, for example, power circuits; these
meters are more rugged mechanically. Some measurements in signal circuits require
higher sensitivity movements so as not to load the circuit under test with the meter
impedance.
Sometimes sensitivity is confused with resolution of a meter, which is defined as the
lowest voltage, current or resistance change that can change the observed reading.
For general-purpose digital multimeters, the lowest voltage range is typically several
hundred millivolts AC or DC, but the lowest current range may be several hundred
milliamperes, although instruments with greater current sensitivity are available.
Measurement of low resistance requires lead resistance (measured by touching the test
probes together) to be subtracted for best accuracy.
The upper end of multimeter measurement ranges varies considerably; measurements
over perhaps 600 volts, 10 amperes, or 100 megohms may require a specialized test
instrument.
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Burden voltage
Any ammeter, including a multimeter in a current range, has a certain resistance. Most
multimeters inherently measure voltage, and pass a current to be measured through a
shunt resistance, measuring the voltage developed across it. The voltage drop is known as
the burden voltage, specified in volts per ampere. The value can change depending on the
range the meter selects, since different ranges usually use different shunt resistors.
The burden voltage can be significant in very low-voltage circuit areas. To check for its
effect on accuracy and on external circuit operation the meter can be switched to different
ranges; the current reading should be the same and circuit operation should not be
affected if burden voltage is not a problem. If this voltage is significant it can be reduced
(also reducing the inherent accuracy and precision of the measurement) by using a higher
current range.
WT
Alternating current sensing
Since the basic indicator system in either an analog or digital meter responds to DC only,
a multimeter includes an AC to DC conversion circuit for making alternating current
measurements. Basic meters utilize a rectifier circuit to measure the average or peak
absolute value of the voltage, but are calibrated to show the calculated root mean square
(RMS) value for a sinusoidal waveform; this will give correct readings for alternating
current as used in power distribution. User guides for some such meters give correction
factors for some simple non-sinusoidal waveforms, to allow the correct root mean square
(RMS) equivalent value to be calculated. More expensive multimeters include an AC to
DC converter that measures the true RMS value of the waveform within certain limits;
the user manual for the meter may indicate the limits of the crest factor and frequency for
which the meter calibration is valid. RMS sensing is necessary for measurements on nonsinusoidal periodic waveforms, such as found in audio signals and variable-frequency
drives.
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Digital multimeters (DMM or DVOM)
WT
A bench-top multimeter from Hewlett-Packard
Modern multimeters are often digital due to their accuracy, durability and extra features.
In a digital multimeter the signal under test is converted to a voltage and an amplifier
with electronically controlled gain preconditions the signal. A digital multimeter displays
the quantity measured as a number, which eliminates parallax errors.
Modern digital multimeters may have an embedded computer, which provides a wealth
of convenience features. Measurement enhancements available include:
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Auto-ranging, which selects the correct range for the quantity under test so that
the most significant digits are shown. For example, a four-digit multimeter would
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automatically select an appropriate range to display 1.234 instead of 0.012, or
overloading. Auto-ranging meters usually include a facility to 'freeze' the meter to
a particular range, because a measurement that causes frequent range changes is
distracting to the user. Other factors being equal, an auto-ranging meter will have
more circuitry than an equivalent, non-auto-ranging meter, and so will be more
costly, but will be more convenient to use.
Auto-polarity for direct-current readings, shows if the applied voltage is positive
(agrees with meter lead labels) or negative (opposite polarity to meter leads).
Sample and hold, which will latch the most recent reading for examination after
the instrument is removed from the circuit under test.
Current-limited tests for voltage drop across semiconductor junctions. While not a
replacement for a transistor tester, this facilitates testing diodes and a variety of
transistor types.
A graphic representation of the quantity under test, as a bar graph. This makes
go/no-go testing easy, and also allows spotting of fast-moving trends.
A low-bandwidth oscilloscope.
Automotive circuit testers, including tests for automotive timing and dwell
signals.
Simple data acquisition features to record maximum and minimum readings over
a given period, or to take a number of samples at fixed intervals.
Integration with tweezers for surface-mount technology.
A combined LCR meter for small-size SMD and through-hole components.
WT
Modern meters may be interfaced with a personal computer by IrDA links, RS-232
connections, USB, or an instrument bus such as IEEE-488. The interface allows the
computer to record measurements as they are made. Some DMMs can store
measurements and upload them to a computer.
The first digital multimeter was manufactured in 1955 by Non Linear Systems.
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Analog multimeters
WT
Inexpensive analog multimeter with a galvanometer needle display
A multimeter may be implemented with a galvanometer meter movement, or less often
with a bar-graph or simulated pointer such as an LCD or vacuum fluorescent display.
Analog multimeters are common; a quality analog instrument will cost about the same as
a DMM. Analog multimeters have the precision and reading accuracy limitations
described above, and so are not built to provide the same accuracy as digital instruments.
Analog meters are able to display a changing reading in real time, whereas digital meters
present such data in a manner that's either hard to follow or more often incomprehensible.
Also an intelligible digital display can follow changes far more slowly than an analogue
movement, so often fails to show what's going on clearly. Some digital multimeters
include a fast-responding bar-graph display for this purpose, though the resolution of
these is usually low.
Analog meters are also useful in situations where its necessary to pay attention to
something other than the meter, and the swing of the pointer can be seen without looking
at it. This can happen when accessing awkward locations, or when working on cramped
live circuitry.
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Analog displays are also used to very roughly read currents well above the maximum
rated current of the meter. For this, the probes are just touched to the circuit momentarily,
and how fast the pointer speeds towards fsd is noted. This is often done when testing state
of charge of dry batteries.
Analogue meter movements are inherently much more fragile physically and electrically
than digital meters. Many analogue meters have been instantly broken by connecting to
the wrong point in a circuit, or while on the wrong range, or by dropping onto the floor.
The ARRL handbook also says that analog multimeters, with no electronic circuitry, are
less susceptible to radio frequency interference.
The meter movement in a moving pointer analog multimeter is practically always a
moving-coil galvanometer of the d'Arsonval type, using either jeweled pivots or taut
bands to support the moving coil. In a basic analog multimeter the current to deflect the
coil and pointer is drawn from the circuit being measured; it is usually an advantage to
minimize the current drawn from the circuit. The sensitivity of an analog multimeter is
given in units of ohms per volt. For example, a very low cost multimeter with a
sensitivity of 1000 ohms per volt would draw 1 milliampere from a circuit at full scale
deflection. More expensive, (and mechanically more delicate) multimeters typically have
sensitivities of 20,000 ohms per volt and sometimes higher, with a 50,000 ohms per volt
meter (drawing 20 microamperes at full scale) being about the upper limit for a portable,
general purpose, non-amplified analog multimeter.
WT
To avoid the loading of the measured circuit by the current drawn by the meter
movement, some analog multimeters use an amplifier inserted between the measured
circuit and the meter movement. While this increased the expense and complexity of the
meter, by use of vacuum tubes or field effect transistors the input resistance can be made
very high and independent of the current required to operate the meter movement coil.
Such amplified multimeters are called VTVMs (vacuum tube voltmeters), TVMs
(transistor volt meters), FET-VOMs, and similar names.
Probes
A multimeter can utilize a variety of test probes to connect to the circuit or device under
test. Crocodile clips, retractable hook clips, and pointed probes are the three most
common attachments. Tweezer probes are used for closely-spaced test points, as in
surface-mount devices. The connectors are attached to flexible, thickly-insulated leads
that are terminated with connectors appropriate for the meter. Probes are connected to
portable meters typically by shrouded or recessed banana jacks, while benchtop meters
may use banana jacks or BNC connectors. 2mm plugs and binding posts have also been
used at times, but are less common today.
Clamp meters clamp around a conductor carrying a current to measure without the need
to connect the meter in series with the circuit, or make metallic contact at all. For all
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except the most specialized and expensive types they are suitable to measure only large
(from several amps up) and alternating currents.
Safety
All but the most inexpensive multimeters include a fuse, or two fuses, which will
sometimes prevent damage to the multimeter from a current overload on the highest
current range. A common error when operating a multimeter is to set the meter to
measure resistance or current and then connect it directly to a low-impedance voltage
source. Unfused meters are often quickly destroyed by such errors, fused meters often
survive. Fuses used in meters will carry the maximum measuring current of the
instrument, but are intended to clear if operator error exposes the meter to a lowimpedance fault. Meters with unsafe fusing are not uncommon, this situation has led to
the creation of the IEC61010 categories.
WT
Digital meters are rated into categories based on their intended application, as set forth by
IEC 61010 -1 and echoed by country and regional standards groups such as the CEN
EN61010 standard. There are four categories:
•
•
•
•
Category I: used where equipment is not directly connected to the mains.
Category II: used on single phase mains final sub-circuits.
Category III: used on permanently installed loads such as distribution panels,
motors, and 3 phase appliance outlets.
Category IV: used on locations where fault current levels can be very high, such
as supply service entrances, main panels, supply meters and primary over-voltage
protection equipment.
Each category also specifies maximum transient voltages for selected measuring ranges
in the meter. Category-rated meters also feature protections from over-current faults.
On meters that allow interfacing with computers, optical isolation may protect attached
equipment against high voltage in the measured circuit.
DMM alternatives
A general-purpose DMM is generally considered adequate for measurements at signal
levels greater than one millivolt or one milliampere, or below about 100 megohms—
levels far from the theoretical limits of sensitivity. Other instruments—essentially similar,
but with higher sensitivity—are used for accurate measurements of very small or very
large quantities. These include nanovoltmeters, electrometers (for very low currents, and
voltages with very high source resistance, such as one teraohm) and picoammeters. These
measurements are limited by available technology, and ultimately by inherent thermal
noise.
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Battery
Hand-held meters use batteries for continuity and resistance readings. This allows the
meter to test a device that is not connected to a power source, by supplying its own low
voltage for the test. A 1.5 volt AA battery is typical; more sophisticated meters with
added capabilities instead or also use a 9 volt battery for some types of readings, or even
higher-voltage batteries for very high resistance testing. Meters intended for testing in
hazardous locations or for use on blasting circuits may require use of a manufacturerspecified battery to maintain their safety rating. A battery is also required to power the
electronics of a digital multimeter or FET-VOM.
WT
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Chapter 3
Network Analyzer (electrical)
WT
ZVA40 vector network analyser from Rohde & Schwarz
A network analyzer is an instrument that measures the network parameters of electrical
networks. Today, network analyzers commonly measure s–parameters because reflection
and transmission of electrical networks are easy to measure at high frequencies, but there
are other network parameter sets such as y-parameters, z-parameters, and h-parameters.
Network analyzers are often used to characterize two-port networks such as amplifiers
and filters, but they can be used on networks with an arbitrary number of ports.
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Overview
WT
A microwave network analyzer (Agilent Technologies N5245A PNA-X) showing a
Smith chart.
Network analyzers are used mostly at high frequencies; operating frequencies can range
from 9 kHz to 110 GHz. Special types of network analyzers can also cover lower
frequency ranges down to 1 Hz. These network analyzers can be used for example for the
stability analysis of open loops or for the measurement of audio and ultrasonic components.
The two main types of network analyzers are
•
•
Scalar Network Analyzer (SNA) — measures amplitude properties only
Vector Network Analyzer (VNA) — measures both amplitude and phase
properties
A VNA may also be called a gain-phase meter or an Automatic Network Analyzer. An
SNA is functionally identical to a spectrum analyzer in combination with a tracking
generator. As of 2007, VNAs are the most common type of network analyzers, and so
references to an unqualified “network analyzer” most often mean a VNA. The three
biggest VNA manufacturers are Agilent, Anritsu, and Rohde & Schwarz.
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A new category of network analyzer is the Microwave Transition Analyzer (MTA) or
Large Signal Network Analyzer (LSNA), which measure both amplitude and phase of
the fundamental and harmonics. The MTA was commercialized before the LSNA, but
was lacking some of the user-friendly calibration features now available with the LSNA.
Architecture
WT
The basic parts of a vector network analyzer
The basic architecture of a network analyzer involves a signal generator, a test set, and
one or more receivers. In some setups, these units are distinct instruments.
Signal generator
The network analyzer needs a test signal, and a signal generator or signal source will
provide one. Older network analyzers did not have their own signal generator, but had the
ability to control a stand alone signal generator using, for example, a GPIB connection.
Nearly all modern network analyzers have a built-in signal generator. High-performance
network analyzers have two built-in sources. Two built-in sources are useful for
applications such as mixer test, where one source provides the RF signal, another the LO,
or amplifier intermodulation testing, where two tones are required for the test.
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Test set
The test set takes the signal generator output and routes it to the device under test, and it
routes the signal to be measured to the receivers.
It often splits off a reference channel for the incident wave. In a SNA, the reference
channel may go to a diode detector (receiver) whose output is sent to the signal
generator's automatic level control. The result is better control of the signal generator's
output and better measurement accuracy. In a VNA, the reference channel goes to the
receivers; it is needed to serve as a phase reference.
Directional coupler. Two resistor power divider.
Some microwave test sets include the front end mixers for the receivers (e.g., test sets for
HP 8510).
WT
The test sets may also contain directional couplers to measure reflected waves.
Transmission/reflection test set.
S-parameter test set.
Receiver
The receivers make the measurements. A network analyzer will have one or more
receivers connected to its test ports. The reference test port is usually lableled R, and the
primary test ports are A, B, C,.... Some analyzers will dedicate a separate receiver to each
test port, but others share one or two receivers among the ports. The R receiver may be
less sensitive than the receivers used on the test ports.
For the SNA, the receiver only measures the magnitude of the signal. A receiver can be a
detector diode that operates at the test frequency. The simplest SNA will have a single
test port, but more accurate measurements are made when a reference port is also used.
The reference port will compensate for amplitude variations in the test signal at the
measurement plane. It is possible to share a single detector and use it for both the
reference port and the test port by making two measurement passes.
For the VNA, the receiver measures both the magnitude and the phase of the signal. It
needs a reference channel (R) to determine the phase, so a VNA needs at least two
receivers. The usual method down converts the reference and test channels to make the
measurements at a lower frequency. The phase may be measured with a quadrature
detector. A VNA requires at least two receivers, but some will have three or four
receivers to permit simultaneous measurement of different parameters.
There are some VNA architectures (six-port) that infer phase and magnitude from just
power measurements.
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Calibration
The accuracy and repeatability of measurements can be improved with calibration.
Calibration involves measuring known standards and using those measurements to
compensate for systematic errors. After making these measurements, the network
analyzer can compute some correction values to produce the expected answer. For
answers that are supposed to be zero, the analyzer can subtract the residual. For non-zero
values, the analyzer could calculate complex factors that will compensate for both phase
and amplitude errors. Calibrations can be simple (such as compensating for transmission
line length) or involved methods that compensate for losses, mismatches, and
feedthroughs.
A network analyzer (or its test set) will have connectors on its front panel, but the
measurements are seldom made at the front panel. Usually some test cables will go from
the front panel to the device under test (DUT) such as a two-port filter or amplifier. The
length of those cables will introduce a time delay and corresponding phase shift
(affecting VNA measurements); the cables may also introduce some attenuation
(affecting SNA and VNA measurements).
WT
S-parameter measurements have a notion of a reference plane. The goal is to refer all
measurements to the reference plane.
Using ideal shorts, opens, and loads makes calibration easy, but ideal standards are
difficult to make. Modern network analyzers will account for the imperfections in the
standards. (Agilent 2006)
Automated calibration fixtures
A calibration using a mechanical calibration kit may take a significant amount of time.
Not only must the operator sweep through all the frequencies of interest, but the operator
must also disconnect and reconnect the various standards. (Agilent 2003, p. 9) To avoid
that work, network analyzers can employ automated calibration standards. (Agilent 2003)
The operator connects one box to the network analyzer. The box has a set of standards
inside and some switches that have already been characterized. The network analyzer can
read the characterization and control the configuration using a digital bus such as USB.
AC power systems
From 1929 to the late 1960s, large alternating current power systems were modelled and
studied on AC network analyzers ( or Transient network analyzers). These were an
outgrowth of the DC calculating boards used in the very earliest power system analysis.
These systems were essentially models of the power system, with generators,
transmission lines, and loads represented by miniature electrical components with scale
values in proportion to the modeled system. Model components were interconnected with
flexible cords to represent the schematic of the modelled system. To reduce the size of
the model components, the network analyzer was energized at a higher frequency than
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the 50 Hz or 60 Hz utility frequency, and model circuits were energized at relatively low
voltages to allow for safe measurement with adequate precision.
AC network analyzers were much used for power flow studies, short circuit calculations,
and system stability studies, but were ultimately replaced by numerical solutions running
on digital computers. Since the multiple elements of the AC network analyzer formed a
powerful analog computer, occasionally problems in physics and chemistry were
modelled (by such researchers as Gabriel Kron of General Electric), during the period up
to the late 1940s prior to the ready availability of general-purpose digital computers.
WT
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Chapter 4
Signal Generator and Voltmeter
Signal generator
A signal generator, also known variously as function generator, pitch generator,
arbitrary waveform generator, digital pattern generator or frequency generator is
an electronic device that generates repeating or non-repeating electronic signals (in either
the analog or digital domains). They are generally used in designing, testing,
troubleshooting, and repairing electronic or electroacoustic devices; though they often
have artistic uses as well.
WT
There are many different types of signal generators, with different purposes and
applications (and at varying levels of expense); in general, no device is suitable for all
possible applications.
Traditionally, signal generators have been embedded hardware units, but since the age of
multimedia-PCs, flexible, programmable software tone generators have also been
available.
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General purpose signal generators
Function generators
WT
Leader Instruments LSG-15 signal generator
A function generator is a device which produces simple repetitive waveforms. Such
devices contain an electronic oscillator, a circuit that is capable of creating a repetitive
waveform. (Modern devices may use digital signal processing to synthesize waveforms,
followed by a digital to analog converter, or DAC, to produce an analog output). The
most common waveform is a sine wave, but sawtooth, step (pulse), square, and triangular
waveform oscillators are commonly available as are arbitrary waveform generators
(AWGs). If the oscillator operates above the audio frequency range (>20 kHz), the
generator will often include some sort of modulation function such as amplitude
modulation (AM), frequency modulation (FM), or phase modulation (PM) as well as a
second oscillator that provides an audio frequency modulation waveform.
Function generators are typically used in simple electronics repair and design; where they
are used to stimulate a circuit under test. A device such as an oscilloscope is then used to
measure the circuit's output. Function generators vary in the number of outputs they
feature, frequency range, frequency accuracy and stability, and several other parameters.
Arbitrary waveform generators
Arbitrary waveform generators, or AWGs, are sophisticated signal generators which
allow the user to generate arbitrary waveforms, within published limits of frequency
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range, accuracy, and output level. Unlike function generators, which are limited to a
simple set of waveforms; an AWG allows the user to specify a source waveform in a
variety of different ways. AWGs are generally more expensive than function generators,
and are often more highly limited in available bandwidth; as a result, they are generally
limited to higher-end design and test applications.
Special purpose signal generators
WT
A pitch generator and a probe for locating a specific pair of wires amongst many, for
example in a punch block.
In addition to the above general-purpose devices, there are several classes of signal
generators designed for specific applications.
Pitch generators and audio generators
A pitch generator is a type of signal generator optimized for use in audio and acoustics
applications. Pitch generators typically include sine waves over the audio frequency
range (20 Hz–20 kHz). Sophisticated pitch generators will also include sweep generators
(a function which varies the output frequency over a range, in order to make frequencydomain measurements), multipitch generators (which output several pitches
simultaneously, and are used to check for intermodulation distortion and other non-linear
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effects), and tone bursts (used to measure response to transients). Pitch generators are
typically used in conjunction with sound level meters, when measuring the acoustics of a
room or a sound reproduction system, and/or with oscilloscopes or specialized audio
analyzers.
Many pitch generators operate in the digital domain, producing output in various digital
audio formats such as AES-3, or SPDIF. Such generators may include special signals to
stimulate various digital effects and problems, such as clipping, jitter, bit errors; they also
often provide ways to manipulate the metadata associated with digital audio formats.
The term synthesizer is used for a device that generates audio signals for music, or that
uses slightly more intricate methods.
Computer programs
WT
Whilst professional signal generators can be expensive, casual hobbyists can make use of
computer programs which generate signals and use the sound card to output the signal as
audio. These programs can be fun for experimentation but are often limited by the
hardware capabilities of the sound card to generate signals only within the aural band.
Video signal generators
A video signal generator is a device which outputs predetermined video and/or
television waveforms, and other signals used to stimulate faults in, or aid in parametric
measurements of, television and video systems. There are several different types of video
signal generators in widespread use. Regardless of the specific type, the output of a video
generator will generally contain synchronization signals appropriate for television,
including horizontal and vertical sync pulses (in analog) or sync words (in digital).
Generators of composite video signals (such as NTSC and PAL) will also include a
colorburst signal as part of the output. Video signal generators are available for a wide
variety of applications, and for a wide variety of digital formats; many of these also
include audio generation capability (as the audio track is an important part of any video
or television program or motion picture).
Technical Trends Driving the ARB Industry
New high-speed DACs provide up to 16-bit resolution at sample rates in excess of 1
GS/s. These devices provide the foundation for an AWG with the bandwidth and
dynamic range to address modern radio and communication applications. In combination
with a quadrature modulator and advanced digital signal processing, high-speed DACs
can be applied to create a full-featured vector signal generator with very high modulation
bandwidth. Example applications include commercial wireless standards such as Wi-Fi
(IEEE 802.11), WiMAX (IEEE 802.16) and LTE, in addition to military standards such
as those specified in the Joint Tactical Radio System (JTRS) initiative. Also, broad
modulation bandwidth allows multi-carrier signal generation, necessary for testing
receiver adjacent channel rejection.
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Voltmeter
WT
Demonstration voltmeter from a physics class
A voltmeter is an instrument used for measuring the electrical potential difference
between two points in an electric circuit. Analog voltmeters move a pointer across a scale
in proportion to the voltage of the circuit; digital voltmeters give a numerical display of
voltage by use of an analog to digital converter.
Voltmeters are made in a wide range of styles. Instruments permanently mounted in a
panel are used to monitor generators or other fixed apparatus. Portable instruments,
usually equipped to also measure current and resistance in the form of a multimeter, are
standard test instruments used in electrical and electronics work. Any measurement that
can be converted to a voltage can be displayed on a meter that is suitably calibrated; for
example, pressure, temperature, flow or level in a chemical process plant.
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General purpose analog voltmeters may have an accuracy of a few percent of full scale,
and are used with voltages from a fraction of a volt to several thousand volts. Digital
meters can be made with high accuracy, typically better than 1%. Specially calibrated test
instruments have higher accuracies, with laboratory instruments capable of measuring to
accuracies of a few parts per million. Meters using amplifiers can measure tiny voltages
of microvolts or less.
Part of the problem of making an accurate voltmeter is that of calibration to check its
accuracy. In laboratories, the Weston Cell is used as a standard voltage for precision
work. Precision voltage references are available based on electronic circuits.
Analog voltmeter
WT
A moving coil galvanometer of the d'Arsonval type
•
•
•
The red wire carries the current to be measured.
The restoring spring is shown in green.
N and S are the north and south poles of the magnet.
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A moving coil galvanometer can be used as a voltmeter by inserting a resistor in series
with the instrument. It employs a small coil of fine wire suspended in a strong magnetic
field. When an electric current is applied, the galvanometer's indicator rotates and
compresses a small spring. The angular rotation is proportional to the current through the
coil. For use as a voltmeter, a series resistance is added so that the angular rotation
becomes proportional to the applied voltage.
One of the design objectives of the instrument is to disturb the circuit as little as possible
and so the instrument should draw a minimum of current to operate. This is achieved by
using a sensitive ammeter or microammeter in series with a high resistance.
The sensitivity of such a meter can be expressed as "ohms per volt", the number of ohms
resistance in the meter circuit divided by the full scale measured value. For example a
meter with a sensitivity of 1000 ohms per volt would draw 1 milliampere at full scale
voltage; if the full scale was 200 volts, the resistance at the instrument's terminals would
be 200,000 ohms and at full scale the meter would draw 1 milliampere from the circuit
under test. For multi-range instruments, the input resistance varies as the instrument is
switched to different ranges.
WT
Moving-coil instruments with a permanent-magnet field respond only to direct current.
Measurement of AC voltage requires a rectifier in the circuit so that the coil deflects in
only one direction. Moving-coil instruments are also made with the zero position in the
middle of the scale instead of at one end; these are useful if the voltage reverses its
polarity.
Voltmeters operating on the electrostatic principle use the mutual repulsion between two
charged plates to deflect a pointer attached to a spring. Meters of this type draw
negligible current but are sensitive to voltages over about 100 volts and work with either
alternating or direct current.
VTVMs and FET-VMs
The sensitivity and input resistance of a voltmeter can be increased if the current required
to deflect the meter pointer is supplied by an amplifier and power supply instead of by the
circuit under test. The electronic amplifier between input and meter gives two benefits; a
rugged moving coil instrument can be used, since its sensitivity need not be high, and the
input resistance can be made high, reducing the current drawn from the circuit under test.
Amplified voltmeters often have an input resistance of 1, 10, or 20 megohms which is
independent of the range selected. A once-popular form of this instrument used a vacuum
tube in the amplifer circuit and so was called the vacuum tube voltmeter, or VTVM.
These were almost always powered by the local AC line current and so were not
particularly portable. Today these circuits use a solid-state amplifier using field-effect
transistors, hence FET-VM, and appear in handheld digital multimeters as well as in
bench and laboratory instruments. These are now so ubiquitous that they have largely
replaced non-amplified multimeters except in the least expensive price ranges.
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Most VTVMs and FET-VMs handle DC voltage, AC voltage, and resistance
measurements; modern FET-VMs add current measurements and often other functions as
well. A specialized form of the VTVM or FET-VM is the AC voltmeter. These
instruments are optimized for measuring AC voltage. They have much wider bandwidth
and better sensitivity than a typical multifunction device.
Digital voltmeters
WT
Two digital voltmeters. Note the 40 microvolt difference between the two measurements,
an offset of 34 parts per million.
The first digital voltmeter was invented and produced by Andrew Kay of Non-Linear
Systems (and later founder of Kaypro) in 1954.
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Digital voltmeters (DVMs) are usually designed around a special type of analog-todigital converter called an integrating converter. Voltmeter accuracy is affected by many
factors, including temperature and supply voltage variations. To ensure that a digital
voltmeter's reading is within the manufacturer's specified tolerances, they should be
periodically calibrated against a voltage standard such as the Weston cell.
Digital voltmeters necessarily have input amplifiers, and, like vacuum tube voltmeters,
generally have a constant input resistance of 10 megohms regardless of set measurement
range.
WT
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Chapter 5
Wattmeter and Vectorscope
Wattmeter
WT
Wattmeter
The wattmeter is an instrument for measuring the electric power (or the supply rate of
electrical energy) in watts of any given circuit.
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Electrodynamic
WT
Early wattmeter on display at the Historic Archive and Museum of Mining in Pachuca,
Mexico.
The traditional analog wattmeter is an electrodynamic instrument. The device consists of
a pair of fixed coils, known as current coils, and a movable coil known as the potential
coil.
The current coils connected in series with the circuit, while the potential coil is connected
in parallel. Also, on analog wattmeters, the potential coil carries a needle that moves over
a scale to indicate the measurement. A current flowing through the current coil generates
an electromagnetic field around the coil. The strength of this field is proportional to the
line current and in phase with it. The potential coil has, as a general rule, a high-value
resistor connected in series with it to reduce the current that flows through it.
The result of this arrangement is that on a dc circuit, the deflection of the needle is
proportional to both the current and the voltage, thus conforming to the equation W=VA
or P=VI.
For AC power, curent and voltage may not be in step, owing to the delaying effects of
circuit inductance or capacitance. On an ac circuit the deflection is proportional to the
average instantaneous product of voltage and current, thus measuring true power, P=VI
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cos φ. Here, cosφ represents the power factor which shows that the power transmitted
may be less than the apparent power obtained by multiplying the readings of a voltmeter
and ammeter in the same circuit.
The two circuits of a wattmeter can be damaged by excessive current. The ammeter and
voltmeter are both vulnerable to overheating — in case of an overload, their pointers will
be driven off scale — but in the wattmeter, either or even both the current and potential
circuits can overheat without the pointer approaching the end of the scale! This is because
the position of the pointer depends on the power factor, voltage and current. Thus, a
circuit with a low power factor will give a low reading on the wattmeter, even when both
of its circuits are loaded to the maximum safety limit. Therefore, a wattmeter is rated not
only in watts, but also in volts and amperes.
A typical wattmeter in educational labs has two voltage coils (pressure coils) and a
current coil. We can connect the two pressure coils in series or parallel to each other to
change the ranges of the wattmeter. Another feature is that the pressure coil can also be
tapped to change the meter's range. If the pressure coil has range of 300 volts, the half of
it can be used so that the range becomes 150 Volts.
WT
Electrodynamometer
Siemens electrodynamometer, circa 1910. F = Fixed coil, D = Movable coil, S = Spiral
spring, T = Torsion head, MM = Mercury cups, I = Index needle
An early current meter was the electrodynamometer. Used in the early 20th century, the
Siemens electrodynamometer, for example, is a form of an electrodynamic ammeter, that
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has a fixed coil which is surrounded by another having its axis at right angles to that of
the fixed coil. This second coil is suspended by a number of silk fibres, and to the coil is
also attached a spiral spring the other end of which is fastened to a torsion head. If then
the torsion head is twisted, the suspended coil experiences a torque and is displaced
through an angle equal to that of the torsion head. The current can be passed into and out
of the movable coil by permitting the ends of the coil to dip into two mercury cups.
If a current is passed through the fixed coil and movable coil in series with one another,
the movable coil tends to displace itself so as to bring the axes of the coils, which are
normally at right angles, more into the same direction. This tendency can be resisted by
giving a twist to the torsion head and so applying to the movable coil through the spring a
restoring torque, which opposes the torque due to the dynamic action of the currents. If
then the torsion head is provided with an index needle, and also if the movable coil is
provided with an indicating point, it is possible to measure the torsional angle through
which the head must be twisted to bring the movable coil back to its zero position. In
these circumstances, the torsional angle becomes a measure of the torque and therefore of
the product of the strengths of the currents in the two coils, that is to say, of the square of
the strength of the current passing through the two coils if they are joined up in series.
The instrument can therefore be graduated by passing through it known and measured
continuous currents, and it then becomes available for use with either continuous or
alternating currents. The instrument can be provided with a curve or table showing the
current corresponding to each angular displacement of the torsion head.
WT
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Electronic wattmeter
WT
Prodigit Model 2000MU (UK version), shown in use and displaying a reading of 10
Watts being consumed by the appliance.
Electronic wattmeters are used for direct, small power measurements or for power
measurements at frequencies beyond the range of electrodynamometer-type instruments.
Digital
A modern digital electronic wattmeter/energy meter samples the voltage and current
thousands of times a second. For each sample, the instantaneous voltage should then be
multiplied by the current of the same instant, and the average of this outcome is the real
power. The real power divided by the apparent volt-amperes (VA) is the power factor. A
computer circuit uses the sampled values to calculate RMS voltage, RMS current, VA,
power (watts), power factor, and kilowatt-hours. The simple models display that
information on LCD. More sophisticated models retain the information over an extended
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period of time, and can transmit it to field equipment or a central location. Wattmeters
vary considerable in correctly calculating energy consumptions, especially when real
power is much lower than VA.
Radio frequency
Instruments with moving coils can be calibrated for direct current or power frequency
currents up to a few hundred Hz. At radio frequencies a common method is a rectifier
circuit arranged to respond to current in a transmission line; the system is calibrated for
the known circuit impedance.
WT
Vectorscope
The diagonal direction of the color burst vector is indicative of a PAL signal
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WT
The graticule of an NTSC vectorscope
A vectorscope is a special type of oscilloscope used in both audio and video applications.
Whereas an oscilloscope or waveform monitor normally displays a plot of signal vs. time,
a vectorscope displays an X-Y plot of two signals, which can reveal details about the
relationship between these two signals. Vectorscopes are highly similar in operation to
oscilloscopes operated in X-Y mode; however those used in video applications have
specialized graticules, and accept standard television or video signals as input
(demodulating and demultiplexing the two components to be analyzed internally).
Applications
Video
In video applications, a vectorscope supplements a waveform monitor for the purpose of
measuring and testing television signals, regardless of format (NTSC, PAL, SECAM or
any number of digital television standards). While a waveform monitor allows a
broadcast technician to measure the overall characteristics of a video signal, a
vectorscope is used to visualize chrominance, which is encoded into the video signal as a
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subcarrier of specific frequency. The vectorscope locks exclusively to the chrominance
subcarrier in the video signal (at 3.58 MHz for NTSC, or at 4.43 MHz for PAL) to drive
its display. In digital applications, a vectorscope instead plots the Cb and Cr channels
against each other (these are the two channels in digital formats which contain chroma
information).
A vectorscope uses an overlaid circular reference display, or graticule, for visualizing
chrominance signals, which is the best method of referring to the QAM scheme used to
encode color into a video signal. The actual visual pattern that the incoming chrominance
signal draws on the vectorscope is called the trace. Chrominance is measured using two
methods—color saturation, encoded as the amplitude, or gain, of the color red, subcarrier
signal, and hue, encoded as the subcarrier's phase. The vectorscope's graticule roughly
represents saturation as distance from the center of the circle, and hue as the angle, in
standard position, around it. The graticule is also embellished with several elements
corresponding to the various components of the standard color bars video test signal,
including boxes around the circles for the colors in the main bars, and perpendicular lines
corresponding to the U and V components of the chrominance signal (and additionally on
an NTSC vectorscope, the I and Q components). NTSC vectorscopes have one set of
boxes for the color bars, while their PAL counterparts have two sets of boxes, because
the R-Y chrominance component in PAL reverses in phase on alternating lines. Another
element in the graticule is a fine grid at the nine-o'clock, or -U position, used for
measuring differential gain and phase.
WT
Often two sets of bar targets are provided: one for colorbars at 75% amplitude and one
for colorbars at 100% amplitude. The 100% bars represent the maximum amplitude (of
the composite signal) that composite encoding allows for. 100% bars are not suitable for
broadcast and are not broadcast safe. 75% bars have reduced amplitude and are broadcast
safe.
In some vectorscope models, only one set of bar targets is provided. The vectorscope can
be setup for 75% or 100% bars by adjusting the gain so that the color burst vector extends
to the "75%" or "100%" marking on the graticule.
The reference signal used for the vectorscope's display is the color burst that is
transmitted before each line of video, which for NTSC is defined to have a phase of 180°,
corresponding to the nine-o'clock position on the graticule. The actual color burst signal
shows up on the vectorscope as a straight line pointing to the left from the center of the
graticule. In the case of PAL, the color burst phase alternates between 135° and 225°,
resulting in two vectors pointing in the half-past-ten and half-past-seven positions on the
graticule, respectively. In digital (and component analog) vectorscopes, colorburst doesn't
exist; hence the phase relationship between the colorburst signal and the chroma
subcarrier is simply not an issue. A vectorscope for SECAM uses a demodulator similar
to the one found in a SECAM-receiver to retrieve the U and V colour signals since they
are transmitted one at a time (Thomson 8300 Vecamscope).
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On older vectorscopes implemented with CRTs, the graticule was often implemented as a
silkscreened overlay which was superimposed over the front surface of the CRT. One
notable exception was the Tektronix WFM601 series of instruments, which are combined
waveform monitors/vectorscopes used to measure CCIR 601 television signals. The
waveform-mode graticules of these instruments is implemented with a silkscreen;
whereas the vectorscope graticule (consisting only of bar targets, as this family did not
support composite video) was drawn on the CRT by the electron beam. Modern
instruments have graticules drawn using computer graphics, and both graticule and trace
are rendered on an external VGA monitor or an internal VGA-compatible LCD display.
Most modern waveform monitors include vectorscope functionality built in; and many
allow the two modes to be displayed side-by-side. The combined device is typically
referred to as a waveform monitor, and standalone vectorscopes are rapidly becoming
obsolete.
Audio
WT
In audio applications, a vectorscope is used to measure the difference between channels
of stereo audio signals. One stereo channel drives the horizontal deflection of the display,
and the other drives the vertical deflection. A monoaural signal, consisting of identical
left and right signals, results in a straight line with a slope of positive one. Any stereo
separation is visible as a deviation from this line, creating a Lissajous figure. If the
straight line (or deviation from it) appears with a slope of negative one, this indicates that
the left and right channels are 180° out of phase.
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Chapter 6
Oscilloscope
WT
Illustration showing the interior of a cathode-ray tube for use in an oscilloscope. Numbers
in the picture indicate: 1. Deflection voltage electrode; 2. Electron gun; 3. Electron beam;
4. Focusing coil; 5. Phosphor-coated inner side of the screen
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WT
A Tektronix model 475A portable analog oscilloscope, a very typical instrument of the
late 1970s
An oscilloscope (also known as a scope, CRO, DSO or, an O-scope) is a type of
electronic test instrument that allows observation of constantly varying signal voltages,
usually as a two-dimensional graph of one or more electrical potential differences using
the vertical or 'Y' axis, plotted as a function of time, (horizontal or 'x' axis). Although an
oscilloscope displays voltage on its vertical axis, any other quantity that can be converted
to a voltage can be displayed as well. In most instances, oscilloscopes show events that
repeat with either no change, or change slowly.
Oscilloscopes are commonly used to observe the exact wave shape of an electrical signal.
In addition to the amplitude of the signal, an oscilloscope can show distortion, the time
between two events (such as pulse width, period, or rise time) and relative timing of two
related signals.
Oscilloscopes are used in the sciences, medicine, engineering, and telecommunications
industry. General-purpose instruments are used for maintenance of electronic equipment
and laboratory work. Special-purpose oscilloscopes may be used for such purposes as
analyzing an automotive ignition system, or to display the waveform of the heartbeat as
an electrocardiogram.
Originally all oscilloscopes used cathode ray tubes as their display element and linear
amplifiers for signal processing, (commonly referred to as CROs) however, modern
oscilloscopes have LCD or LED screens, fast analog-to-digital converters and digital
signal processors. Although not as commonplace, some oscilloscopes used storage CRTs
to display single events for a limited time. Oscilloscope peripheral modules for general
purpose laptop or desktop personal computers use the computer's display, allowing them
to be used as test instruments.
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Features and uses
WT
Basic Oscilloscope
Description
Display and general external appearance
The basic oscilloscope, as shown in the illustration, is typically divided into four sections:
the display, vertical controls, horizontal controls and trigger controls. The display is
usually a CRT or LCD panel which is laid out with both horizontal and vertical reference
lines referred to as the graticule. In addition to the screen, most display sections are
equipped with three basic controls, a focus knob, an intensity knob and a beam finder
button.
The vertical section controls the amplitude of the displayed signal. This section carries a
Volts-per-Division (Volts/Div) selector knob, an AC/DC/Ground selector switch and the
vertical (primary) input for the instrument. Additionally, this section is typically equipped
with the vertical beam position knob.
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The horizontal section controls the time base or “sweep” of the instrument. The primary
control is the Seconds-per-Division (Sec/Div) selector switch. Also included is a
horizontal input for plotting dual X-Y axis signals. The horizontal beam position knob is
generally located here.
The trigger section controls the start event of the sweep. The trigger can be set to
automatically restart after each sweep or it can be configured to respond to an internal or
external event. The principal controls of this section will be the source and coupling
selector switches. An external trigger input (EXT Input) and level adjustment will also be
included.
In addition to the basic instrument, most oscilloscopes are supplied with a probe as
shown. The probe will connect to any input on the instrument and typically has a resistor
of ten times the oscilloscope's input impedance. This results in a .1 (-10X) attenuation
factor, but helps to isolate the capacitive load presented by the probe cable from the
signal being measured. Some probes have a switch allowing the operator to bypass the
resistor when appropriate.
WT
Size and portability
Most modern oscilloscopes are lightweight, portable instruments that are compact enough
to be easily carried by a single person. In addition to the portable units, the market offers
a number of miniature battery-powered instruments for field service applications.
Laboratory grade oscilloscopes, especially older units which use vacuum tubes, are
generally bench-top devices or may be mounted into dedicated carts. Special-purpose
oscilloscopes may be rack-mounted or permanently mounted into a custom instrument
housing.
Inputs
The signal to be measured is fed to one of the input connectors, which is usually a coaxial
connector such as a BNC or UHF type. Binding posts or banana plugs may be used for
lower frequencies. If the signal source has its own coaxial connector, then a simple
coaxial cable is used; otherwise, a specialised cable called a "scope probe", supplied with
the oscilloscope, is used. In general, for routine use, an open wire test lead for connecting
to the point being observed is not satisfactory, and a probe is generally necessary.
General-purpose oscilloscopes usually present an input impedance of 1 megohm in
parallel with a small but known capacitance such as 20 picofarads. This allows the use of
standard oscilloscope probes. Scopes for use with very high frequencies may have 50ohm inputs, which must be either connected directly to a 50-ohm signal source or used
with Z0 or active probes.
Less-frequently-used inputs include one (or two) for triggering the sweep, horizontal
deflection for X-Y mode displays, and trace brightening/darkening, sometimes called "Zaxis" inputs.
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Probes
Open wire test leads (flying leads) are likely to pick up interference, so they are not
suitable for low level signals. Furthermore, the leads have a high inductance, so they are
not suitable for high frequencies. Using a shielded cable (i.e., coaxial cable) is better for
low level signals. Coaxial cable also has lower inductance, but it has higher capacitance:
a typical 50 ohm cable has about 90 pF per meter. Consequently, a one meter direct (1X)
coaxial probe will load a circuit with a capacitance of about 110 pF and a resistance of
1 megohm.
To minimize loading, attenuator probes (e.g., 10X probes) are used. A typical probe uses
a 9 megohm series resistor shunted by a low-value capacitor to make an RC compensated
divider with the cable capacitance and scope input. The RC time constants are adjusted to
match. For example, the 9 megohm series resistor is shunted by a 12.2 pF capacitor for a
time constant of 110 microseconds. The cable capacitance of 90 pF in parallel with the
scope input of 20 pF and 1 megohm (total capacitance 110 pF) also gives a time constant
of 110 microseconds. In practice, there will be an adjustment so the operator can
precisely match the low frequency time constant (called compensating the probe).
Matching the time constants makes the attenuation independent of frequency. At low
frequencies (where the resistance of R is much less than the reactance of C), the circuit
looks like a resistive divider; at high frequencies (resistance much greater than
reactance), the circuit looks like a capacitive divider.
WT
The result is a frequency compensated probe for modest frequencies that presents a load
of about 10 megohms shunted by 12 pF. Although such a probe is an improvement, it
does not work when the time scale shrinks to several cable transit times (transit time is
typically 5 ns). In that time frame, the cable looks like its characteristic impedance, and
there will be reflections from the transmission line mismatch at the scope input and the
probe that causes ringing. The modern scope probe uses lossy low capacitance
transmission lines and sophisticated frequency shaping networks to make the 10X probe
perform well at several hundred megahertz. Consequently, there are other adjustments for
completing the compensation.
Probes with 10:1 attenuation are by far the most common; for large signals (and slightlyless capacitive loading), 100:1 probes are not rare. There are also probes that contain
switches to select 10:1 or direct (1:1) ratios, but one must be aware that the 1:1 setting
has significant capacitance (tens of pF) at the probe tip, because the whole cable's
capacitance is now directly connected.
Good oscilloscopes allow for probe attenuation, easily showing effective sensitivity at the
probe tip. Some of the best ones have indicator lamps behind translucent windows in the
panel to prompt the user to read effective sensitivity. The probe connectors (modified
BNC's) have an extra contact to define the probe's attenuation. (A certain value of
resistor, connected to ground, "encodes" the attenuation.)
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There are special high-voltage probes which also form compensated attenuators with the
oscilloscope input; the probe body is physically large, and one made by Tektronix
requires partly filling a canister surrounding the series resistor with volatile liquid
fluorocarbon to displace air. At the oscilloscope end is a box with several waveformtrimming adjustments. For safety, a barrier disc keeps one's fingers distant from the point
being examined. Maximum voltage is in the low tens of kV. (Observing a high-voltage
ramp can create a staircase waveform with steps at different points every repetition, until
the probe tip is in contact. Until then, a tiny arc charges the probe tip, and its capacitance
holds the voltage (open circuit). As the voltage continues to climb, another tiny arc
charges the tip further.)
There are also current probes, with cores that surround the conductor carrying current to
be examined. One type has a hole for the conductor, and requires that the wire be passed
through the hole; it's for semi-permanent or permanent mounting. However, other types,
for testing, have a two-part core that permit them to be placed around a wire. Inside the
probe, a coil wound around the core provides a current into an appropriate load, and the
voltage across that load is proportional to current. However, this type of probe can sense
AC, only.
WT
A more-sophisticated probe (originally made by Tektronix) includes a magnetic flux
sensor in the magnetic circuit. The probe connects to an amplifier, which feeds (low
frequency) current into the coil to cancel the sensed field; the magnitude of that current
provides the low-frequency part of the current waveform, right down to DC. The coil still
picks up high frequencies. There is a combining network akin to a loudspeaker crossover
network.
Front panel controls
Focus control
This control adjusts CRT focus to obtain the sharpest, most-detailed trace. In practice,
focus needs to be adjusted slightly when observing quite-different signals, which means
that it needs to be an external control. Flat-panel displays do not need a focus control;
their sharpness is always optimum.
Intensity control
This adjusts trace brightness. Slow traces on CRT oscilloscopes need less, and fast ones,
especially if not often repeated, require more. On flat panels, however, trace brightness is
essentially independent of sweep speed, because the internal signal processing effectively
synthesizes the display from the digitized data.
Beam finder
Modern oscilloscopes have direct-coupled deflection amplifiers, which means the trace
could be deflected off-screen. They also might have their CRT beam blanked without the
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operator knowing it. In such cases, the screen is blank. To help in restoring the display
quickly and without experimentation, the beam finder circuit overrides any blanking and
ensures that the beam will not be deflected off-screen; it limits the deflection. With a
display, it's usually very easy to restore a normal display. (While active, beam-finder
circuits might temporarily distort the trace severely, however this is acceptable.)
Graticule
The graticule is a grid of squares that serve as reference marks for measuring the
displayed trace. These markings, whether located directly on the screen or on a
removable plastic filter, usually consist of a 1 cm grid with closer tick marks (often at
2 mm) on the centre vertical and horizontal axis. One expects to see ten major divisions
across the screen; the number of vertical major divisions varies. Comparing the grid
markings with the waveform permits one to measure both voltage (vertical axis) and time
(horizontal axis). Frequency can also be determined by measuring the waveform period
and calculating its reciprocal.
WT
On old and lower-cost CRT oscilloscopes the graticule is a sheet of plastic, often with
light-diffusing markings and concealed lamps at the edge of the graticule. The lamps had
a brightness control. Higher-cost instruments have the graticule marked on the inside face
of the CRT, to eliminate parallax errors; better ones also had adjustable edge illumination
with diffusing markings. (Diffusing markings appear bright.) Digital oscilloscopes,
however, generate the graticule markings on the display in the same way as the trace.
External graticules also protect the glass face of the CRT from accidental impact. Some
CRT oscilloscopes with internal graticules have an unmarked tinted sheet plastic light
filter to enhance trace contrast; this also serves to protect the faceplate of the CRT.
Accuracy and resolution of measurements using a graticule is relatively limited; better
instruments sometimes have movable bright markers on the trace that permit internal
circuits to make more refined measurements.
Both calibrated vertical sensitivity and calibrated horizontal time are set in 1 - 2 - 5 - 10
steps. This leads, however, to some awkward interpretations of minor divisions. At 2,
each of the five minor divisions is 0.4, so one has to think 0.4, 0.8, 1.2, and 1.6, which is
rather awkward. One Tektronix plug-in used a 1 - 2.5 - 5 - 10 sequence, which simplified
estimating. The "2.5" didn't look as "neat", but was very welcome.
Timebase Controls
These select the horizontal speed of the CRT's spot as it creates the trace; this process is
commonly referred to as the sweep. In all but the least-costly modern oscilloscopes, the
sweep speed is selectable and calibrated in units of time per major graticule division.
Quite a wide range of sweep speeds is generally provided, from seconds to as fast as
picoseconds (in the fastest) per division. Usually, a continuously-variable control (often
a knob in front of the calibrated selector knob) offers uncalibrated speeds, typically
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slower than calibrated. This control provides a range somewhat greater than that of
consecutive calibrated steps, making any speed available between the extremes.
Holdoff control
Found on some better analog oscilloscopes, this varies the time (holdoff) during which
the sweep circuit ignores triggers. It provides a stable display of some repetitive events in
which some triggers would create confusing displays. It is usually set to minimum,
because a longer time decreases the number of sweeps per second, resulting in a dimmer
trace.
Vertical sensitivity, coupling, and polarity controls
To accommodate a wide range of input amplitudes, a switch selects calibrated
sensitivity of the vertical deflection. Another control, often in front of the calibratedselector knob, offers a continuously-variable sensitivity over a limited range from
calibrated to less-sensitive settings.
WT
Often the observed signal is offset by a steady component, and only the changes are of
interest. A switch (AC position) connects a capacitor in series with the input that passes
only the changes (provided that they are not too slow -- "slow" would mean visible).
However, when the signal has a fixed offset of interest, or changes quite slowly, the input
is connected directly (DC switch position). Most oscilloscopes offer the DC input option.
For convenience, to see where zero volts input currently shows on the screen, many
oscilloscopes have a third switch position (GND) that disconnects the input and grounds
it. Often, in this case, the user centers the trace with the Vertical Position control.
Better oscilloscopes have a polarity selector. Normally, a positive input moves the trace
upward, but this permits inverting—positive deflects the trace downward.
Horizontal sensitivity control
This control is found only on more elaborate oscilloscopes; it offers adjustable sensitivity
for external horizontal inputs.
Vertical position control
The vertical position control moves the whole displayed trace up and down. It is used to
set the no-input trace exactly on the center line of the graticule, but also permits offsetting
vertically by a limited amount. With direct coupling, adjustment of this control can
compensate for a limited DC component of an input.
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Horizontal position control
The horizontal position control moves the display sidewise. It usually sets the left end of
the trace at the left edge of the graticule, but it can displace the whole trace when desired.
This control also moves the X-Y mode traces sidewise in some instruments, and can
compensate for a limited DC component as for vertical position.
Dual-trace controls
Each input channel usually has its own set of sensitivity, coupling, and position controls,
although some four-trace oscilloscopes have only minimal controls for their third and
fourth channels.
Dual-trace oscilloscopes have a mode switch to select either channel alone, both
channels, or (in some) an X-Y display, which uses the second channel for X deflection.
When both channels are displayed, the type of channel switching can be selected on
some oscilloscopes; on others, the type depends upon timebase setting. If manually
selectable, channel switching can be free-running (asynchronous), or between
consecutive sweeps. Some Philips dual-trace analog oscilloscopes had a fast analog
multiplier, and provided a display of the product of the input channels.
WT
Multiple-trace oscilloscopes have a switch for each channel to enable or disable display
of that trace's signal.
Delayed-sweep controls
These include controls for the delayed-sweep timebase, which is calibrated, and often
also variable. The slowest speed is several steps faster than the slowest main sweep
speed, although the fastest is generally the same. A calibrated multiturn delay time
control offers wide range, high resolution delay settings; it spans the full duration of the
main sweep, and its reading corresponds to graticule divisions (but with much finer
precision). Its accuracy is also superior to that of the display.
A switch selects display modes: Main sweep only, with a brightened region showing
when the delayed sweep is advancing, delayed sweep only, or (on some) a combination
mode.
Good CRT oscilloscopes include a delayed-sweep intensity control, to allow for the
dimmer trace of a much-faster delayed sweep that nevertheless occurs only once per main
sweep. Such oscilloscopes also are likely to have a trace separation control for
multiplexed display of both the main and delayed sweeps together.
Sweep trigger controls
A switch selects the Trigger Source. It can be an external input, one of the vertical
channels of a dual or multiple-trace oscilloscope, or the AC line (mains) frequency.
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Another switch enables or disables Auto trigger mode, or selects single sweep, if
provided in the oscilloscope. Either a spring-return switch position or a pushbutton arms
single sweeps.
A Level control varies the voltage on the waveform which generates a trigger, and the
Slope switch selects positive-going or negative-going polarity at the selected trigger
level.
Basic types of sweeps
Triggered sweeps
WT
Type 465 Tektronix oscilloscope. This was a very popular analog oscilloscope, portable,
and is an excellent representative example.
To display events with unchanging or slowly (visibly) changing waveforms, but
occurring at times that may not be evenly spaced, modern oscilloscopes have triggered
sweeps. Compared to simpler oscilloscopes with sweep oscillators that are always
running, triggered-sweep oscilloscopes are markedly more versatile.
A triggered sweep starts at a selected point on the signal, providing a stable display. In
this way, triggering allows the display of periodic signals such as sine waves and square
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waves, as well as nonperiodic signals such as single pulses, or pulses that don't recur at a
fixed rate.
With triggered sweeps, the scope will blank the beam and start to reset the sweep circuit
each time the beam reaches the extreme right side of the screen. For a period of time,
called holdoff, (extendable by a front-panel control on some better oscilloscopes), the
sweep circuit resets completely and ignores triggers. Once holdoff expires, the next
trigger starts a sweep. The trigger event is usually the input waveform reaching some
user-specified threshold voltage (trigger level) in the specified direction (going positive
or going negative—trigger polarity).
In some cases, variable holdoff time can be really useful to make the sweep ignore
interfering triggers that occur before the events one wants to observe. In the case of
repetitive, but quite-complex waveforms, variable holdoff can create a stable display that
can't otherwise practically be obtained.
WT
Automatic sweep mode
Triggered sweeps can display a blank screen if there are no triggers. To avoid this, these
sweeps include a timing circuit that generates free-running triggers so a trace is always
visible. Once triggers arrive, the timer stops providing pseudo-triggers. Automatic sweep
mode can be de-selected when observing low repetition rates.
Recurrent sweeps
If the input signal is periodic, the sweep repetition rate can be adjusted to display a few
cycles of the waveform. Early (tube) oscilloscopes and lowest-cost oscilloscopes have
sweep oscillators that run continuously, and are uncalibrated. Such oscilloscopes are very
simple, comparatively inexpensive, and were useful in radio servicing and some TV
servicing. Measuring voltage or time is possible, but only with extra equipment, and is
quite inconvenient. They are primarily qualitative instruments.
They have a few (widely spaced) frequency ranges, and relatively wide-range continuous
frequency control within a given range. In use, the sweep frequency is set to slightly
lower than some submultiple of the input frequency, to display typically at least two
cycles of the input signal (so all details are visible). A very simple control feeds an
adjustable amount of the vertical signal (or possibly, a related external signal) to the
sweep oscillator. The signal triggers beam blanking and a sweep retrace sooner than it
would occur free-running, and the display becomes stable.
Single sweeps
Some oscilloscopes offer these—the sweep circuit is manually armed (typically by a
pushbutton or equivalent) "Armed" means it's ready to respond to a trigger. Once the
sweep is complete, it resets, and will not sweep until re-armed. This mode, combined
with a oscilloscope camera, captures single-shot events.
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Types of trigger include:
•
•
•
•
external trigger, a pulse from an external source connected to a dedicated input on
the scope.
edge trigger, an edge-detector that generates a pulse when the input signal crosses
a specified threshold voltage in a specified direction. These are the most-common
types of triggers; the level control sets the threshold voltage, and the slope control
selects the direction (negative or positive-going). (The first sentence of the
description also applies to the inputs to some digital logic circuits; those inputs
have fixed threshold and polarity response.)
video trigger, a circuit that extracts synchronizing pulses from video formats such
as PAL and NTSC and triggers the timebase on every line, a specified line, every
field, or every frame. This circuit is typically found in a waveform monitor
device, although some better oscilloscopes include this function.
delayed trigger, which waits a specified time after an edge trigger before starting
the sweep. As described under delayed sweeps, a trigger delay circuit (typically
the main sweep) extends this delay to a known and adjustable interval. In this
way, the operator can examine a particular pulse in a long train of pulses.
WT
Delayed sweeps
These are found on more-sophisticated oscilloscopes, which contain a second set of
timebase circuits for a delayed sweep. A delayed sweep provides a very-detailed look at
some small selected portion of the main timebase. The main timebase serves as a
controllable delay, after which the delayed timebase starts. This can start when the delay
expires, or can be triggered (only) after the delay expires. Ordinarily, the delayed
timebase is set for a faster sweep, sometimes much faster, such as 1000:1. At extreme
ratios, jitter in the delays on consecutive main sweeps degrades the display, but delayedsweep triggers can overcome that.
The display shows the vertical signal in one of several modes—the main timebase, or the
delayed timebase only, or a combination. When the delayed sweep is active, the main
sweep trace brightens while the delayed sweep is advancing. In one combination mode,
provided only on some oscilloscopes, the trace changes from the main sweep to the
delayed sweep once the delayed sweep starts, although less of the delayed fast sweep is
visible for longer delays. Another combination mode multiplexes (alternates) the main
and delayed sweeps so that both appear at once; a trace separation control displaces them.
Dual and multiple-trace oscilloscopes
Oscilloscopes with two vertical inputs, referred to as dual-trace oscilloscopes, are
extremely useful and commonplace. Using a single-beam CRT, they multiplex the inputs,
usually switching between them fast enough to display two traces apparently at once.
Less common are oscilloscopes with more traces; four inputs are common among these,
but a few (Kikusui, for one) offered a display of the sweep trigger signal if desired. Some
multi-trace oscilloscopes use the external trigger input as an optional vertical input, and
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some have third and fourth channels with only minimal controls. In all cases, the inputs,
when independently displayed, are time-multiplexed, but dual-trace oscilloscopes often
can add their inputs to display a real-time analog sum. (Inverting one channel provides a
difference, provided that neither channel is overloaded. This difference mode can provide
a moderate-performance differential input.)
Switching channels can be asynchronous, that is, free-running, with trace blanking while
switching, or after each horizontal sweep is complete. Asynchronous switching is usually
designated "Chopped", while sweep-synchronized is designated "Alt[ernate]". A given
channel is alternately connected and disconnected, leading to the term "chopped". Multitrace oscilloscopes also switch channels either in chopped or alternate modes.
In general, chopped mode is better for slower sweeps. It is possible for the internal
chopping rate to be a multiple of the sweep repetition rate, creating blanks in the traces,
but in practice this is rarely a problem; the gaps in one trace are overwritten by traces of
the following sweep. A few oscilloscopes had a modulated chopping rate to avoid this
occasional problem. Alternate mode, however, is better for faster sweeps.
WT
True dual-beam CRT oscilloscopes did exist, but were not common. One type (Cossor,
U.K.) had a beam-splitter plate in its CRT, and single-ended deflection following the
splitter. Beam-splitter types had horizontal deflection common to both vertical channels,
but dual-gun oscilloscopes could have separate time bases, or use one time base for both
channels. Multiple-gun CRTs (up to ten guns) were made in past decades. With ten guns,
the envelope (bulb) was cylindrical throughout its length.
The vertical amplifier
In an analog oscilloscope, the vertical amplifier acquires the signal[s] to be displayed. In
better oscilloscopes, it delays them by a fraction of a microsecond, and provides a signal
large enough to deflect the CRT's beam. That deflection is at least somewhat beyond the
edges of the graticule, and more typically some distance off-screen. The amplifier has to
have low distortion to display its input accurately (it must be linear), and it has to recover
quickly from overloads. As well, its time-domain response has to represent transients
accurately—minimal overshoot, rounding, and tilt of a flat pulse top.
A vertical input goes to a frequency-compensated step attenuator to reduce large signals
to prevent overload. The attenuator feeds a low-level stage (or a few), which in turn feed
gain stages (and a delay-line driver if there is a delay). Following are more gain stages,
up to the final output stage which develops a large signal swing (tens of volts, sometimes
over 100 volts) for CRT electrostatic deflection.
In dual and multiple-trace oscilloscopes, an internal electronic switch selects the
relatively low-level output of one channel's amplifiers and sends it to the following stages
of the vertical amplifier, which is only a single channel, so to speak, from that point on.
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In free-running ("chopped") mode, the oscillator (which may be simply a different
operating mode of the switch driver) blanks the beam before switching, and unblanks it
only after the switching transients have settled.
Part way through the amplifier is a feed to the sweep trigger circuits, for internal
triggering from the signal. This feed would be from an individual channel's amplifier in a
dual or multi-trace oscilloscope, the channel depending upon the setting of the trigger
source selector.
This feed precedes the delay (if there is one), which allows the sweep circuit to unblank
the CRT and start the forward sweep, so the CRT can show the triggering event. Highquality analog delays add a modest cost to a oscilloscope, and are omitted in oscilloscopes that are cost-sensitive.
The delay, itself, comes from a special cable with a pair of conductors wound around a
flexible magnetically-soft core. The coiling provides distributed inductance, while a
conductive layer close to the wires provides distributed capacitance. The combination is a
wideband transmission line with considerable delay per unit length. Both ends of the
delay cable require matched impedances to avoid reflections.
X-Y mode
WT
Most modern oscilloscopes have several inputs for voltages, and thus can be used to plot
one varying voltage versus another. This is especially useful for graphing I-V curves
(current versus voltage characteristics) for components such as diodes, as well as
Lissajous patterns. Lissajous figures are an example of how an oscilloscope can be used
to track phase differences between multiple input signals. This is very frequently used in
broadcast engineering to plot the left and right stereophonic channels, to ensure that the
stereo generator is calibrated properly. Historically, stable Lissajous figures were used to
show that two sine waves had a relatively simple frequency relationship, a numericallysmall ratio. They also indicated phase difference between two sine waves of the same
frequency.
Complete loss of signal in an X-Y display means that the CRT's beam strikes a small
spot, which risks burning the phosphor. Older phosphors burned more easily. Some
dedicated X-Y displays reduce beam current greatly, or blank the display entirely, if there
are no inputs present.
Bandwidth
Bandwidth is a measure of the range of frequencies that can be displayed; it refers
primarily to the vertical amplifier, although the horizontal deflection amplifier has to be
fast enough to handle the fastest sweeps. The bandwidth of the oscilloscope is limited by
the vertical amplifiers and the CRT (in analog instruments) or by the sampling rate of the
analog to digital converter in digital instruments. The bandwidth is defined as the
frequency at which the sensitivity is 0.707 of the sensitivity at lower frequency (a drop of
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3 dB). The rise time of the fastest pulse that can be resolved by the scope is related to its
bandwidth approximately:
Bandwidth in Hz x rise time in seconds = 0.35
For example, a oscilloscope intended to resolve pulses with a rise time of 1 nanosecond
would have a bandwidth of 350 MHz.
For a digital oscilloscope, a rule of thumb is that the continuous sampling rate should be
ten times the highest frequency desired to resolve; for example a 20 megasample/second
rate would be applicable for measuring signals up to about 2 megahertz.
Other features
WT
Some oscilloscopes have cursors, which are lines that can be moved about the screen to
measure the time interval between two points, or the difference between two voltages. A
few older oscilloscopes simply brightened the trace at movable locations. These cursors
are more accurate than visual estimates referring to graticule lines.
Better quality general purpose oscilloscopes include a calibration signal for setting up the
compensation of test probes; this is (often) a 1 kHz square-wave signal of a definite peakto-peak voltage available at a test terminal on the front panel. Some better oscilloscopes
also have a squared-off loop for checking and adjusting current probes.
Sometimes the event that the user wants to see may only happen occasionally. To catch
these events, some oscilloscopes, known as "storage scopes", preserve the most recent
sweep on the screen. This was originally achieved by using a special CRT, a "storage
tube", which would retain the image of even a very brief event for a long time.
Some digital oscilloscopes can sweep at speeds as slow as once per hour, emulating a
strip chart recorder. That is, the signal scrolls across the screen from right to left. Most
oscilloscopes with this facility switch from a sweep to a strip-chart mode at about one
sweep per ten seconds. This is because otherwise, the scope looks broken: it's collecting
data, but the dot cannot be seen.
In current oscilloscopes, digital signal sampling is more often used for all but the simplest
models. Samples feed fast analog-to-digital converters, following which all signal
processing (and storage) is digital.
Many oscilloscopes have different plug-in modules for different purposes, e.g., highsensitivity amplifiers of relatively narrow bandwidth, differential amplifiers, amplifiers
with four or more channels, sampling plugins for repetitive signals of very high
frequency, and special-purpose plugins, including audio/ultrasonic spectrum analyzers,
and stable-offset-voltage direct-coupled channels with relatively high gain.
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Examples of use
WT
Lissajous figures on an oscilloscope, with 90 degrees phase difference between x and y
inputs.
One of the most frequent uses of scopes is troubleshooting malfunctioning electronic
equipment. One of the advantages of a scope is that it can graphically show signals:
where a voltmeter may show a totally unexpected voltage, a scope may reveal that the
circuit is oscillating. In other cases the precise shape or timing of a pulse is important.
In a piece of electronic equipment, for example, the connections between stages (e.g.
electronic mixers, electronic oscillators, amplifiers) may be 'probed' for the expected
signal, using the scope as a simple signal tracer. If the expected signal is absent or
incorrect, some preceding stage of the electronics is not operating correctly. Since most
failures occur because of a single faulty component, each measurement can prove that
half of the stages of a complex piece of equipment either work, or probably did not cause
the fault.
Once the faulty stage is found, further probing can usually tell a skilled technician exactly
which component has failed. Once the component is replaced, the unit can be restored to
service, or at least the next fault can be isolated. This sort of troubleshooting is typical of
radio and TV receivers, as well as audio amplifiers, but can apply to quite-different
devices such as electronic motor drives.
Another use is to check newly designed circuitry. Very often a newly designed circuit
will misbehave because of design errors, bad voltage levels, electrical noise etc. Digital
electronics usually operate from a clock, so a dual-trace scope which shows both the
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clock signal and a test signal dependent upon the clock is useful. Storage scopes are
helpful for "capturing" rare electronic events that cause defective operation.
Pictures of use
WT
Heterodyne
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WT
AC hum on sound
Sum of a low-frequency and a high-frequency signal
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WT
Bad filter on sine
Dual trace, showing different time bases on each trace
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Selection
Oscilloscopes generally have a checklist of some set of the above features. The basic
measure of virtue is the bandwidth of its vertical amplifiers. Typical scopes for general
purpose use should have a bandwidth of at least 100 MHz, although much lower
bandwidths are acceptable for audio-frequency applications. A useful sweep range is
from one second to 100 nanoseconds, with triggering and delayed sweep.
The chief benefit of a quality oscilloscope is the quality of the trigger circuit. If the
trigger is unstable, the display will always be fuzzy. The quality improves roughly as the
frequency response and voltage stability of the trigger increase.
Analog oscilloscopes have been almost totally displaced by digital storage scopes except
for the low bandwidth (< 60 MHz) segment of the market. Greatly increased sample rates
have eliminated the display of incorrect signals, known as "aliasing", that was sometimes
present in the first generation of digital scopes. The used test equipment market,
particularly on-line auction venues, typically have a wide selection of older analog scopes
available. However it is becoming more difficult to obtain replacement parts for these
instruments and repair services are generally unavailable from the original manufacturer.
WT
As of 2007, a 350 MHz bandwidth (BW), 2.5 giga-samples per second (GS/s), dualchannel digital storage scope costs about US$7000 new. The current true real-time analog
bandwidth record, as of April 2010, is held by the Agilent Infiniium 90000X series of
oscilloscopes with a 32 GHz BW and a sample rate of 80 GSa/s. The current equivalent
time sampling bandwidth record for sampling digital storage oscilloscopes, as of June
2006, is held by the LeCroy WaveExpert series with a 100 GHz bandwidth.
On the lowest end, an inexpensive hobby-grade single-channel DSO can now be
purchased for under $100 as of August 2010. These often have limited bandwidth but
fulfill the basic functions of an oscilloscope.
Software
Many oscilloscopes today provide one or more external interfaces to allow remote
instrument control by external software. These interfaces (or buses) include GPIB,
Ethernet, serial port, and USB.
Types and models
The following section is a brief summary of various types and models available. For a
detailed discussion, refer to the other article.
Cathode-ray oscilloscope (CRO)
The earliest and simplest type of oscilloscope consisted of a cathode ray tube, a vertical
amplifier, a timebase, a horizontal amplifier and a power supply. These are now called
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'analog' scopes to distinguish them from the 'digital' scopes that became common in the
1990s and 2000s.
Dual-beam oscilloscope
The dual-beam analog oscilloscope can display two signals simultaneously. A special
dual-beam CRT generates and deflects two separate beams. Although multi-trace analog
oscilloscopes can simulate a dual-beam display with chop and alternate sweeps, those
features do not provide simultaneous displays. (Real time digital oscilloscopes offer the
same benefits of a dual-beam oscilloscope, but they do not require a dual-beam display.)
Analog storage oscilloscope
Trace storage is an extra feature available on some analog scopes; they used direct-view
storage CRTs. Storage allows the trace pattern that normally decays in a fraction of a
second to remain on the screen for several minutes or longer. An electrical circuit can
then be deliberately activated to store and erase the trace on the screen.
WT
Digital oscilloscopes
While analog devices make use of continually varying voltages, digital devices employ
binary numbers which correspond to samples of the voltage. In the case of digital
oscilloscopes, an analog-to-digital converter (ADC) is used to change the measured
voltages into digital information..
Digital storage oscilloscope
The digital storage oscilloscope, or DSO for short, is now the preferred type for most
industrial applications, although simple analog CROs are still used by hobbyists. It
replaces the unreliable storage method used in analog storage scopes with digital
memory, which can store data as long as required without degradation. It also allows
complex processing of the signal by high-speed digital signal processing circuits.
Digital sampling oscilloscopes
Digital sampling oscilloscopes operate on the same principle as analog sampling
oscilloscopes and like their analog partners, are of great use when analyzing high
frequency signals. That is, signals whose frequencies are higher than the oscilloscope's
sampling rate.
Digital phosphor oscilloscopes
Digital phosphor oscilloscopes (DPOs) are the most recently developed type of digital
scope. DPOs employ a unique processing architecture in order to overcome the
limitations of DSOs and digital sampling oscilloscopes. This unique architecture is a
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parallel processing setup rather than the serial processing setups of the other two types of
digital scopes.
Mixed-signal oscilloscopes
A mixed-signal oscilloscope (or MSO) has two kinds of inputs, a small number (typically
two or four) of analog channels, and a larger number (typically sixteen) of digital
channels.
Handheld oscilloscopes
Handheld oscilloscopes (also called scopemeters) are useful for many test and field
service applications. Today, a hand held oscilloscope is usually a digital sampling
oscilloscope, using a liquid crystal display.
WT
PC-based oscilloscopes (PCO)
A new type of "oscilloscope" is emerging that consists of a specialized signal acquisition
board (which can be an external USB or Parallel port device, or an internal add-on PCI or
ISA card).
Related instruments
A large number of instruments used in a variety of technical fields are really
oscilloscopes with inputs, calibration, controls, display calibration, etc., specialized and
optimized for a particular application. Examples of such oscilloscope-based instruments
include television waveform analyzers and medical devices such as vital function
monitors and electrocardiogram and electroencephalogram instruments. In automobile
repair, an ignition analyzer is used to show the spark waveforms for each cylinder. All of
these are essentially oscilloscopes, performing the basic task of showing the changes in
one or more input signals over time in an X-Y display.
Other instruments convert the results of their measurements to a repetitive electrical
signal, and incorporate an oscilloscope as a display element. Such complex measurement
systems include spectrum analyzers, transistor analyzers, and time domain reflectometers
(TDRs). Unlike an oscilloscope, these instruments automatically generate stimulus or
sweep a measurement parameter.
History
The Braun tube was known in 1897, and in 1899 Jonathan Zenneck equipped it with
beam-forming plates and a magnetic field for sweeping the trace. Early cathode ray tubes
had been applied experimentally to laboratory measurements as early as the 1920s,but
suffered from poor stability of the vacuum and the cathode emitters. V. K. Zworykin
described a permanently sealed, high-vacuum cathode ray tube with a thermionic emitter
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in 1931. This stable and reproducible component allowed General Radio to manufacture
an oscilloscope that was usable outside a laboratory setting.
WT
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Chapter 7
Atomic Clock
Atomic clock
WT
FOCS 1, a continuous cold caesium fountain atomic
clock in Switzerland, started operating in 2004 at an
uncertainty of one second in 30 million years
Classification
Clock
Industry
Telecommunications, Science
Application
Fuel Source
Powered
Inventor
Invented
GPS
Electricity
Yes
US National Bureau of
Standards
1949
The master atomic clock ensemble at the U.S. Naval Observatory in Washington D.C.,
which provides the time standard for the U.S. Department of Defense. The rack mounted
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units in the background are HP 5071A caesium beam clocks. The black units in the
foreground are Sigma-Tau MHM-2010 hydrogen maser standards.
An atomic clock is a clock that uses an electronic transition frequency in the microwave
optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency
standard for its timekeeping element. Atomic clocks are the most accurate time and
frequency standards known, and are used as primary standards for international time
distribution services, to control the frequency of television broadcasts, and in global
navigation satellite systems such as GPS.
The principle of operation of an atomic clock is not based on nuclear physics, but rather
on atomic physics and using the microwave signal that electrons in atoms emit when they
change energy levels. Early atomic clocks were based on masers at room temperature.
Currently, the most accurate atomic clocks first cool the atoms to near absolute zero
temperature by slowing them with lasers and probing them in atomic fountains in a
microwave-filled cavity. An example of this is the NIST-F1 atomic clock, the U.S.
national primary time and frequency standard.
WT
The accuracy of an atomic clock depends on the temperature of the sample atoms—
cooler atoms move more slowly, allowing longer probe times, as well as having reduced
collision rates—and on the frequency and intrinsic width of the electronic transition.
Higher frequencies and narrow lines increase precision.
National standards agencies maintain an accuracy of 10−9 seconds per day (approximately
1 part in 1014), and a precision set by the radio transmitter pumping the maser. These
clocks collectively define a continuous and stable time scale, International Atomic Time
(TAI). For civil time, another time scale is disseminated, Coordinated Universal Time
(UTC). UTC is derived from TAI, but approximately synchronized, by using leap
seconds, to UT1, which is based on actual rotations of the earth with respect to the solar
time.
History
The idea of using atomic transitions to measure time was first suggested by Lord Kelvin
in 1879. The practical method for doing this became magnetic resonance, developed in
the 1930s by Isidor Rabi. In 1945, Rabi first publicly suggested that atomic beam
magnetic resonance might be used as the basis of a clock. The first atomic clock was an
ammonia maser device built in 1949 at the U.S. National Bureau of Standards (NBS, now
NIST). It was less accurate than existing quartz clocks, but served to demonstrate the
concept. The first accurate atomic clock, a caesium standard based on a certain transition
of the caesium-133 atom, was built by Louis Essen in 1955 at the National Physical
Laboratory in the UK. Calibration of the caesium standard atomic clock was carried out
by the use of the astronomical time scale ephemeris time (ET). This led to the
internationally agreed definition of the latest SI second being based on atomic time.
Equality of the ET second with the (atomic clock) SI second has been verified to within 1
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part in 1010. The SI second thus inherits the effect of decisions by the original designers
of the ephemeris time scale, determining the length of the ET second.
WT
May 2009- JILA's strontium optical atomic clock is based on neutral atoms. Shining a
blue laser onto ultracold strontium atoms in an optical trap tests how efficiently a
previous burst of light from a red laser has boosted the atoms to an excited state. Only
those atoms that remain in the lower energy state respond to the blue laser, causing the
fluorescence seen here.
Since the beginning of development in the 1950s, atomic clocks have been based on the
hyperfine (microwave) transitions in hydrogen-1, caesium-133, and rubidium-87. The
first commercial atomic clock was the Atomichron, manufactured by the National
Company. More than 50 were sold between 1956 and 1960. This bulky and expensive
instrument was subsequently replaced by much smaller rack-mountable devices, such as
the Hewlett-Packard model 5060 caesium frequency standard, released in 1964.
In the late 1990s four factors contributed to major advances in clocks:
•
•
•
•
Laser cooling and trapping of atoms
So-called high-finesse Fabry–Pérot cavities for narrow laser line widths
Precision laser spectroscopy
Convenient counting of optical frequencies using optical combs
In August 2004, NIST scientists demonstrated a chip-scaled atomic clock. According to
the researchers, the clock was believed to be one-hundredth the size of any other. It was
also claimed that it requires just 75 mW, making it suitable for battery-driven
applications. This device could conceivably become a consumer product.
Mechanism
Since 1967, the International System of Units (SI) has defined the second as the duration
of 9192631770cycles of radiation corresponding to the transition between two energy
levels of the caesium-133 atom.
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This definition makes the caesium oscillator the primary standard for time and frequency
measurements, called the caesium standard. Other physical quantities, e.g., the volt and
the metre, rely on the definition of the second in their own definitions.
The actual time-reference of an atomic clock consists of an electronic oscillator operating
at microwave frequency. The oscillator is arranged so that its frequency-determining
components include an element that can be controlled by a feedback signal. The feedback
signal keeps the oscillator tuned in resonance with the frequency of the electronic
transition of caesium or rubidium.
The core of the atomic clock is a tunable microwave cavity containing the gas. In a
hydrogen maser clock the gas emits microwaves (the gas mases) on a hyperfine
transition, the field in the cavity oscillates, and the cavity is tuned for maximum
microwave amplitude. Alternatively, in a caesium or rubidium clock, the beam or gas
absorbs microwaves and the cavity contains an electronic amplifier to make it oscillate.
For both types the atoms in the gas are prepared in one electronic state prior to filling
them into the cavity. For the second type the number of atoms which change electronic
state is detected and the cavity is tuned for a maximum of detected state changes.
WT
Most of the complexity of the clock lies in this adjustment process. The adjustment tries
to correct for unwanted side-effects, such as frequencies from other electron transitions,
temperature changes, and the spreading in frequencies caused by ensemble effects. One
way of doing this is to sweep the microwave oscillator's frequency across a narrow range
to generate a modulated signal at the detector. The detector's signal can then be
demodulated to apply feedback to control long-term drift in the radio frequency. In this
way, the quantum-mechanical properties of the atomic transition frequency of the
caesium can be used to tune the microwave oscillator to the same frequency, except for a
small amount of experimental error. When a clock is first turned on, it takes a while for
the oscillator to stabilize. In practice, the feedback and monitoring mechanism is much
more complex than described above.
Historical accuracy of atomic clocks from NIST
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A number of other atomic clock schemes are in use for other purposes. Rubidium
standard clocks are prized for their low cost, small size (commercial standards are as
small as 400 cm3) and short-term stability. They are used in many commercial, portable
and aerospace applications. Hydrogen masers (often manufactured in Russia) have
superior short-term stability compared to other standards, but lower long-term accuracy.
Often, one standard is used to fix another. For example, some commercial applications
use a rubidium standard periodically corrected by a global positioning system receiver.
This achieves excellent short-term accuracy, with long-term accuracy equal to (and
traceable to) the U.S. national time standards.
The lifetime of a standard is an important practical issue. Modern rubidium standard
tubes last more than ten years, and can cost as little as US$50. Caesium reference tubes
suitable for national standards currently last about seven years and cost about US$35,000.
The long-term stability of hydrogen maser standards decreases because of changes in the
cavity's properties over time.
WT
Modern clocks use magneto-optical traps to cool the atoms for improved precision.
Physical package realizations
There exists a number of methods of utilizing the hyperfine splitting. These methods have
their benefits and draw-backs and have influenced the development of commercial
devices and laboratory standards. By tradition the hardware which is used to probe the
atoms is called the physical package.
Atomic beam standard
The atomic beam standard is a direct extension of the Stern-Gerlach atomic splitting
experiment. The atoms of choice are heated in an oven to create gas, which is collimated
into a beam. This beam passes through a state-selector magnet A, where atoms of the
wrong state are separated out from the beam. The beam is exposed to an RF field at or
near the transition. The beam then passes through a space before it is again exposed to the
RF field. The RF field and a static homogeneous magnetic field from the C-field coil will
change the state of the atoms. After the second RF field exposure the atomic beam passes
through a second state selector magnet B, where the atom state being selected out of the
beam at the A magnet is being selected. This way, the detected amount of atoms will
relate to the ability to match the atomic transition. After the second state-selector a massspectrometer using an ionizer will detect the rate of atoms being received.
Modern variants of this beam mechanism use optical pumping to transition all atoms to
the same state rather than dumping half the atoms. Optical detection using scintillation
can also be used.
The most common isotope for beam devices is caesium (133Cs), but rubidium (87Rb) and
thallium (205Tl) are examples of others used in early research.
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The frequency errors can be made very small for a beam device, or predicted (such as the
magnetic field pull of the C-coil) in such a way that a high degree of repeatability and
stability can be achieved. This is why an atomic beam can be used as a primary standard.
Atomic gas cell standard
The atomic gas cell standard builds on a confined reference isotope (often an alkali metal
such as Rubidium (87Rb)) inside an RF cavity. The atoms are excited to a common state
using optical pumping; when the applied RF field is swept over the hyperfine spectrum,
the gas will absorb the pumping light, and a photodetector provides the response. The
absorption peak steers the fly-wheel oscillator.
A typical rubidium gas-cell uses a rubidium (87Rb) lamp heated to 108-110 degrees
Celsius, and an RF field to excite it to produce light, where the D1 and D2 lines are the
significant wavelengths. An 85Rb cell filters out the D1 line so that only the D2 line
pumps the 87Rb gas cell in the RF cavity.
WT
Among the significant frequency pulling mechanisms inherent to the gas cell are wallshift, buffer-gas shift, cavity-shift and light-shift. The wall-shift occurs as the gas bumps
into the wall of the glass container. Wall-shift can be reduced by wall coating and
compensation by buffer gas. The buffer gas shift comes from the reference atoms which
bounce into buffer gas atoms such as neon and argon; these shifts can be both positive
and negative. The cavity shift comes from the RF cavity, which can deform the resonance
amplitude response; this depends upon cavity center frequency and resonator Q-value.
Light-shift is an effect where frequency is pulled differently depending on the light
intensity, which often is modulated by the temperature shift of the rubidium lamp and
filter cell.
There are thus many factors in which temperature and aging can shift frequency over
time, and this is why a gas cell standard is unfit for a primary standard, but can become a
very inexpensive, low-power and small-size solution for a secondary standard or where
better stability compared to crystal oscillators is needed, but not the full performance of a
caesium beam standard. The rubidium gas standards have seen use in telecommunications
systems and portable instruments.
Active maser standard
The active maser standard is a development from the atomic beam standard in which the
observation time was incremented by using a bounce-box. By controlling the beam
intensity spontaneous emission will provide sufficient energy to provide a continuous
oscillation, which is being tapped and used as a reference for a fly-wheel oscillator.
The active maser is sensitive to wall-shift and cavity pulling. The wall-shift is mitigated
by using PTFE coating (or other suitable coating) to reduce the effect. The cavity pulling
effect can be reduced by automatic cavity tuning. In addition the magnetic field pulls the
frequency.
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While not being long-term stable as caesium beams, it remains one of the most stable
sources available. The inherent pulling effects makes repeatability troublesome and does
prohibits its use as being primary standard, but it makes an excellent secondary standard.
It is used as low-noise fly-wheel standard for caesium beam standards.
Fountain standard
The fountain standard is a development from the beam standard where the beam has been
folded back to itself such that the first and second RF field becomes the same RF cavity.
A ball of atoms is laser cooled, which reduces black body temperature shifts. Phase errors
between RF cavities are essentially removed. The length of the beam is longer than many
beams, but the speed is also much slower such that the observation time becomes
significantly longer and hence a higher Q value is achieved in the Ramsay fringes.
WT
Caesium fountains has been implemented in many laboratories, but rubidium has even
greater ability to provide stability in the fountain configuration.
Ion trap standard
The ion trap standard is a set of different approaches, but their common property is that
atoms used in their ion form is confined in a electrostatic field and cooled down. The
hyperfine region of the available electron is then being tracked similar to that of a gas cell
standard.
Ion traps has been tried for numerous ions, where mercury 199Hg+ was an early candidate.
Power consumption
The power consumption of atomic clocks varies with their size. Chip scale atomic clocks
require power on the order of 100 mW; NIST-F1 uses power orders of magnitude greater.
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Research
WT
Chip-scale atomic clock unveiled by NIST
Most research focuses on the often conflicting goals of making the clocks smaller,
cheaper, more accurate, and more reliable.
New technologies, such as femtosecond frequency combs, optical lattices and quantum
information, have enabled prototypes of next generation atomic clocks. These clocks are
based on optical rather than microwave transitions. A major obstacle to developing an
optical clock is the difficulty of directly measuring optical frequencies. This problem has
been solved with the development of self-referenced mode-locked lasers, commonly
referred to as femtosecond frequency combs. Before the demonstration of the frequency
comb in 2000, terahertz techniques were needed to bridge the gap between radio and
optical frequencies, and the systems for doing so were cumbersome and complicated.
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With the refinement of the frequency comb these measurements have become much more
accessible and numerous optical clock systems are now being developed around the
world.
Like in the radio range, absorption spectroscopy is used to stabilize an oscillator—in this
case a laser. When the optical frequency is divided down into a countable radio frequency
using a femtosecond comb, the bandwidth of the phase noise is also divided by that
factor. Although the bandwidth of laser phase noise is generally greater than stable
microwave sources, after division it is less.
The two primary systems under consideration for use in optical frequency standards are
single ions isolated in an ion trap and neutral atoms trapped in an optical lattice. These
two techniques allow the atoms or ions to be highly isolated from external perturbations,
thus producing an extremely stable frequency reference.
WT
Optical clocks have already achieved better stability and lower systematic uncertainty
than the best microwave clocks. This puts them in a position to replace the current
standard for time, the caesium fountain clock.
Atomic systems under consideration include Al+, Hg+/2+, Hg, Sr, Sr+/2+, In+/3+, Ca, Ca+,
Yb+/2+/3+ and Yb.
Quantum clocks
In March 2008, physicists at NIST described a quantum logic clock based on individual
ions of beryllium and aluminium. This clock was compared to NIST's mercury ion clock.
These were the most accurate clocks that had been constructed, with neither clock
gaining nor losing time at a rate that would exceed a second in over a billion years. In
February 2010, NIST physicists described a second, enhanced version of the quantum
logic clock based on individual ions of magnesium and aluminium. Considered the
world's most precise clock, it offers more than twice the precision of the original.
Applications
The development of atomic clocks has led to many scientific and technological advances
such as a worldwide system of precise position measurement (Global Positioning
System), and applications in the Internet, which depend critically on frequency and time
standards. Atomic clocks are installed at sites of time signal radio transmitters. They are
used at some long wave and medium wave broadcasting stations to deliver a very precise
carrier frequency. Atomic clocks are used in many scientific disciplines, such as for longbaseline interferometry in radioastronomy.
Global Positioning System
The Global Positioning System (GPS) provides very accurate timing and frequency
signals. A GPS receiver works by measuring the relative time delay of signals from a
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minimum of three, but usually more GPS satellites, each of which has three or four
onboard caesium or rubidium atomic clocks. The relative times are mathematically
transformed into three absolute spatial coordinates and one absolute time coordinate. The
time is accurate to within about 50 nanoseconds. However, inexpensive GPS receivers
may not assign a high priority to updating the display, so the displayed time may differ
perceptibly from the internal time. Precision time references that use GPS are marketed
for use in computer networks, laboratories, and cellular communications networks, and
do maintain accuracy to within about 50ns.
Time signal radio transmitters
A radio clock is a clock that automatically synchronizes itself by means of government
radio time signals received by a radio receiver. Many retailers market radio clocks
inaccurately as atomic clocks; although the radio signals they receive originate from
atomic clocks, they are not atomic clocks themselves. They are inexpensive time-keeping
devices with an accuracy of about a second. Instrument grade time receivers provide
higher accuracy. Such devices incur a transit delay of approximately 1 ms for every 300
kilometres (186 mi) of distance from the radio transmitter. Many governments operate
transmitters for time-keeping purposes.
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Chapter 8
Breadboard
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A solderless breadboard with a completed circuit
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WT
This 1920s TRF radio manufactured by Signal is constructed on a wooden breadboard
A breadboard (protoboard) is a construction base for a one-of-a-kind electronic circuit,
a prototype. In modern times the term is commonly used to refer to a particular type of
breadboard, the solderless breadboard (plugboard).
Because the solderless breadboard does not require soldering, it is reusable, and thus can
be used for temporary prototypes and experimenting with circuit design more easily.
Other, often historic, breadboard types don't have this property. This is also in contrast to
stripboard (veroboard) and similar prototyping printed circuit boards, which are used to
build more permanent soldered prototypes or one-offs, and cannot easily be reused. A
variety of electronic systems may be prototyped by using breadboards, from small analog
and digital circuits to complete central processing units (CPUs).
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A binary counter wired up on a large solderless breadboard
The hole pattern for a typical etched prototyping PCB (printed circuit board) is similar to
the node pattern of the solderless breadboards shown above.
Evolution
In the early days of radio, amateurs would nail bare copper wires or terminal strips to a
wooden board (often literally a cutting board for bread) and solder electronic components
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to them. Sometimes a paper schematic diagram was first glued to the board as a guide to
placing terminals, then components and wires were installed over their symbols on the
schematic. Using thumbtacks or small nails as mounting posts was also common.
Over time, breadboards have evolved greatly, with the term being used for all kinds of
prototype electronic devices. For example, US Patent 3,145,483, filed in 1961 and
granted in 1964, describes a wooden plate breadboard with mounted springs and other
facilities. Six years later, US Patent 3,496,419, granted in 1970 after a 1967 filing, refers
to a particular printed circuit board layout as a Printed Circuit Breadboard. Both
examples also refer to and describe other types of breadboards as prior art.
The solderless breadboard
Typical specifications
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A modern solderless breadboard consists of a perforated block of plastic with numerous
tin plated phosphor bronze or nickel silver alloy spring clips under the perforations. The
clips are often called tie points or contact points. The number of tie points is often given
in the specification of a solderless breadboard.
The spacing between the clips (lead pitch) is typically 0.1" (2.54 mm). Integrated circuits
(ICs) in dual in-line packages (DIPs) can be inserted to straddle the centerline of the
block. Interconnecting wires and the leads of discrete components (such as capacitors,
resistors, inductors, etc.) can be inserted into the remaining free holes to complete the
circuit. Where ICs are not used, discrete components and connecting wires may use any
of the holes. Typically the spring clips are rated for 1 ampere at 5 volts and 0.333
amperes at 15 volts (5 watts).
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Bus and terminal strips
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Logical 4-bits adder where sums are linked to LEDs on a typical breadboard
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Example breadboard drawing. Two bus strips and one terminal strip in one block. 25
consecutive terminals in a bus strip connected (indicated by gaps in the red and blue
lines). Four binding posts depicted at the top.
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WT
Close-up of a solderless breadboard. An IC straddling the centerline is probed with an
oscilloscope probe. The solderless breadboard is mounted on a blue painted metal sheet.
Red and black binding posts are present. The black one partly obscured by the
oscilloscope probe.
Solderless breadboards are available from several different manufacturers, but most share
a similar layout. The layout of a typical solderless breadboard is made up from two types
of areas, called strips. Strips consist of interconnected electrical terminals.
terminal strips
The main area, to hold most of the electronic components.
In the middle of a terminal strip of a breadboard, one typically finds a notch
running in parallel to the long side. The notch is to mark the centerline of the
terminal strip and provides limited airflow (cooling) to DIP ICs straddling the
centerline. The clips on the right and left of the notch are each connected in a
radial way; typically five clips (i.e., beneath five holes) in a row on each side of
the notch are electrically connected. The five clip columns on the left of the notch
are often marked as A, B, C, D, and E, while the ones on the right are marked F,
G, H, I and J. When a "skinny" Dual Inline Pin package (DIP) integrated circuit
(such as a typical DIP-14 or DIP-16, which have a 0.3 inch separation between
the pin rows) is plugged into a breadboard, the pins of one side of the chip are
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supposed to go into column E while the pins of the other side go into column F on
the other side of the notch.
bus strips
To provide power to the electronic components.
A bus strip usually contains two columns: one for ground and one for a supply
voltage. However, some breadboards only provide a single-column power
distributions bus strip on each long side. Typically the column intended for a
supply voltage is marked in red, while the column for ground is marked in blue or
black. Some manufacturers connect all terminals in a column. Others just connect
groups of e.g. 25 consecutive terminals in a column. The latter design provides a
circuit designer with some more control over crosstalk (inductively coupled noise)
on the power supply bus. Often the groups in a bus strip are indicated by gaps in
the color marking.
Bus strips typically run down one or both sides of a terminal strip or between
terminal strips. On large breadboards additional bus strips can often be found on
the top and bottom of terminal strips.
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Some manufacturers provide separate bus and terminal strips. Others just provide
breadboard blocks which contain both in one block. Often breadboard strips or blocks of
one brand can be clipped together to make a larger breadboard.
In a more robust and slightly easier to handle variant, one or more breadboard strips are
mounted on a sheet of metal. Typically, that backing sheet also holds a number of
binding posts. These posts provide a clean way to connect an external power supply.
Diagram
A "full size" terminal breadboard strip typically consists of around 56 to 65 rows of
connectors, each row containing the above mentioned two sets of connected clips (A to E
and F to J). Together with bus strips on each side this makes up a typical 784 to 910 tie
point solderless breadboard. "Small size" strips typically come with around 30 rows.
Miniature solderless breadboards as small as 17 rows (no bus strips, 170 tie points) can
be found. These are more kind of a novelty item than of great practical use.
Jump wires
The jump wires for solderless breadboarding can be obtained in ready-to-use jump wire
sets or can be manually manufactured. The latter can become tedious work for larger
circuits. Ready-to-use jump wires come in different qualities, some even with tiny plugs
attached to the wire ends. Jump wire material for ready-made or home-made wires should
usually be 22 AWG (0.33 mm²) solid copper, tin-plated wire - assuming no tiny plugs are
to be attached to the wire ends. The wire ends should be stripped 3/16" to 5/16" (approx.
5 mm to 8 mm). Shorter stripped wires might result in bad contact with the board's spring
clips (insulation being caught in the springs). Longer stripped wires increase the
likelihood of short-circuits on the board. Needle-nose pliers and tweezers are helpful
when inserting or removing wires, particularly on crowded boards.
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Differently colored wires and color coding discipline are often adhered to for
consistency. However, the number of available colors is typically far less than the
number of signal types or paths. So typically a few wire colors get reserved for the supply
voltages and ground (e.g. red, blue, black), some more for main signals, while the rest
often get random colors. There are ready-to-use jump wire sets on the market where the
color indicates the length of the wires; however, these sets do not allow applying a
meaningful color coding schema.
Inside a breadboard: construction
The following images show the inside of a bus strip.
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inside breadboard 1
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inside breadboard 2
inside breadboard 3
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inside breadboard 4
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inside breadboard 5
inside breadboard 6
Advanced solderless breadboards
Some manufacturers provide high-end versions of solderless breadboards. These are
typically high-quality breadboard modules mounted on some flat casing. The casing
contains additional equipment for breadboarding, for example, a power supply, one or
more signal generators, serial interfaces, LED or LCD modules, logic probes, etc.
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Solderless breadboard modules can also be found mounted on devices like
microcontroller evaluation boards. They provide an easy way to add additional periphery
circuits to the evaluation board.
Limitations
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An example of a complex circuit built on a breadboard. The circuit is an Intel 8088 single
board computer.
Due to large stray capacitance (from 2-25 pF per contact point), high inductance of some
connections and a relatively high and not very reproducible contact resistance, solderless
breadboards are limited to operate at relatively low frequencies, usually less than
10 MHz, depending on the nature of the circuit. The relative high contact resistance can
already be a problem for DC and very low frequency circuits. Solderless breadboards are
further limited by their voltage and current ratings.
Solderless breadboards usually cannot accommodate surface mount technology devices
(SMD) or non 0.1" (2.54 mm) grid spaced components, like for example those with 2 mm
spacing. Further, they can not accommodate components with multiple rows of
connectors if these connectors don't match the dual in-line layout—it is impossible to
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provide the correct electrical connectivity. Sometimes small PCB adapters called
breakout adapters can be used to fit the component to the board. Such adapters carry one
or more components and have 0.1" (2.54 mm) connectors in a single in-line or dual inline layout. Larger components are usually plugged into a socket on the adapter, while
smaller components (e.g. SMD resistors) are usually soldered directly onto the adapter.
The adapter is then plugged into the breadboard via the 0.1" connectors. However, the
need to solder the components onto the adapter negates some of the advantage of using a
solderless breadboard.
Complex circuits can become unmanageable on a breadboard due to the large amount of
wiring necessary.
Alternatives
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Alternative methods to create prototypes are point-to-point construction, reminiscent of
the original breadboards, wire wrap, wiring pencil, and boards like stripboard. Complicated systems, such as modern computers comprising millions of transistors, diodes
and resistors, do not lend themselves to prototyping using breadboards, as sprawling
designs on breadboards can be difficult to lay out and debug. Modern circuit designs are
generally developed using a schematic capture and simulation system, and tested in
software simulation before the first prototype circuits are built on a printed circuit board.
Integrated circuit designs are a more extreme version of the same process: since
producing prototype silicon is expensive, extensive software simulations are performed
before fabricating the first prototypes. However, prototyping techniques are still used for
some applications such as RF circuits, or where software models of components are
inexact or incomplete.
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Chapter 9
Pliers
WT
Flat-nose pliers
Pliers are a hand tool used to hold objects firmly, for cutting, bending, or physical
compression. Generally, pliers consist of a pair of metal first-class levers joined at a
fulcrum positioned closer to one end of the levers, creating short jaws on one side of the
fulcrum, and longer handles on the other side. This arrangement creates a mechanical
advantage, allowing the force of the hand's grip to be amplified and focused on an object
with precision. The jaws can also be used to manipulate objects too small or unwieldy to
be manipulated with the fingers.
There are many kinds of pliers made for various general and specific purposes.
History
As pliers in the general sense are an ancient and simple invention, no single point in
history, or single inventor, can be credited. Early metal working processes from several
millennia BCE would have required plier-like devices to handle hot materials in the
process of smithing or casting. Development from wooden to bronze pliers would have
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probably happened sometime prior to 3000 BCE. Among the oldest illustrations of pliers
are those showing the Greek god Hephaestus in his forge. Today, pliers intended
principally to be used for safely handling hot objects are usually called tongs. The
number of different designs of pliers grew with the invention of the different objects
which they were used to handle: horseshoes, fasteners, wire, pipes, electrical and
electronic components.
Design
The basic design of pliers has changed little since their origins, with the pair of handles,
the pivot (often formed by a rivet), and the head section with the gripping jaws or cutting
edges forming the three elements. In distinction to a pair of scissors or shears, the plier's
jaws always meet each other at one pivot angle.
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The materials used to make pliers consist mainly of steel alloys with additives such as
vanadium or chromium, to improve strength and prevent corrosion. Often pliers have
insulated grips to ensure better handling and prevent electrical conductivity. In some lines
of fine work (such as jewellery or musical instrument repair), some specialized pliers
feature a layer of comparatively soft metal (such as brass) over the two plates of the head
of the pliers to reduce pressure placed on some fine tools or materials. Making entire
pliers out of softer metals would be impractical, reducing the force required to bend or
break them.
Common types
Needle-nose pliers
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Locking pliers
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Gripping pliers
Lineman's pliers
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Lineman's pliers
Lineman's pliers (US English), also called combination pliers and commonly referred
to in the USA and Canada as Kleins after Klein Tools, are a type of pliers used by
electricians and other tradesmen for gripping small objects, to cut and bend wire and
cable. Lineman's pliers have a gripping joint at their snub nose, and cutting edge in their
craw, and insulating handle grips that reduce (but do not eliminate) the risk of electric
shock from contact with live wires (versions with properly tested and guaranteed
insulation in two colors to make faults visible are also available). Some versions include
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either an additional gripping or crimping device at the crux of the handle side of the
pliers' joint. Lineman's pliers typically are machined from forged steel and the two
handles precisely joined with a heavy-duty rivet that maintains the pliers' accuracy even
after repeated use under extreme force on heavy-gauge wire.
Durability
Lineman's pliers owe their effectiveness to the rigid accuracy of their closing (cutting
/gripping) action, and to the durable, forged steel from which they are machined.
Although the cutting edge may effectively dull with prolonged use or misuse (for
example, cutting large steel screws or wire, cutting live wires that electrically short and
melt the tool's cutting edges), this tool is otherwise virtually indestructible because it does
not depend on a knife-sharp edge, only a 'breaking' edge.
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Typical uses
Cutting
Lineman's (or Linesmans') pliers are an essential item in the electrician's tool complement. They cut, bend, and may be used to strip wire insulation or cable jackets. As with
most pliers and scissors or shears, lineman's pliers apply most force closest to the pivotpoint of the two handles, so for larger materials, the closer one can get the wire or cable
to the joint or 'craw' of the pliers, the easier and cleaner the cut will be. Premium
Lineman's pliers may have the pivot point moved closer to the cutting edge in order to
produce much greater cutting force.
Bending/Straightening
Lineman's pliers can be used to bend or straighten heavy- gauge solid wire or sheet metal
components, especially in cases where smaller pliers don't offer enough mechanical
advantage. The square nose and flat side of Lineman pliars is particularly useful for
creating accurate right angle bends.
Cutting metal-clad (MC) cable
A rotosplit is the ideal tool for this job, but lineman's pliers can be used to first 'crack' the
spiral casing of the cable by bending it sharply, partially exposing the insulated wires,
inside. This creates a place for the pliers to gain purchase, and, with the application of
strong force with two hands, they will cut the cable. To strip the cable, saw through one
wrap of the spiral metal casing using a metal-cutting saw blade (for example, on a hack
saw or powered reciprocating saw) and then use two pliers to twist the casing sharply and
break apart the sections on either side of the saw cut. If no saw or rotosplit is available, it
is possible (though laborious) to use lineman's pliers to grasp the end of the cable and
unwind 12 inches of stiffly-spiralled aluminum to expose the wire inside.
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Gripping
The most common application of the lineman's pliers in gripping is to twist bare
(stripped) wires together, to form a common electrical connection between the wires
(wire nuts can be used to enhance this electrical connection and guard against corrosion
of the contact-points between wires, as well as to insulate the bared wire ends and
provide additional mechanical 'locking' of the junction). The gripping action of lineman's
pliers is also used to pull fish-tape ends in a long (high-friction) wire run through conduit,
to crimp soft metals, or to pull nails and other fasteners.
Other functions
Lineman's pliers are similar to needle-nose pliers: both tools share a typically solid,
machined forged steel construction, durable pivot, gripping nose and cutting craw. The
main differences are that the slender nose of the needle-nose pliers enable it to form small
diameter bends, and position or support items in awkward places. Needle-nose pliers
typically have a lower handle/nose length ratio, reducing the force that can be exerted at
the tip. Also, needle-nose pliers tend be available in smaller sizes (for electronics
applications, they may be found as small as 1/10 scale of the full-size version).
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Lineman's pliers may be used to cut steel screws up to #10, and virtually any dry-wall
screw. Although, unlike some multi-purpose wire-stripping pliers, lineman's pliers will
not always maintain a clean thread-continuity after the cutting, drywall screws typically
will still function in drywall or soft woods such as those used in light-frame construction;
driving the screw in reverse with moderate pressure will 'drill' a starter-hole, allowing the
remaining threads of a cut screw to engage and draw the screw in normally. A machine
screw cut by linemans' pliers may function properly about 60% of the time.
Lineman's pliers sometimes include an integrated crimping device in the craw of the
handle side of the pliers' joint. The nose-end grippers of lineman's pliers are designed
come about 1/16" short of positive contact, when the pliers are fully closed. The pliers'
gripping end may be used to squeeze soft metal flat, and function well as a crimper in
some applications.
Lineman's pliers have a tapered nose suitable for reaming the rough edge of a 1/2" or
larger conduit, or cleaning sharp metal from the inside of a standard metal knock-out in
an electrical enclosure such as a junction box or breaker panel. Some brands(Ideal)
manufacture pliers with a narrower jaw, suitable for reaming smaller conduit.
Professional Lineman's pliers are quite rugged and though not rated for striking, they are
quite often used to sink(but not set) concrete inserts, pound nails, or chip small bits of
concrete. Sometimes nicknamed an electrician's hammer.
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Round-nose pliers
Round nose pliers are a type of specialist hand tool that are most commonly used for
creating loops in pieces of wire. They are generally used by electricians and jewellery
makers and are characterised by their rounded, tapering jaws.
There are a variety of round nose pliers in existence today. Some feature insulated
handles for safe electrical work, with other features including spring fitted joints and
comfort grips for ease of use.
Round nose pliers are sometimes referred to as rosary pliers and snub-nose pliers.
Gallery
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Round nose pliers
Round nose pliers
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Needle-nose pliers
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Needle-nose pliers
Needle-nose pliers (also known as long-nose pliers, pinch-nose pliers, or snipe-nose
pliers) are both cutting and gripping pliers used by electricians and other tradesmen to
bend, re-position and cut wire. Their namesake long gripping nose provides excellent
control and reach for fine work in small or crowded electrical enclosures, while cutting
edges nearer the pliers' joint provide "one-tool" convenience. Given their long shape, they
are useful for reaching into cavities where cables (or other materials) have become stuck
or unreachable to fingers or other means.
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Locking pliers
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Locking pliers
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Small locking pliers holding a pencil
Locking pliers, Mole grips (Mole wrench) or Vise-Grips are pliers that can be locked
into position, using an over-center action. One side of the handle includes a bolt that is
used to adjust the spacing of the jaws, the other side of the handle (especially in larger
models) often includes a lever to push the two sides of the handles apart to unlock the
pliers. "Mole" and "Vise-Grip" are trade names of different brands of locking pliers.
Locking pliers are available in many different configurations, such as needle-nose
locking pliers, locking wrenches, locking clamps and various shapes to fix metal parts for
welding. They also come in many sizes. Leatherman manufactures a multitool, the
Leatherman Crunch, which includes locking pliers instead of regular pliers, and which
folds together to the size of other multitools.
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Operation
The jaws are set to a size slightly smaller than the diameter of what is to be gripped by
turning the bolt in one handle with the jaws closed. When the jaws are opened and the
handles squeezed together, they move a lever over its center point and lock the jaw of the
pliers onto the gripped object. A typical usage would be to hold metal parts in place for
welding.
History
The first locking pliers, named Vise-Grips, were invented by William Petersen in De
Witt, Nebraska in 1924. Mole grips were developed by Thomas Coughtrie in 1955, then
managing director of M. K. Mole and Son.
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Tongue-and-groove pliers
Tongue-and-groove pliers, also known as water pump pliers, adjustable pliers,
groove-joint pliers, and Channellocks, are a type of slip-joint pliers. They have serrated
jaws generally set 45– to 60-degrees from the handles. The lower jaw can be moved to a
number of positions by sliding along a tracking section under the upper jaw. An
advantage of this design is that the pliers can adjust to a number of sizes without the
distance in the handle growing wider. These pliers often have long handles—commonly
9.5 to 12 inches long—for increased leverage.
Tongue-and-groove pliers are commonly used for turning and holding nuts and bolts,
gripping irregularly shaped objects, and clamping materials.
This design of pliers was invented and popularized by the Champion–DeArment Tool
Company in 1934 under the brand name Channellock (after which the company would
eventually take its name) but are also now produced by a number of other manufacturers.
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Gallery
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Special purpose pliers
Circlip
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This diagram illustrates the removal of a snap ring from the rear hub of a bicycle, on
which it is used to retain a single rear sprocket
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Internal circlip
A circlip (a combination of 'circle' and 'clip', and pronounced thus), also known as a CClip, snap ring or Jesus clip, is a type of fastener consisting of a semi-flexible metal
ring with open ends which can be snapped into place, into a machined groove on a dowel
pin or other part to permit rotation but to prevent lateral movement. There are two basic
types: internal and external, referring to whether they are fitted into a tube or over a shaft.
Circlips are often used to secure pinned connections.
Details
WT
The clips are known as wrist pin clips or wrist pin retainers when used to keep retain
piston wrist pins.
Common examples include "E-clips" (e-ring) and the (both internal and external) snap
ring or circlip. This general type of fasteners are sized to provide an interference fit onto
(or into, in the case of an internal fastener) a groove or land when in use, such that they
must be elastically deformed in order to install or remove them.
Installation and lubrication
As they are stamped out of sheetmetals there is a smooth side and a rough side. Install the
circlip with the smooth side facing the part and the rough side facing out. (The rough
edge could dig into the part enough to be pushed off the position by the part's motion.)
This is particularly important when the circlip is installed directly against
rotating(moving) part. Wet or dry lubrication is required for the circlip to maintain its
function.
Circlips which are fitted may be removed with a pair of needle-nosed pliers or a special
snap ring tool if the circlip is designed to include entry points for the pliers or tool.
Alternatively, cautious leverage with a flat-headed screwdriver may be necessary in lieu
of the correct tools or design of snap-ring.
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Breaker-grozier pliers
Breaker-grozier pliers
WT
Breaker-grozier pliers are pliers used by glaziers to break and finish glass in a
controlled manner. They are dual purpose pliers, with a flat jaw that's used for breaking
out scores and a curved jaw that's used for grozing flares from the edge of broken glass.
Both jaws are serrated for removing flares and tiny points of glass.
To break out a score, the glass to be removed is held firmly in the pliers, with the flat jaw
on top of the glass near the score line. A sharp bend downward breaks the glass at the
score.
To remove unwanted glass flares and unwanted points , the glass piece is held with one
hand with the pliers curved side up. A gentle upward rolling scrapes the glass edge
(grozz) against the serrated teeth removes unwanted glass flares. This removal of grozz is
known as grozing.
The tips of these pliers can also be used in a chewing motion to remove small sections of
glass or nibble out deep inside curves.
Breaker/grozier pliers are a combination tool. Glaziers also use single-purpose pliers such
as breaking pliers, with two flat jaws, and running pliers, which apply even pressure on
both sides of a score to make a controlled gentle break on the glass.
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Adjustable pliers
Slip joint pliers
WT
Common, two-position straight slip joint pliers
Slip joint pliers are pliers whose pivot point or fulcrum can be moved to increase the
size range of their jaws. Most slip joint pliers use a mechanism that allows sliding the
pivot point into one of several positions when the pliers are fully opened.
Varieties
Straight slip joint pliers
Straight slip joint pliers are configured similarly to common or lineman's pliers in that
their jaws are in line with their handles. One side of the pliers usually has two holes that
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are connected by a slot for the pivot. The pivot is fastened to the other side and shaped
such that it can slide through the slot when the pliers are fully opened.
Tongue-and-groove pliers
Tongue-and-groove pliers have their jaws offset from their handles and have several
positions at which the lower jaw can be positioned.
Cutting pliers
Diagonal pliers
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Diagonal pliers with uninsulated handles
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Diagonal pliers (or wire cutters or diagonal cutting pliers) are pliers intended for the
cutting of wire (they are generally not used to grab or turn anything). They are sometimes
called side cutting pliers or side cutters, although these terms are shared by other pliers
designs, such as lineman's pliers, and may lead to confusion. The plane defined by the
cutting edges of the jaws intersects the joint rivet at an angle or "on a diagonal", hence
the name. Instead of using a shearing action as with scissors, they cut by indenting and
wedging the wire apart. The jaw edges are ground to a symmetrical "V" shape; thus the
two jaws can be visualized to form the letter "X", as seen end-on when fully occluded.
The pliers are made of tempered steel and inductive heating and quenching are often used
to selectively harden the jaws.
Diags or Dikes (a portmanteau of "Diagonal CutterS" is pronounced "dikes") – as in the
phrase "a pair of dikes" or "hand me those dikes" – is jargon used especially in the
electrical industry, to describe diagonal pliers. Dike can also be used as a verb, such as in
the idiom "when in doubt, dike it out". This jargon has largely fallen out of use due to
confusion with the semi-derogatory Dyke (slang).
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In the United Kingdom and Ireland, diagonal pliers are commonly referred to as snips,
and in Australia and Canada they are often referred to as side cutters.
Applications
The handles of diagonal cutting pliers are commonly insulated with a dip-type or shrink
fit electrically-insulating material for comfort and some protection against electric shock.
However, only tools that are specifically rated for safe live line work should be used on
known live wires.
Diagonal pliers are useful for cutting copper, brass, iron, aluminium and steel wire.
Lower quality versions are generally not suitable for cutting tempered steel, such as piano
wire, as the jaws are not hard enough. Attempting to cut such material will usually cause
indentations to be made in the jaws, or a piece to break out of one or both jaws, thus
ruining the tool. However higher quality side cutters can cut hardened steel, such as
2 mm piano wire.
For electronics work, special diagonal cutters that are ground flush to the apex of the
cutting edge on one side of the jaws are often used. These flush-cutting pliers allow wires
to be trimmed flush or nearly flush to a solder joint, avoiding the sharp tip left by
symmetrical diagonal cutters. It is common for this type of diagonal cutter to be referred
to by another name, such as "flush cutter" to distinguish it from symmetrical cutters.
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Needle-nose pliers
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Needle-nose pliers
Needle-nose pliers (also known as long-nose pliers, pinch-nose pliers, or snipe-nose
pliers) are both cutting and gripping pliers used by electricians and other tradesmen to
bend, re-position and cut wire. Their namesake long gripping nose provides excellent
control and reach for fine work in small or crowded electrical enclosures, while cutting
edges nearer the pliers' joint provide "one-tool" convenience. Given their long shape, they
are useful for reaching into cavities where cables (or other materials) have become stuck
or unreachable to fingers or other means.
Rotational pliers
•
Developed by NASA engineers to enable an astronaut to turn a nut in
weightlessness. The linear motion of the hand is converted to rotational motion to
drive a socket wrench.
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Gallery
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Slip joint pliers
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Diagonal pliers or side cutters
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Lineman's pliers or combination pliers
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Pincers
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Electrical wire stripping and terminal crimping pliers
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Crimptool for N, R-SMA, TNC connectors for RG174, RG58 and HDF/LMR200
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Heavy duty crimping pliers with interchangeable RJ heads
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Hand crimp tool
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Hand crimp tool for insulated terminals and non-insulated terminals; also has a wire
cutter and stripper and screw cutters
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Chapter 10
Semiconductor Curve Tracer
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The Type 575 Transistor-Curve Tracer displays the dynamic characteristic curves of both
NPN and PNP transistors on the screen of a 5-inch cathode-ray tube. Several different
transistor characteristic curves may be displayed, including the collector family in the
common-base and common emitter configuration. In addition to the transistor
characteristic curves, the Type 575 is used to display dynamic characteristics of a wide
range of semiconductor devices." (Tektronix, Catalog, 1967)
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A semiconductor curve tracer is a specialised piece of electronic test equipment used to
analyse the characteristics of discrete semiconductor devices such as diodes, transistors,
and thyristors. Based on an oscilloscope, the device also contains voltage and current
sources that can be used to stimulate the device under test (DUT).
Operation
The basic operating principle of the device is to apply a swept (automatically varying)
voltage to the main terminals of the DUT while measuring the amount of current that the
device permits to flow. This so-called V-I (voltage versus current) graph is displayed on
an oscilloscope screen. The operator can control the maximum amount of voltage applied
to the device, the polarity of the voltage applied (including the automatic application of
both positive and negative polarities), and the load resistance inserted in series with the
device.
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For two terminal devices (such as diodes and DIACs), this is sufficient to fully
characterize the device. The curve tracer can display all of the interesting parameters such
as the diode's forward voltage, reverse leakage current, reverse breakdown voltage, and
so on. For triggerable devices such as DIACs, the forward and reverse trigger voltages
will be clearly displayed. The discontinuity caused by negative resistance devices (such
as tunnel diodes) can also be seen.
The main terminal voltage can often be swept up to several thousand volts with load
currents of tens of amps available at lower voltages.
Three-terminal devices require an additional connection; this is usually supplied from a
stepped voltage or current source attached to the control terminal of the DUT. By
sweeping through the full range of main terminal voltages with each step of the control
signal, a family of V-I curves can be generated. This family of curves makes it very easy
to determine the gain of a transistor or the trigger voltage of a thyristor or TRIAC. For
most devices, a stepped current is used. For field effect transistors, a stepped voltage is
used instead.
Curve tracers usually contain convenient connection arrangements for two- or threeterminal DUTs, often in the form of sockets arranged to allow the plugging-in of the
various common packages used for transistors and diodes. Most curve tracers also allow
the simultaneous connection of two DUTs; in this way, two DUTs can be "matched" for
optimum performance in circuits (such as differential amplifiers) which depend upon the
close matching of device parameters. This can be seen in the image to the right where a
toggle switch allows the rapid switching between the DUT on the left and the DUT on the
right as the operator compared the respective curve families of the two devices.
Kelvin sensing
Curve tracers are usually supplied with various semiconductor device test fixture adapters
that have Kelvin sensing.
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History
The original semiconductor curve tracers were originally based on vacuum tube circuits.
The Scientific Test, Inc. Curve Tracer is unique in its differences from the Tektronix
Model. The Tektronix model 575 curve tracer shown in the gallery was a typical
instrument of this generation. At this time, the available semiconductor devices simply
didn't have enough performance to generate the test waveforms adequate to characterise
themselves.
Nowadays, curve tracers are entirely solid state (with the possible exception of the CRT)
and are substantially automated to ease the workload of the operator, automatically
capture data, and assure the safety of the curve tracer as well as the DUT.
List of curve tracer models
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ABI Electronics Ltd
•
•
•
CircuitMaster 4000M
Counterfeit IC Detector
System 8 Range of V-I Testers
Agilent Technologies
•
B1505A
B&K Precision
•
501A
Heathkit
•
•
IT-1121
IT-3121
Leader Electronics
•
LTC-905
Scientific Test, Inc.
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Series 5000
Tektronix
•
570 - vacuum tube curve tracer
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•
•
•
575
576
577, 577D1, 577D2
Telequipment
•
CT71
Sony/Tektronix
•
•
370, 370A and 370B
371, 371A and 371B (high power)
Availability
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The Agilent B1505A is a new product that is orderable starting April 1, 2009.
Scientific Test, Inc. based in Richardson, TX has curve tracers available in current
production with ongoing support. Scientific Test, Inc. has details of products at
This Tektronix page has details of the 370B and 371B models. Both models have been
discontinued.
Reconditioned curve tracers may still be obtained from various suppliers of used test
equipment.
Safety
Curve tracers are capable of generating lethal voltages and currents and so pose an
electrocution hazard for the operator. Modern curve tracers often contain mechanical
shields and interlocks that make it more difficult for the operator to come into contact
with hazardous voltages or currents. The DUTs can also become hot during operation,
posing a burn hazard to the operator.
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Transistor-curve tracer (detail 1)
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Transistor-curve tracer (detail 2)
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Transistor-curve tracer (parts)
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Chapter 11
Soldering Iron
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A gas fired soldering iron
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Electric soldering iron
A soldering iron is a hand tool most commonly used in soldering. It supplies heat to melt
the solder so that it can flow into the joint between two workpieces.
A soldering iron is composed of a heated metal tip and an insulated handle. Heating is
often achieved electrically, by passing an electric current (supplied through an electrical
cord or battery cables) through the resistive material of a heating element. Another
heating method includes combustion of a suitable gas, which can either be delivered
through a tank mounted on the iron (flameless), or through an external flame.
Less common uses include pyrography (burning designs into wood) and plastic welding.
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Types
Electronics
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Soldering electronic components
For electrical work a low-power iron, a power rating between 15 and 30 watts, is used.
Higher ratings are available, but do not run hotter; instead there is more power available
for larger joints.
Small battery-operated or gas soldering irons are useful when electricity is unavailable.
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Temperature-controlled soldering station
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Temperature-controlled soldering station
A temperature-controlled soldering station consists of an electrical power supply and a
soldering iron. It is most commonly used for soldering electronic components.
A variety of means are used to control temperature. The simplest of these is a variable
power control, much like a light dimmer, which together with the loss of heat from the
iron to the environment roughly sets the temperature. Another type of system uses a
thermostat, often inside the iron's tip, which switches power on and off to the elements. A
more advanced version of this uses a microprocessor to monitor the temperature of the tip
via a thermocouple and adjusts the power to the heating element accordingly.
Another approach to use magnetized soldering tips which lose their magnetic properties
at a certain temperature (the Curie point). As long as the tip is magnetic, it clings to the
heating element. At the design temperature, it loses contact, cooling down. Other
complex irons circulate a high-frequency AC current through the tip, using magnetic
physics to direct heating only where the surface of the tip drops below the Curie point.
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Stands
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Soldering iron stand
A soldering iron stand keeps the iron away from flammable materials, and often also
comes with a cellulose sponge and flux pot for cleaning the tip. Some soldering irons for
continuous and professional use come as part of a soldering station, which allows the
exact temperature of the tip to be adjusted, kept constant, and sometimes displayed.
Tips
Some soldering irons have interchangeable tips, also known as bits, that vary in size and
shape for different types of work. Pyramid tips with a triangular flat face and chisel tips
with a wide flat face are useful for soldering sheet metal. Fine conical or tapered chisel
tips are typically used for electronics work.
Older and very cheap irons typically use a bare copper tip, which is shaped with a file or
sandpaper. This dissolves gradually into the solder, suffering pitting and erosion of the
shape. Copper tips are sometimes filed when worn down. Iron plated copper tips have
become increasingly popular since the 1980s. Because iron is not readily dissolved by
molten solder, the plated tip is more durable than a bare copper one. This is especially
important when working at the higher temperatures needed for modern lead-free solders.
Solid iron & steel tips are seldom used, they store less heat and rusting can break the
heating element.
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Cleaning
When burnt flux and oxidized material begin to accumulate on the tip, they can block
heat transfer and contaminate joints, making soldering difficult or impossible. Many
soldering stations come with cellulose sponges which are dampened and used to wipe a
hot iron's tip clean. Wet denim performs the same job but may wear down the tip. A wire
brush or wire wheel (mounted on a bench grinder) is sometimes carefully used to remove
very severe oxidation, though this may risk damaging the tip's protective iron plating. A
small amount of fresh solder is usually then applied to the clean tip in a process called
tinning. The working surface of the tip is usually kept tinned (coated with wet solder) to
minimize oxidation. Oxidation blocks heat transfer, corrodes the tip and contaminates the
joint.
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