XYZs of Oscilloscopes - Department of Electrical, Computer, and

Primer
XYZs of Oscilloscopes
Analog Oscilloscope
Delay Line
Vert
Amp
Display
Amp
Trigger
Horiz
Amp
Digital Storage Oscilloscope
Amp
A/D
Acquisition
Memory
DeMux
µP
Display
Memory
Display
Digital Phosphor Oscilloscope
DPX
Digital
Phosphor
Amp
A/D
Acquisition
Rasterizer
Display
Memory
Display
µP
Copyright © 2000 Tektronix, Inc. All rights reserved.
X Y Zs of Oscilloscopes
Introduction
The oscilloscope is an essential tool if you plan to design or repair electronic
equipment. It lets you “see” electrical signals.
Energy, vibrating particles, and other invisible forces are everywhere in our
physical universe. Sensors can convert these forces into electrical signals that
you can observe and study with an oscilloscope. Oscilloscopes let you “see”
events that occur in a split second.
Why Read This Book?
If you are a scientist, engineer, technician, or electronics hobbyist, you should
know how to use an oscilloscope. The concepts presented here provide you
with a good starting point.
If you are using an oscilloscope for the first time, read this book to get a solid
understanding of oscilloscope basics. Then, read the manual provided with
your oscilloscope to learn specific information about how to use it in your
work. After reading this book, you will be able to:
• Describe how oscilloscopes work
• Describe the difference between analog, digital storage, and digital phosphor
oscilloscopes
• Describe electrical waveform types
• Understand basic oscilloscope controls
• Take simple measurements
If you come across an unfamiliar term in this book, check the glossary in the
back for a definition.
This book serves as a useful classroom aid. It includes vocabulary and multiple
choice written exercises on oscilloscope theory and controls. You do not need
any mathematical or electronics knowledge. This book emphasizes teaching you
about oscilloscopes – how they work, how to choose the right one, and and how
to make it work for you.
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XYZs of Oscilloscopes
Contents
Introduction· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · i
Why Read This Book? · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · i
The Oscilloscope · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 1
What Can You Do With an Oscilloscope? · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 2
Analog, Digital Storage, and Digital Phosphor Oscilloscopes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 2
How Oscilloscopes Work · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 3
Analog Oscilloscopes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 3
Digital Storage Oscilloscopes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 4
Digital Phosphor Oscilloscopes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 4
Sampling Methods · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 5
Real-Time Sampling with Interpolation · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 5
Oscilloscope Terminology · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 7
Measurement Terms · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 7
Types of Waves · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 7
Sine Waves · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 7
Square and Rectangular Waves · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 8
Sawtooth and Triangle Waves · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 8
Step and Pulse Shape · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 8
Complex Waves · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 8
Waveform Measurements · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 9
Frequency and Period · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 9
Voltage · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 9
Phase · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 9
Performance Terms · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 9
Bandwidth · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 9
Rise Time · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 9
Effective Bits · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 9
Frequency Response · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 10
Vertical Sensitivity · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 10
Sweep Speed · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 10
Gain Accuracy · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 10
Time Base or Horizontal Accuracy · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 10
Sample Rate · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 10
ADC Resolution (or Vertical Resolution) · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 10
Record Length · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 10
Waveform Capture Rate · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 10
Setting Up · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 11
Grounding · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 11
Ground the Oscilloscope · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 11
Ground Yourself · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 11
Setting the Controls · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 11
Probes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 12
“Intelligent” Probe Interfaces · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 12
Using Passive Probes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 12
Using Active Probes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 13
Using Current Probes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 13
Where to Clip the Ground Clip · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 13
Compensating the Probe · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 14
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The Controls · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 15
Display Controls · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 15
Vertical Controls · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 15
Position and Volts per Division · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 15
Input Coupling · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 15
Bandwidth Limit · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 16
Alternate and Chop Display · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 16
Math Operations · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 17
Horizontal Controls · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 17
Position and Seconds per Division · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 17
Time Base Selections · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 17
Trigger Position · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 17
Zoom · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 18
XY Mode · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 18
The Z Axis · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 18
XYZ Mode · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 18
Trigger Controls · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 18
Trigger Level and Slope · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 19
Trigger Sources · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 19
Trigger Modes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 19
Trigger Coupling · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 20
Trigger Holdoff · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 20
Digitizing Oscilloscope Triggers · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 20
Acquisition Controls for Digitizing Oscilloscopes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 21
Acquisition Modes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 21
Stopping and Starting the Acquisition System · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 22
Sampling Methods · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 22
Other Controls · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 22
Measurement Techniques · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 23
The Display · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 23
Voltage Measurements · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 23
Time and Frequency Measurements · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 24
Pulse and Rise Time Measurements · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 24
Phase Shift Measurements · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 25
Waveform Measurements with Digitizing Oscilloscopes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 26
What’s Next? · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 26
Written Exercises · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 27
Part I Exercises · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 28
Part II Exercises · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 30
Answers to Written Exercises · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 34
Glossary · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 35
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The Oscilloscope
What is an oscilloscope, what can you do with it,
and how does it work? This section answers these
fundamental questions.
The oscilloscope is basically a graph-displaying
device – it draws a graph of an electrical signal (see
Figure 1). In most applications the graph shows how
signals change over time: the vertical (Y) axis represents voltage and the horizontal (X) axis represents
time. The intensity or brightness of the display is
sometimes called the Z axis. This simple graph can
tell you many things about a signal. Here are a few:
• You can determine the time and voltage values of
a signal
• You can calculate the frequency of an oscillating
signal
• You can see the “moving parts” of a circuit represented by the signal
• You can tell how often a particular portion of the
signal is occurring relative to other portions
• You can tell if a malfunctioning component is
distorting the signal
• You can find out how much of a signal is direct
current (DC) or alternating current (AC)
• You can tell how much of the signal is noise and
whether the noise is changing with time
An oscilloscope’s front panel includes a display
screen and the knobs, buttons, switches, and indicators used to control signal acquisition and display.
Front-panel controls normally are divided into
Vertical, Horizontal, and Trigger sections, and in
addition, there are display controls and input
connectors. See if you can locate these front-panel
sections in Figures 2 and 3 as well as on your
oscilloscope.
Figure 1. X, Y, and Z Components of a displayed waveform.
Figure 2. The TAS 465 Analog Oscilloscope front panel.
1
Figure 3. The TDS 784D Digital Phosphor Oscilloscope front panel.
What Can You Do With an
Oscilloscope?
Oscilloscopes are used by everyone from television
repair technicians to physicists. They are indispensable for anyone designing or repairing electronic
equipment.
The usefulness of an oscilloscope is not limited to
the world of electronics. With the proper transducer,
an oscilloscope can measure all kinds of phenomena.
A transducer is a device that creates an electrical
signal in response to physical stimuli, such as
sound, mechanical stress, pressure, light, or heat. For
example, a microphone is a transducer that converts
sound to an electrical signal.
An automotive engineer uses an oscilloscope to
measure engine vibrations. A medical researcher
uses an oscilloscope to measure brain waves. The
possibilities are endless.
Analog, Digital Storage, and Digital
Phosphor Oscilloscopes
Electronic equipment can be divided into two types:
analog and digital. Analog equipment works with
continuously variable voltages, while digital equipment works with discrete binary numbers that may
represent voltage samples. For example, a conven-
tional phonograph is an analog device, while a
compact disc player is a digital device.
Oscilloscopes also come in analog and digitizing
types (see Figure 5). Fundamentally an analog oscilloscope works by applying the measured signal
voltage directly to an electron beam moving across
the oscilloscope screen (usually a cathode-ray tube,
CRT). The back side of the screen is treated with a
coating that phosphoresces wherever the electron
beam hits it. The signal voltage deflects the beam up
and down proportionally, tracing the waveform on
the screen. The more frequently the beam hits a
particular screen location, the more brightly it glows.
This gives an immediate picture of the waveform.
The range of frequencies an analog scope can display
is limited by the CRT. At very low frequencies, the
signal appears as a bright, slow-moving dot that’s
difficult to distinguish as a waveform. At high
frequencies, the CRT’s “writing speed” defines the
limit. When the signal frequency exceeds the CRT’s
writing speed, the display becomes too dim to see.
The fastest analog scopes can display frequencies up
to about 1 GHz.
In contrast, a digitizing oscilloscope uses an analogto-digital converter (ADC) to convert the voltage
being measured into digital information. The digitizing scope acquires the waveform as a series of
Figure 4. An example of scientific data gathered by an oscilloscope.
2
samples. It stores these samples until it accumulates
enough samples to describe a waveform, and then reassembles the waveform for viewing on the screen.
The conventional digitizing scope is known as a
DSO – Digital Storage Oscilloscope. Its display
doesn’t rely on luminous phosphor; instead, it uses a
raster-type screen.
Recently a third major oscilloscope architecture has
emerged: the Digital Phosphor Oscilloscope (DPO).
The DPO is a digitizing scope that faithfully
emulates the best display attributes of the analog
scope and provides the benefits of digital acquisition
and processing as well. Like the DSO, the DPO uses a
raster screen. But instead of a phosphor, it employs
special parallel processing circuitry that delivers a
crisp, intensity-graded trace.
For both DSOs and DPOs, the digital approach
means that the scope can display any frequency
within its range with equal stability, brightness, and
clarity. The digitizing oscilloscope’s frequency range
is determined by its sample rate, assuming that its
probes and vertical sections are adequate for the
task.
For many applications either an analog or digitizing
oscilloscope will do. However, each type has unique
characteristics that may make it more or less suitable
for specific tasks.
People often prefer analog oscilloscopes when it’s
important to display rapidly varying signals in “real
time” (as they occur). The analog scope’s chemical
phosphor-based display has a characteristic known
as “intensity grading” which makes the trace brighter
wherever the signal features occur most often. This
makes it easy to distinguish signal details just by
looking at the trace’s intensity levels.
Digital storage oscilloscopes allow you to capture
and view events that may happen only once – “transient” events. Because the waveform information is
in digital form (a series of stored binary values), it
can be analyzed, archived, printed, and otherwise
processed within the scope itself or by an external
computer. The waveform doesn’t need to be continuous; even when the signal disappears, it can be
displayed. However, DSOs have no real-time intensity grading; therefore they cannot express varying
levels of intensity in the live signal.
The Digital Phosphor Oscilloscope breaks down the
barrier between analog and digitizing scope technologies. It’s equally suitable for viewing high
frequencies or low, repetitive waveforms, transients,
and signal variations in real time. Among digitizing
scopes, only the DPO provides the Z (intensity) axis
that’s missing from conventional DSOs.
How Oscilloscopes Work
To better understand the oscilloscope’s many uses,
you need to know a little more about how oscilloscopes display a signal. Although analog oscilloscopes work somewhat differently than digitizing
oscilloscopes, some of the internal systems are
similar. Analog oscilloscopes are simpler in concept
and are described first, followed by a description of
digitizing oscilloscopes.
Figure 5. Analog and digitizing oscilloscopes display waveforms.
Delay Line
Vert
Amp
Display
Amp
Trigger
Horiz
Amp
Figure 6. Analog oscilloscope block diagram.
Analog Oscilloscopes
When you connect an oscilloscope probe to a circuit,
the voltage signal travels through the probe to the
vertical system of the oscilloscope. Figure 6 is a
simple block diagram that shows how an analog
oscilloscope displays a measured signal.
Depending on how you set the vertical scale
(volts/div control), an attenuator reduces the signal
voltage or an amplifier increases the signal voltage.
Next, the signal travels directly to the vertical deflection plates of the cathode ray tube (CRT). Voltage
applied to these deflection plates causes a glowing
dot to move. (An electron beam hitting the phosphor
inside the CRT creates the glowing dot.) A positive
voltage causes the dot to move up while a negative
voltage causes the dot to move down.
The signal also travels to the trigger system to start or
trigger a “horizontal sweep.” Horizontal sweep is a
term referring to the action of the horizontal system
causing the glowing dot to move across the screen.
Triggering the horizontal system causes the horizontal time base to move the glowing dot across the
screen from left to right within a specific time
interval. Many sweeps in rapid sequence cause the
movement of the glowing dot to blend into a solid
line. At higher speeds, the dot may sweep across the
screen up to 500,000 times each second.
Together, the horizontal sweeping action and the
vertical deflection action traces a graph of the signal
3
Figure 7. Triggering stabilizes a repeating waveform.
on the screen. The trigger is necessary to stabilize a
repeating signal. It ensures that the sweep begins at
the same point of a repeating signal, resulting in a
clear picture as shown in Figure 7.
In summary, when using an analog oscilloscope (or
any type of oscilloscope), you need to adjust three
basic settings to accommodate an incoming signal:
• The attenuation or amplification of the signal.
Use the volts/div control to adjust the amplitude
of the signal to the desired measurement range
• The time base. Use the sec/div control to set the
amount of time per division represented horizontally across the screen
• The triggering of the oscilloscope. Use the trigger
level to stabilize a repeating signal, or for triggering on a single event
In addition, analog scopes have focus and intensity
controls that can be adjusted to create a sharp,
legible display.
Digital Storage Oscilloscopes
Some of the systems that make up DSOs are the same
as those in analog oscilloscopes; however, digitizing
oscilloscopes contain additional data processing
systems (see Figure 8). With the added systems, the
digitizing oscilloscope collects data for the entire
waveform and then displays it.
The first (input) stage of a DSO is a vertical amplifier,
just like the analog oscilloscope’s. Vertical attenuation controls allow you to adjust the amplitude range
of this stage.
Next, the analog-to-digital converter (ADC) in the
acquisition system samples the signal at discrete
points in time and converts the signal’s voltage at
these points to digital values called sample points.
The horizontal system’s sample clock determines
how often the ADC takes a sample. The rate at which
Amp
A/D
DeMux
the clock “ticks” is called the sample rate and is
expressed in samples per second.
The sample points from the ADC are stored in
memory as waveform points. More than one sample
point may make up one waveform point.
Together, the waveform points make up one waveform record. The number of waveform points used to
make a waveform record is called the record length.
The trigger system determines the start and stop
points of the record. The display receives these
record points after being stored in memory.
Depending on the capabilities of your oscilloscope,
additional processing of the sample points may take
place, enhancing the display. Pretrigger may be
available, allowing you to see events before the
trigger point.
Note that the DSO’s signal path includes a microprocessor. The measured signal passes through this
device on its way to the display. In addition to
processing the signal, the microprocessor coordinates display activities, manages the front-panel
controls, and more. This is known as a “serial
processing” architecture.
Digital Phosphor Oscilloscopes
The Digital Phosphor Oscilloscope (DPO) offers a
new approach to oscilloscope architecture. Like the
analog oscilloscope, its first stage is a vertical amplifier; like the DSO, its second stage is an ADC. But
after the analog-to-digital conversion, the DPO looks
quite different from the DSO. It has special features
designed to recreate the intensity grading of an
analog CRT.
Rather than relying on a chemical phosphor as an
analog scope does, the DPO has a purely electronic
Digital Phosphor that’s actually a continuously
updated data base. This data base has a separate
Acquisition
Memory
Figure 8. Digital storage oscilloscope block diagram – “serial processing.”
4
µP
Display
Memory
Display
DPX
Digital
Phosphor
Amp
A/D
Acquisition
Rasterizer
Display
Memory
Display
µP
Figure 9. Digital phosphor oscilloscope block diagram – “Parallel Processing.”
“cell” of information for every single pixel in the
scope’s display. Each time a waveform is captured
(in other words, every time the scope triggers), it is
mapped into the Digital Phosphor database’s cells.
Each cell representing a screen location that is
touched by the waveform gets reinforced with intensity information. Others do not. Thus intensity information builds up in cells where the waveform passes
most often.
When the Digital Phosphor database is fed to the
oscilloscope’s display, the display reveals intensified
waveform areas, in proportion to the signal’s
frequency of occurrence at each point – much like
the intensity grading characteristics of an analog
oscilloscope (unlike an analog scope, though, the
DPO allows the varying levels to be expressed in
contrasting colors if you wish). With a DPO, it’s easy
to see the difference between a waveform that occurs
on almost every trigger and one that occurs, say,
every 100th trigger.
Importantly, the DPO uses a parallel processing
architecture to achieve all this manipulation without
slowing down the whole acquisition process. Like
the DSO, the DPO uses a microprocessor for display
management, measurement automation, and
analysis. But the DPO’s microprocessor is outside the
acquisition/display signal path (see Figure 9), where
it doesn’t affect the acquisition speed.
Sampling Methods
one sweep. There are two solutions for accurately
acquiring signals whose frequency exceeds half the
oscilloscope’s sample rate:
• Collect a few sample points of the signal in a
single pass (in real-time mode) and use interpolation to fill in the gaps. Interpolation is a
processing technique to estimate what the waveform looks like based on a few points
• Build a picture of the waveform by acquiring
samples from successive cycles of the waveform,
assuming the signal repeats itself (equivalent-time
sampling mode)
Real-Time Sampling with Interpolation
Digitizing oscilloscopes take discrete samples of the
signal which can be displayed. However, it can be
difficult to visualize the signal represented as dots,
especially because there may be only a few dots
representing high-frequency portions of the signal.
To aid in the visualization of signals, digitizing oscilloscopes typically have interpolation display modes.
In simple terms, interpolation “connects the dots.”
Using this process, a signal that is sampled only a
few times in each cycle can be accurately displayed.
However, for accurate representation of the signal,
the sample rate should be at least four times the
bandwidth of the signal.
Linear interpolation connects sample points with
straight lines. This approach is limited to recon-
Digitizing oscilloscopes – DSO or DPO – can use
either real-time, interpolated real-time, or equivalent-time sampling to collect sample points. Realtime sampling is ideal for signals whose frequency is
less than half the scope’s maximum sample rate.
Here, the oscilloscope can acquire more than enough
points in one “sweep” of the waveform to construct
an accurate picture (see Figure 10). Note that realtime sampling is the only way to capture single-shot
transient signals with a digitizing scope.
When measuring high-frequency signals, the oscilloscope may not be able to collect enough samples in
Figure 10. Real-time sampling.
5
structing straight-edged signals such as square
waves.
The more versatile sin x/x interpolation connects
sample points with curves (see Figure 11). Sin x/x
interpolation is a mathematical process in which
points are calculated to fill in the time between the
real samples.
This form of interpolation lends itself to curved and
irregular signal shapes, which are far more common
in the real world than pure square waves and pulses.
Consequently, sin x /x interpolation is the preferred
method for most applications.
Figure 11. Linear and sine interpolation.
Figure 12. Equivalent-time sampling.
6
Some digitizing oscilloscopes can use equivalenttime sampling to capture very fast repeating signals.
Equivalent-time sampling constructs a picture of a
repetitive signal by capturing a little bit of information from each repetition (see Figure 12). The waveform slowly builds up like a string of lights going on
one-by-one. With sequential sampling, the points
appear from left to right in sequence; with random
sampling, the points appear randomly along the
waveform.
Oscilloscope Terminology
Learning a new skill often involves learning a new
vocabulary. This idea holds true for learning how to
use an oscilloscope. This section describes some
useful measurement and oscilloscope performance
terms.
Measurement Terms
The generic term for a pattern that repeats over time
is a wave – sound waves, brain waves, ocean waves,
and voltage waves are all repeating patterns. An
oscilloscope measures voltage waves. One cycle of a
wave is the portion of the wave that repeats. A waveform is a graphic representation of a wave. A voltage
waveform shows time on the horizontal axis and
voltage on the vertical axis.
waveforms and Figure 14 shows some common
sources of waveforms.
Types of Waves
You can classify most waves into these types:
• Sine waves
• Square and rectangular waves
• Triangle and sawtooth waves
• Step and pulse shapes
• Complex waves
Sine Waves
Waveform shapes tell you a great deal about a signal.
Any time you see a change in the height of the waveform, you know the voltage has changed. Any time
there’s a flat horizontal line, you know that there’s
no change for that length of time. Straight diagonal
lines mean a linear change – rise or fall of voltage at
a steady rate. Sharp angles on a waveform mean
sudden change. Figure 13 shows some common
The sine wave is the fundamental wave shape for
several reasons. It has harmonious mathematical
properties – it’s the same sine shape you may have
studied in high school trigonometry class. The
power line voltage at your wall outlet varies as a sine
wave. Test signals produced by the oscillator circuit
of a signal generator are often sine waves. Most AC
power sources produce sine waves. (AC stands for
alternating current, although the voltage alternates
too. DC stands for direct current, which means a
Figure 13. Common waveforms.
Figure 14. Sources of common waveforms.
7
steady current and voltage, such as a battery
produces.)
The damped sine wave is a special case you may see
in a circuit that oscillates but winds down over time.
The rectangular wave is like the square wave except
that the high and low time intervals are not of equal
length. It is particularly important when analyzing
digital circuitry.
Figure 15 shows examples of sine and damped sine
waves.
Figure 16 shows examples of square and rectangular
waves.
Square and Rectangular Waves
Sawtooth and Triangle Waves
The square wave is another common wave shape.
Basically, a square wave is a voltage that turns on
and off (or goes high and low) at regular intervals. It’s
a standard wave for testing amplifiers – good amplifiers increase the amplitude of a square wave with
minimum distortion. Television, radio, and
computer circuitry often use square waves for timing
signals.
Sawtooth and triangle waves result from circuits
designed to control voltages linearly, such as the
horizontal sweep of an analog oscilloscope or the
raster scan of a television. The transitions between
voltage levels of these waves change at a constant
rate. These transitions are called ramps.
Figure 17 shows examples of sawtooth and triangle
waves.
Step and Pulse Shape
Figure 15. Sine and damped sine waves.
Figure 16. Square and rectangular waves.
Signals such as steps and pulses that only occur
once are called single-shot or transient signals. The
step indicates a sudden change in voltage, like what
you would see if you turned on a power switch. The
pulse indicates what you would see if you turned a
power switch on and then off again. It might represent one bit of information traveling through a
computer circuit or it might be a glitch (a defect) in a
circuit.
A collection of pulses travelling together creates a
pulse train. Digital components in a computer
communicate with each other using pulses. Pulses
are also common in x-ray and communications
equipment.
Figure 18 shows examples of step and pulse shapes
and a pulse train.
Complex Waves
Figure 17. Sawtooth and triangle waves.
Figure 18. Step, pulse, and pulse train shapes.
Some waveforms combine the characteristics of
sines, squares, steps, and pulses to produce a waveshape that challenges many oscilloscopes. The signal
information may be embedded in the form of amplitude, phase, and/or frequency variations. For
example, look at Figure 19 – although it’s an ordinary composite video signal, it is made up of many
cycles of higher-frequency waveforms embedded in a
lower-frequency “envelope.” In this example it’s
usually most important to understand the relative
levels and timing relationships of the steps. What’s
needed to view this signal is an oscilloscope that
captures the low-frequency envelope and blends in
the higher-frequency waves in an intensity-graded
fashion so you can see their overall level.
Analog instruments and DPOs are most suited to
viewing complex waves such as video signals. Their
displays provide the necessary intensity grading.
Often, the frequency-of-occurrence information that
their displays express is essential to understanding
what the waveform is really doing.
Figure 19. Complex wave (NTSC composite video signal).
8
Waveform Measurements
You use many terms to describe the types of
measurements that you take with your oscilloscope.
This section describes some of the most common
measurements and terms.
Frequency and Period
If a signal repeats, it has a frequency. The frequency
is measured in Hertz (Hz) and equals the number of
times the signal repeats itself in one second (the
cycles per second). A repeating signal also has a
period – this is the amount of time it takes the signal
to complete one cycle. Period and frequency are reciprocals of each other, so that 1/period equals the
frequency and 1/frequency equals the period. So, for
example, the sine wave in Figure 20 has a frequency
of 3 Hz and a period of 1/3 second.
Voltage
Voltage is the amount of electric potential (a kind of
signal strength) between two points in a circuit.
Usually one of these points is ground (zero volts) but
not always – you may want to measure the voltage
from the maximum peak to the minimum peak of a
waveform, referred to as the peak-to-peak voltage.
The word amplitude commonly refers to the
maximum voltage of a signal measured from ground
or zero volts. The waveform shown in Figure 21 has
an amplitude of one volt and a peak-to-peak voltage
of two volts.
Phase
Phase is best explained by looking at a sine wave.
The voltage level of sine waves is based on circular
motion, and a circle has 360 degrees (°). One cycle of
a sine wave has 360°, as shown in Figure 21. Using
degrees, you can refer to the phase angle of a sine
wave when you want to describe how much of the
period has elapsed.
Phase shift describes the difference in timing
between two otherwise similar signals. In Figure 22,
the waveform labeled “current” is said to be 90° out
of phase with the waveform labeled “voltage,” since
the waves reach similar points in their cycles exactly
1/4 of a cycle apart (360°/4 = 90°). Phase shifts are
common in electronics.
Performance Terms
The terms described in this section may come up in
your discussions about oscilloscope performance.
Understanding these terms will help you evaluate
and compare your oscilloscope with other models.
Bandwidth
The bandwidth specification tells you the frequency
range the oscilloscope accurately measures.
As signal frequency increases, the capability of the
oscilloscope to accurately respond decreases. By
convention, the bandwidth tells you the frequency at
which the displayed signal reduces to 70.7% of the
applied sine wave signal. (This 70.7% point is
referred to as the “–3 dB point” – a term based on a
logarithmic scale.)
Rise Time
Rise time is another way of describing the useful
frequency range of an oscilloscope. Rise time may be
a more appropriate performance consideration when
you expect to measure pulses and steps. An oscilloscope cannot accurately display pulses with rise
times faster than the specified rise time of the oscilloscope.
Effective Bits
Effective bits is a measure of a digitizing oscilloscope’s ability to accurately reconstruct a signal by
considering the quality of the oscilloscope’s ADC
and amplifiers. This measurement compares the
oscilloscope’s actual error to that of an ideal digitizer. Because the actual errors include noise and
distortion, the frequency and amplitude of the signal
as well as the bandwidth of the instrument must be
specified.
Figure 20. Frequency and period.
Figure 21. Sine wave degrees.
Figure 22. Phase shift.
9
Frequency Response
Bandwidth alone is not enough to ensure that an
oscilloscope can accurately capture a high frequency
signal. The goal of oscilloscope design is to have
Maximally Flat Envelope Delay (MFED). A
frequency response of this type has excellent pulse
fidelity with minimum overshoot and ringing. Since
a digitizing oscilloscope is composed of real amplifiers, attenuators, ADCs, interconnect and relays, the
MFED response is a goal which can only be
approached. Pulse fidelity varies considerably with
model and manufacturer.
Vertical Sensitivity
The vertical sensitivity indicates how much the
vertical amplifier can amplify a weak signal. Vertical
sensitivity is usually given in millivolts (mV) per
division. The smallest voltage a general purpose
oscilloscope can detect is typically about 1 mV per
vertical screen division.
Sweep Speed
For analog oscilloscopes, this specification indicates
how fast the trace can sweep across the screen,
allowing you to see fine details. The fastest sweep
speed of an oscilloscope is usually given in nanoseconds/div.
Gain Accuracy
The gain accuracy indicates how accurately the
vertical system attenuates or amplifies a signal. This
is usually listed as a percentage error.
Time Base or Horizontal Accuracy
The time base or horizontal accuracy indicates how
accurately the horizontal system displays the timing
of a signal. This is usually listed as a percentage
error.
Sample Rate
On digitizing oscilloscopes, the sample rate indicates
how many samples per second the ADC (and therefore the oscilloscope) can acquire. Maximum sample
rates are usually given in megasamples per second
(MS/s). The faster the oscilloscope can sample, the
more accurately it can represent fine details in a fast
signal. The minimum sample rate may also be
important if you need to look at slowly changing
10
signals over long periods of time. Typically, the
sample rate changes with changes made to the
vertical sensitivity control to maintain a constant
number of waveform points in the waveform record.
ADC Resolution (or Vertical Resolution)
The resolution, in bits, of the ADC (and therefore the
digitizing oscilloscope) indicates how precisely it
can turn input voltages into digital values.
Calculation techniques can improve the effective
resolution.
Record Length
The record length of a digitizing oscilloscope indicates how many waveform points the oscilloscope is
able to acquire for one waveform record. Some digitizing oscilloscopes let you adjust the record length.
The maximum record length depends on the amount
of memory in your oscilloscope and its ability to
combine memory length from unused channels.
Since the oscilloscope can only store a finite number
of waveform points, there is a trade-off between
record detail and record length. You can acquire
either a detailed picture of a signal for a short period
of time (the oscilloscope “fills up” on waveform
points quickly) or a less detailed picture for a longer
period of time. Some oscilloscopes let you add more
memory to increase the record length for special
applications.
Waveform Capture Rate
Waveform capture rate is the rate at which an oscilloscope triggers, acquires, and displays waveforms.
On DSOs, the rate is a few hundred times per second
at the most, due to their serial processing architecture. All in all, most DSOs sample about 1% of the
total time the signal is available to them. The limitation of this approach is that signal activity continues
even though the oscilloscope isn’t sampling very
often. And an important waveform aberration might
occur during that lapse. A new digitizing oscilloscope architecture, the DPO, has emerged to solve
this problem. On a DPO, signal acquisition is
repeated hundreds of thousands of times per second
– as fast as an analog oscilloscope. The DPO’s
extremely high waveform capture rate (as well as
their digital phosphor technology) makes it possible
to view rare, erratic signal events.
Setting Up
This section briefly describes how to set up and start
using an oscilloscope – specifically, how to ground
the oscilloscope, set the controls in standard positions, and compensate the probe.
Proper grounding is an important step when setting
up to take measurements or work on a circuit.
Properly grounding the oscilloscope protects you
from a hazardous shock and grounding yourself
protects your circuits from damage.
Grounding
Ground the Oscilloscope
Grounding the oscilloscope is necessary for safety. If
a high voltage contacts the case of an ungrounded
oscilloscope, any part of the case including knobs
that appear insulated, can give you a shock.
However, with a properly grounded oscilloscope, the
current travels through the grounding path to earth
ground rather than through you to earth ground.
To ground the oscilloscope means to connect it to an
electrically neutral reference point (such as earth
ground). Ground your oscilloscope by plugging its
three-pronged power cord into an outlet grounded to
earth ground.
Grounding is also necessary for taking accurate
measurements with your oscilloscope. The oscilloscope needs to share the same ground as any circuits
you are testing.
Some oscilloscopes do not require the separate
connection to earth ground. These oscilloscopes
have insulated cases and controls, which keeps any
possible shock hazard away from the user.
Ground Yourself
If you are working with integrated circuits (ICs), you
also need to ground yourself. Integrated circuits have
tiny conduction paths that can be damaged by static
electricity that builds up on your body. You can ruin
an expensive IC simply by walking across a carpet or
taking off a sweater and then touching the leads of
the IC. To solve this problem, wear a grounding strap
(see Figure 23). This strap safely sends static charges
on your body to earth ground.
Setting the Controls
After plugging in the oscilloscope, take a look at the
front panel. It is divided into three main sections
labeled Vertical, Horizontal, and Trigger (see Figure
24). Your oscilloscope may have other sections,
depending on the model and type (analog or digitizing).
Figure 23. Typical wrist type grounding strap.
Notice the input connectors on your oscilloscope.
This is where you attach probes. Most oscilloscopes
have at least two input channels and each channel
Figure 24. Front-panel control sections of a typical oscilloscope.
11
can display a waveform on the screen. Multiple
channels are handy for comparing waveforms.
Some oscilloscopes have an AUTOSET or PRESET
button that sets up the controls in one step to accommodate a signal. If your oscilloscope does not have
this feature, it is helpful to set the controls to standard positions before taking measurements.
Standard positions include the following:
• Set the oscilloscope to display channel 1
• Set the volts/division scale to a mid-range position
• Turn off the variable volts/division
• Turn off all magnification settings
• Set the channel 1 input coupling to DC
• Set the trigger mode to auto
• Set the trigger source to channel 1
• Turn trigger holdoff to minimum or off
• Set the intensity control to a nominal viewing
level
• Adjust the focus control for a sharp display
These are general instructions for setting up your
oscilloscope. If you are not sure how to do any of
these steps, refer to the manual that came with your
oscilloscope. The Controls section describes the
controls in more detail.
Probes
Now you are ready to connect a probe to your oscilloscope. It’s important to use a probe designed to
work with your oscilloscope. A probe is more than a
cable with a clip-on tip. It’s a high-quality connector,
carefully designed not to pick up stray radio and
power-line noise.
Probes are designed not to influence the behavior of
the circuit you are testing. However, no measurement device can act as a perfectly invisible observer.
The unintentional interaction of the probe and oscilloscope with the circuit being tested is called circuit
loading. To minimize circuit loading, you will probably use a 10X attenuator (passive) probe.
Your oscilloscope probably arrived with a passive
probe as a standard accessory. Passive probes
provide you with an excellent tool for generalpurpose testing and troubleshooting. For more
specific measurements or tests, many other types of
probes exist. Two examples are active and current
probes.
Descriptions of these probes follow, with more
emphasis given to the passive probe since this is the
probe type that allows the most flexibility of use.
12
“Intelligent” Probe Interfaces
Many modern oscilloscopes provide special automated features built into the input and mating probe
connectors. The act of connecting the probe to the
instrument notifies the oscilloscope about the
probe’s attenuation factor, which in turn scales the
display so that the probe’s attenuation is figured into
the readout on the screen.
Some probe interfaces also recognize the type of
probe; that is, passive, active, or current. Lastly, the
interface may act as a DC power source for probes.
Active probes have their own amplifier and buffer
circuitry that requires DC power.
Using Passive Probes
Most passive probes have some attenuation factor,
such as 10X, 100X, and so on. By convention, attenuation factors, such as for the 10X attenuator probe,
have the X after the factor. In contrast, magnification
factors like X10 have the X first.
The 10X (read as “ten times”) attenuator probe minimizes circuit loading and is an excellent generalpurpose passive probe. Circuit loading becomes
more pronounced at higher frequencies, so be sure to
use this type of probe when measuring signals above
5 kHz. The 10X attenuator probe improves the accuracy of your measurements, but it also reduces the
amplitude of the signal seen on the screen by a factor
of 10.
Because it attenuates the signal, the 10X attenuator
probe makes it difficult to look at signals less than 10
millivolts. The 1X probe is similar to the 10X attenuator probe but lacks the attenuation circuitry.
Without this circuitry, more interference is introduced into the circuit being tested. Use the 10X
attenuator probe as your standard probe, but keep
the 1X probe handy for measuring weak signals.
Some probes have a convenient feature for switching
between 1X and 10X attenuation at the probe tip. If
your probe has this feature, make sure you are using
the correct setting before taking measurements.
Many oscilloscopes can detect whether you are using
a 1X or 10X probe and adjust their screen readouts
accordingly. However with some oscilloscopes, you
must set the type of probe you are using or read from
the proper 1X or 10X marking on the volts/div
control.
The 10X attenuator probe works by balancing the
probe’s electrical properties against the oscilloscope’s electrical properties. Before using a 10X
attenuator probe, you need to adjust this balance for
your particular oscilloscope. This adjustment is
called compensating the probe and is further
described elsewhere in this document. Figure 25
shows a simple diagram of the internal workings of a
probe, its adjustment, and the input of an oscilloscope.
Figure 26 shows a typical passive probe and some
accessories to use with the probe.
Using Active Probes
Active probes provide their own amplification or
perform some other type of operation to process the
signal before applying it to the oscilloscope. These
types of probes can solve problems such as circuit
loading or can perform tests on signals, sending the
results to the oscilloscope. Active probes require a
power source for their operation.
Using Current Probes
Current probes enable you to directly observe and
measure current waveforms. They are available for
measuring both AC and DC current. Current probes
use jaws that clip around the wire carrying the
current. This makes them unique since they are not
connected in series with the circuit; therefore, they
cause little or no interference in the circuit.
Where to Clip the Ground Clip
Measuring a signal requires two connections: the
probe tip connection and a ground connection.
Probes come with an alligator-clip attachment for
grounding the probe to the circuit under test. In
practice, you clip the grounding clip to a known
ground point in the circuit, such as the metal chassis
of a stereo you are repairing, and touch the probe tip
to a test point in the circuit.
Figure 25. Typical probe/oscilloscope 10-to-1 divider network.
Figure 26. A typical passive probe with accessories.
13
Compensating the Probe
Before using a passive probe, you need to compensate it – to balance its electrical properties to a
particular oscilloscope. You should get into the habit
of compensating the probe every time you set up
your oscilloscope. A poorly adjusted probe can make
your measurements less accurate. Figure 27 shows
what happens to measured waveforms when using a
probe that is not properly compensated.
Most oscilloscopes have a square-wave reference
signal available at a terminal on the front panel
which can be used to compensate the probe. You
compensate a probe by:
Figure 27. The effects of improper probe compensation.
14
• Attaching the probe to an input connector
• Connecting the probe tip to the probe compensation signal
• Attaching the ground clip of the probe to ground
• Viewing the square wave reference signal
• Making the proper adjustments on the probe so
that the corners of the square wave are square
When you compensate the probe, always first attach
any accessory tips you will use and connect the
probe to the vertical channel you plan to use. This
way, the oscilloscope has the same electrical properties for compensation as it does when you make
measurements.
The Controls
This section briefly describes the basic controls
found on analog and digitizing oscilloscopes.
Remember that some controls differ between analog
and digitizing oscilloscopes; your oscilloscope probably has controls not discussed here.
Display Controls
Display systems vary between analog oscilloscopes
and digitizing scopes, including both DSOs and
Digital Phosphor Oscilloscopes (DPOs). Common
controls include:
• An intensity control to adjust the brightness of
the waveform. As you increase the sweep speed
of an analog oscilloscope, you need to increase
the intensity level.
• A focus control to adjust the sharpness of the
waveform. Digitizing oscilloscopes may not have
a focus control.
• A trace rotation control to align the waveform
trace with the screen’s horizontal axis. The position of your oscilloscope in the earth’s magnetic
field affects waveform alignment. Digitizing oscilloscopes may not have a trace rotation control.
• On DPOs, a contrast control.
• On many DSOs and on DPOs, a color palette
control to select trace colors and intensity grading
color levels.
• Other display controls may let you adjust the
intensity of the graticule lights and turn on or off
any on-screen information (such as menus).
Vertical Controls
Use the vertical controls to position and scale the
waveform vertically. Your oscilloscope also has
controls for setting the input coupling and other
signal conditioning, described later in this section.
Figure 28 shows a typical front panel and on-screen
menus for the vertical controls.
Position and Volts per Division
The vertical position control lets you move the
waveform up or down to exactly where you want it
on the screen.
The volts per division (usually written volts/div)
setting varies the size of the waveform on the screen.
A good general purpose oscilloscope can accurately
display signal levels from about 4 millivolts to 40
volts.
The volts/div setting is a scale factor. For example, if
the volts/div setting is 5 volts, then each of the eight
vertical divisions represents 5 volts and the entire
screen can show 40 volts from bottom to top
(assuming a graticule with eight major divisions). If
the setting is 0.5 volts/div, the screen can display 4
volts from bottom to top, and so on. The maximum
voltage you can display on the screen is the volts/div
setting times the number of vertical divisions.
(Recall that the probe you use, 1X or 10X, also influences the scale factor. You must divide the volts/div
scale by the attenuation factor of the probe if the
oscilloscope does not do it for you.)
Often, the volts/div scale has either a variable gain or
a fine gain control for scaling a displayed signal to a
certain number of divisions. Use this control to take
rise time measurements.
Input Coupling
Coupling means the method used to connect an electrical signal from one circuit to another. In this case,
the input coupling is the connection from your test
circuit to the oscilloscope. The coupling can be set to
DC, AC, or ground. DC coupling shows all of an
input signal. AC coupling blocks the DC component
of a signal so that you see the waveform centered at
Figure 28. Typical Vertical controls.
15
zero volts. Figure 29 illustrates this difference. The
AC coupling setting is handy when the entire signal
(alternating plus constant components) is too large
for the volts/div setting.
The ground setting disconnects the input signal from
the vertical system, which lets you see where the
zero-volt level is on the screen. With grounded input
coupling and auto trigger mode, you see a horizontal
line on the screen that represents zero volts.
Switching from DC to ground and back again is a
handy way of measuring signal voltage levels with
respect to ground.
Bandwidth Limit
Most oscilloscopes have a circuit that limits the
bandwidth of the oscilloscope. By limiting the bandwidth, you reduce the high-frequency noise that
sometimes appears on the displayed waveform,
providing you with a more refined signal display.
Figure 29. AC and DC input coupling.
Figure 30. Multi-channel display modes.
16
Alternate and Chop Display
On analog scopes, multiple channels are displayed
using either an alternate or chop mode. (Digitizing
oscilloscopes can present multiple channels simultaneously without the need for chop or alternate
modes.)
Alternate mode draws each channel alternately – the
oscilloscope completes one sweep on channel 1,
then one sweep on channel 2, a second sweep on
channel 1, and so on. Use this mode with mediumto high-speed signals, when the sec/div scale is set to
0.5 ms or faster.
Chop mode causes the oscilloscope to draw small
parts of each signal by switching back and forth
between them. The switching rate is too fast for you
to notice, so the waveform looks whole. You typically use this mode with slow signals requiring
sweep speeds of 1 ms per division or less. Figure 30
shows the difference between the two modes. It is
often useful to view the signal both ways, to make
sure you have the best view.
Math Operations
Your oscilloscope may also have operations to allow
you to add waveforms together, creating a new waveform display. Analog oscilloscopes combine the
signals while digitizing oscilloscopes create new
waveforms mathematically. Subtracting waveforms
is another math operation. Subtraction is possible
with analog oscilloscopes by using the channel
invert function on one signal and then using the add
operation. Digitizing oscilloscopes typically have a
subtraction operation available. Figure 31 illustrates
a third waveform created by adding two different
signals together.
Horizontal Controls
Use the horizontal controls to position and scale the
waveform horizontally. Figure 32 shows a typical
front panel and on-screen menus for the horizontal
controls.
Using the power of their internal processors, digitizing oscilloscopes offer many advanced math operations: multiplication, division, integration, Fast
Fourier Transform, and more.
Position and Seconds per Division
The horizontal position control moves the waveform
left and right to exactly where you want it on the
screen.
The seconds per division (usually written as sec/div)
setting lets you select the rate at which the waveform
is drawn across the screen (also known as the time
base setting or sweep speed). This setting is a scale
factor. For example, if the setting is 1 ms, each horizontal division represents 1 ms and the total screen
width represents 10 ms (ten divisions). Changing the
sec/div setting lets you look at longer or shorter time
intervals of the input signal.
As with the vertical volts/div scale, the horizontal
sec/div scale may have variable timing, allowing you
to set the horizontal time scale in between the
discrete settings.
Figure 31. Adding channels.
Time Base Selections
Your oscilloscope has a time base usually referred to
as the main time base and it is probably the most
useful. Many oscilloscopes have what is called a
delayed time base – a time base sweep that starts
after a pre-determined time from the start of the
main time base sweep. Using a delayed time base
sweep allows you to see events more clearly or even
see events not visible with the main time base sweep
alone.
The delayed time base requires the setting of a delay
time and possibly the use of delayed trigger modes
and other settings not described in this book. Refer
to the manual supplied with your oscilloscope for
information on how to use these features.
Trigger Position
Horizontal trigger position control is only available
on digitizing oscilloscopes. The trigger position
control may be located in the horizontal control
section of your oscilloscope. It actually represents
“the horizontal position of the trigger in the waveform record.”
Varying the horizontal trigger position allows you to
capture what a signal did before a trigger event
Figure 32. Typical Horizontal controls.
17
(called pretrigger viewing). Thus it determines the
length of the viewable signal both preceding and
following a trigger point.
Digitizing oscilloscopes can provide pretrigger
viewing because they constantly process the input
signal whether a trigger has been received or not. A
steady stream of data flows through the oscilloscope;
the trigger merely tells the oscilloscope to save the
present data in memory. In contrast, analog oscilloscopes only display the signal (that is, write it on the
CRT) after receiving the trigger.
Pretrigger viewing is a valuable troubleshooting aid.
For example, if a problem occurs intermittently, you
can trigger on the problem, record the events that led
up to it and, possibly, find the cause.
Zoom
Your oscilloscope may have special horizontal
magnification settings that let you display a magnified section of the waveform on-screen. On a DSO,
the operation is performed on stored digitized data.
XY Mode
Most analog oscilloscopes have the capability of
displaying a second channel signal along the X-axis
Figure 33. Typical Trigger controls.
18
(instead of time). This is known as XY mode. The XY
mode is further explained in the Measurement
Techniques section of this document.
The Z Axis
The Z axis brings a third dimension – intensity – to
the traditional waveform display. One application of
the Z-axis is to feed special timed signals into the
separate Z input to create highlighted “marker” dots
at known intervals in the waveform.
XYZ Mode
DPOs can use the Z input to create an XY display
with intensity grading. In this case, the DPO samples
the instantaneous data value at the Z input and uses
that value to intensify a specific part of the waveform. XYZ is especially useful for displaying the
polar patterns commonly used in testing wireless
communication devices.
Trigger Controls
The trigger controls let you stabilize repeating waveforms and capture single-shot waveforms. Figure 33
shows a typical front panel and on-screen menus for
the trigger controls.
The trigger makes repeating waveforms appear static
on the oscilloscope display by repeatedly displaying
the same portion of the input signal. Imagine the
jumble on the screen that would result if each sweep
started at a different place on the signal (see Figure
34).
Trigger Level and Slope
Your oscilloscope may have several different types of
triggers, such as edge, video, pulse, or logic. Edge
triggering is the basic and most common type.
For edge triggering, the trigger level and slope
controls provide the basic trigger point definition.
The trigger circuit acts as a comparator. You select
the slope and voltage level of one side of the
comparator. When the trigger signal matches your
settings, the oscilloscope generates a trigger.
• The slope control determines whether the trigger
point is on the rising or the falling edge of a
signal. A rising edge is a positive slope and a
falling edge is a negative slope.
• The level control determines where on the edge
the trigger point occurs.
Figure 35 shows the effect the trigger slope and level
settings have on how a waveform is displayed.
Trigger Sources
The oscilloscope does not necessarily have to trigger
on the signal being measured. Several sources can
trigger the sweep:
• Any input channel
• An external source other than the signal applied
to an input channel
• The power source signal
• A signal internally generated by the oscilloscope
Most of the time, you can leave the oscilloscope set
to trigger on the channel displayed. Many oscilloscopes provide a trigger output that delivers the
trigger signal to another instrument.
Note that the oscilloscope can use an alternate
trigger source whether displayed or not. So you have
to be careful not to unwittingly trigger on, for
example, channel 1 while displaying channel 2.
Trigger Modes
The trigger mode determines whether or not the
oscilloscope draws a waveform if it does not detect a
trigger. Common trigger modes include normal and
auto.
In normal mode, the oscilloscope only sweeps if the
input signal reaches the set trigger point; otherwise
(on an analog oscilloscope) the screen is blank or (on
a digitizing oscilloscope) frozen on the last acquired
waveform. Normal mode can be disorienting since
you may not see the signal at first if the level control
is not adjusted correctly.
Auto mode causes the oscilloscope to sweep, even
without a trigger. If no signal is present, a timer in
the oscilloscope triggers the sweep. This ensures that
the display will not disappear if the signal drops to
small voltages. It is also the best mode to use if you
are looking at many signals and do not want to
bother setting the trigger each time.
Figure 34. Untriggered display.
Figure 35. Positive and negative slope triggering.
19
In practice, you will probably use both modes:
normal mode because it lets you select just the signal
area you need to see, and auto mode because it
requires less adjustment.
Some oscilloscopes also include special modes for
single sweeps, triggering on video signals, or automatically setting the trigger level.
Trigger Coupling
Just as you can select either AC or DC coupling for
the vertical system, you can choose the kind of
coupling for the trigger signal.
Besides AC and DC coupling, your oscilloscope may
also have high frequency rejection, low frequency
rejection, and noise rejection trigger coupling. These
special settings are useful for eliminating noise from
the trigger signal to prevent false triggering.
Trigger Holdoff
Sometimes getting an oscilloscope to trigger on the
correct part of a signal requires great skill. Many
oscilloscopes have special features to make this task
easier.
Trigger holdoff is an adjustable period of time during
which the oscilloscope cannot trigger. This feature is
useful when you are triggering on complex waveform shapes, so that the oscilloscope only triggers on
the first eligible trigger point. Figure 36 shows how
using trigger holdoff helps create a usable display.
Figure 36. Trigger holdoff.
20
Digitizing Oscilloscope Triggers
In addition to the usual threshold triggering, many
digitizing oscilloscopes offer a host of specialized
trigger settings which have no equivalents on analog
instruments. These triggers respond to specific
conditions in the incoming signal, making it easy to
detect, for example, a pulse that is narrower than it
should be. Such a condition would be impossible to
detect with a voltage threshold trigger alone.
Following is a partial list of the digital triggers found
on advanced DSOs and DPOs, along with a brief
definition of each:
• Pulse Width and Glitch trigger
Detects pulses either within or exceeding specified widths.
• Runt Pulse trigger
Detects a pulse that crosses the lesser but not the
greater of two threshold levels.
• Logic (Boolean) trigger
Uses multiple oscilloscope inputs as binary
inputs that must meet logical conditions such as
NAND or NOR to produce a trigger.
• Serial Data trigger
Detects specific data combinations in digital
telecom signals
• Setup and Hold Violation trigger
Detects violations of digital setup and hold time
when clock and data signals are acquired on two
different inputs.
Acquisition Controls for Digitizing
Oscilloscopes
Digitizing oscilloscopes have settings that let you
control how the acquisition system processes a
signal. Look over the acquisition options on your
digitizing oscilloscope while you read this description. Figure 37 shows an example of an acquisition
menu.
Acquisition Modes
Acquisition modes control how waveform points are
produced from sample points. Recall from the first
section that sample points are the digital values that
come directly out of the Analog-to-Digital-Converter
(ADC). The time between sample points is called the
sample interval. Waveform points are the digital
values that are stored in memory and displayed to
form the waveform. The time value difference
between waveform points is called the waveform
interval. The sample interval and the waveform
interval may be, but need not be, the same. This fact
leads to the existence of several different acquisition
modes in which one waveform point is made up
from several sequentially acquired sample points.
Additionally, waveform points can be created from a
composite of sample points taken from multiple
acquisitions, which leads to another set of acquisition modes. A description of the most commonly
used acquisition modes follows.
• Sample Mode: This is the simplest acquisition
mode. The oscilloscope creates a waveform point
by saving one sample point during each waveform interval.
• Peak Detect Mode: The oscilloscope saves the
minimum and maximum value sample points
taken during two waveform intervals and uses
these samples as the two corresponding waveform points. Digitizing oscilloscopes with peak
detect mode run the ADC at a fast sample rate,
even at very slow time base settings (long waveform interval), and are able to capture fast signal
changes that would occur between the waveform
points if in sample mode. Peak detect mode is
particularly useful for seeing narrow pulses
spaced far apart in time.
• Hi Res Mode: Like peak detect, hi res mode is a
way of getting more information in cases when
the ADC can sample faster than the time base
setting requires. In this case, multiple samples
taken within one waveform interval are averaged
together to produce one waveform point. The
result is a decrease in noise and an improvement
in resolution for low-speed signals.
• Envelope Mode: Envelope mode is similar to peak
detect mode. However, in envelope mode, the
minimum and maximum waveform points from
multiple acquisitions are combined to form a
waveform that shows min/max changes over
time. Peak detect mode is usually used to acquire
the records that are combined to form the envelope waveform.
• Average Mode: In average mode, the oscilloscope
saves one sample point during each waveform
interval as in sample mode. However, waveform
points from consecutive acquisitions are then
averaged together to produce the final displayed
waveform. Average mode reduces noise without
loss of bandwidth but requires a repeating signal.
A special note about DPO acquisition: The Digital
Phosphor Oscilloscope has a high display sample
density and an innate ability to capture intensity
(Z-axis) information. With its intensity axis, the DPO
is able to provide the same type of 3-dimensional,
real-time display that analog scopes are known for.
As you look at the waveform trace on a DPO, you can
see brightened areas. These are the areas where the
signal occurs most often. This makes it easy to
distinguish, for example, the basic signal shape from
a transient that occurs only once in a while. The
basic signal would appear much brighter.
DPOs also include all the acquisition modes
described above.
Figure 37. Example of an acquisition menu.
21
Stopping and Starting the Acquisition System
One of the greatest advantages of digitizing oscilloscopes is their ability to store waveforms for later
viewing. To this end, there are usually one or more
buttons on the front panel that allow you to stop and
start the acquisition system so you can analyze
waveforms at your leisure. Additionally, you may
want the oscilloscope to automatically stop
acquiring after one acquisition is complete or after
one set of records has been turned into an envelope
or average waveform. This feature is commonly
called single sweep or single sequence and its
controls are usually found either with the other
acquisition controls or with the trigger controls.
Sampling Methods
In digitizing oscilloscopes that can use either realtime sampling or equivalent-time sampling as
described earlier, the acquisition controls will allow
you to choose which one to use for acquiring signals.
22
Note that this choice makes no difference for slow
time base settings and only has an effect when the
ADC cannot sample fast enough to fill the record
with waveform points in one pass.
Other Controls
So far, we have described the basic controls that a
beginner needs to know about. Your oscilloscope
may have other controls for various functions. Some
of these may include:
• Measurement cursors
• Keypads for mathematical operations or data
entry
• Printing capabilities
• Interfaces for connecting your oscilloscope to a
computer
Look over the other options available to you and
read your oscilloscope’s manual to find out more
about these other controls.
Measurement Techniques
This section teaches you basic measurement techniques. The two most basic measurements you can
make are voltage and time measurements. Just about
every other measurement is based on one of these
two fundamental techniques.
Another handy formula is the power law: the power
of a DC signal equals the voltage times the current.
Calculations are more complicated for AC signals,
but the point here is that measuring the voltage is the
first step toward calculating other quantities.
This section discusses methods for making measurements visually with the oscilloscope screen. This is a
common technique with analog instruments, and
also may be useful for “at-a-glance” interpretation of
DSO or DPO displays.
Figure 39 shows the voltage of one peak (Vp) and the
peak-to-peak voltage (Vp-p), which is usually twice
Vp. Use the RMS (root-mean-square) voltage (VRMS)
to calculate the power of an AC signal.
Note that most digitizing oscilloscopes include automated measurement tools. Knowing how to make
measurements manually as described here will help
you understand and check the automatic measurements of DSOs and DPOs. Automated measurements
are explained later in this section.
The most basic method of taking voltage measurements is to count the number of divisions a waveform spans on the oscilloscope’s vertical scale.
Adjusting the signal to cover most of the screen
The Display
Take a look at the oscilloscope display. Notice the
grid markings on the screen – these markings create
the graticule. Each vertical and horizontal line
constitutes a major division. The graticule is usually
laid out in an 8-by-10 division pattern. Labeling on
the oscilloscope controls (such as volts/div and
sec/div) always refers to major divisions. The tick
marks on the center horizontal and vertical graticule
lines (see Figure 38) are called minor divisions.
Many oscilloscopes display on the screen how many
volts each vertical division represents and how
many seconds each horizontal division represents.
Voltage Measurements
Voltage is the amount of electric potential, expressed
in volts, between two points in a circuit. Usually one
of these points is ground (zero volts) but not always.
Voltages can also be measured from peak-to-peak –
from the maximum point of a signal to its minimum
point. You must be careful to specify which voltage
you mean.
Figure 38. An oscilloscope graticule.
The oscilloscope is primarily a voltage-measuring
device. Once you have measured the voltage, other
quantities are just a calculation away. For example,
Ohm’s law states that voltage between two points in
a circuit equals the current times the resistance.
From any two of these quantities, you can calculate
the third using the following formula:
Ohm’s Law:
Voltage = Current * Resistance
Current =
Voltage
Resistance
Resistance =
Voltage
Current
Figure 39. Voltage peak and peak-to-peak voltage.
Power Law:
Power = Voltage * Current
23
vertically, then taking the measurement along the
center vertical graticule line having the smaller divisions makes for the best voltage measurements (see
Figure 40). The more screen area you use, the more
accurately you can read from the screen.
Many oscilloscopes have on-screen cursors that let
you take waveform measurements automatically onscreen, without having to count graticule marks. A
cursor is simply a line that you can move across the
screen. Two horizontal cursor lines can be moved up
and down to bracket a waveform’s amplitude for
voltage measurements, and two vertical lines move
right and left for time measurements. A readout
shows the voltage or time at the positions of the
cursors.
Time and Frequency Measurements
You take time measurements using the horizontal
scale of the oscilloscope. Time measurements
include measuring the period, pulse width, and
timing of pulses. Frequency is the reciprocal of the
period, so once you know the period, the frequency
is one divided by the period. Like voltage measurements, time measurements are more accurate when
you adjust the portion of the signal to be measured to
cover a large area of the screen. Taking time measurements along the center horizontal graticule line,
having smaller divisions, makes for the best time
measurements (see Figure 41).
Pulse and Rise Time Measurements
Figure 40. Measure voltage on the center vertical graticule line.
In many applications, the details of a pulse’s shape
are important. Pulses can become distorted and
cause a digital circuit to malfunction, and the timing
of pulses in a pulse train is often significant.
Standard pulse measurements are pulse width and
pulse rise time. Rise time is the amount of time a
pulse takes to go from the low to high voltage. By
convention, the rise time is measured from 10% to
90% of the full voltage of the pulse. This eliminates
any irregularities at the pulse’s transition corners.
This also explains why most oscilloscopes have 10%
and 90% markings on their screen. Pulse width is
the amount of time the pulse takes to go from low to
high and back to low again. By convention, the pulse
width is measured at 50% of full voltage. See Figure
42 for these measurement points.
Pulse measurements often require fine-tuning the
triggering. To become an expert at capturing pulses,
you should learn how to use trigger holdoff and how
to set the digitizing oscilloscope to capture pretrigger
data, as described earlier in The Controls section.
Horizontal magnification is another useful feature for
measuring pulses, since it allows you to see fine
details of a fast pulse.
Figure 41. Measure time on the center horizontal graticule line.
Figure 42. Rise time and pulse width measurement points.
24
Phase Shift Measurements
The horizontal control section may have an XY
mode that lets you display an input signal rather
than the time base on the horizontal axis. This mode
of operation opens up a whole new area of phaseshift measurement techniques.
The phase of a wave is the amount of time that
passes from the beginning of a cycle to the beginning
of the next cycle, measured in degrees. Phase shift
describes the difference in timing between two
otherwise identical periodic signals.
One method for measuring phase shift is to use XY
mode. This involves connecting one signal to the
vertical system as usual and then another signal to
the horizontal system. (This method only works if
both signals are sinusoidal.) This set up is called an
XY measurement because both the X and Y axis are
tracing voltages. The waveform resulting from this
arrangement is called a Lissajous pattern (named for
French physicist Jules Antoine Lissajous and
pronounced LEE-sa-zhoo). From the shape of the
Lissajous pattern, you can tell the phase difference
between the two signals. You can also tell their
frequency ratio. Figure 43 shows Lissajous patterns
for various frequency ratios and phase shifts.
The XY measurement mode originated with analog
oscilloscopes. Due to their relatively low sample
density, DSOs may have difficulty creating real-time
XY displays. Some DSOs create an XY image by
accumulating data points over time, then displaying
the composite. Digital Phosphor Oscilloscopes, on
the other hand, are able to acquire and display a
genuine XY mode image in real-time, using a continuous stream of digitized data. DPOs can also display
an XYZ image with intensified areas.
Figure 43. Lissajous patterns.
25
Waveform Measurements with
Digitizing Oscilloscopes
Digitizing oscilloscopes have functions that make
waveform measurements easier. Modern DSOs and
DPOs have front-panel buttons or screen-based
menus from which you can select fully automated
measurements. These include amplitude, period,
rise/fall time, and much more. Many digitizing
instruments also provide mean and RMS calculations, duty cycle, and other math operations.
Automated measurements appear as on-screen
alphanumeric readouts. Typically these readings are
more accurate than it’s possible to obtain with direct
graticule interpretation.
Following is a list of the fully automated waveform
measurements available on the TDS 500D/700D
Series Digital Phosphor Oscilloscopes.
Period
Frequency
Width +
Width –
Rise time
Fall time
Amplitude
Extinction ratio
Mean optical power
26
Duty cycle +
Duty cycle –
Delay
Phase
Burst width
Peak-to-peak
Mean
Cycle mean
Cycle area
High
Low
Minimum
Maximum
Overshoot +
Overshoot –
RMS
Cycle RMS
What’s Next?
This section has covered basic measurement techniques. Other measurement techniques involve
setting up the oscilloscope to test electrical components on an assembly line, subtracting noise from a
signal, capturing elusive transient signals, and many
others that would take too much room to list. The
measurement techniques you will use depend on
your application, but you have learned enough to get
started. Practice using your oscilloscope and read
more about it. Soon its operation will be second
nature to you.
Written Exercises
This section contains written exercises that cover information in this book. The
exercises are divided into two parts, Part I and Part II.
Part I covers information presented in these sections:
• The Oscilloscope
• Oscilloscope Terminology
Part II covers information presented in sections:
• Setting Up
• The Controls
• Measurement Techniques
27
Part I Exercises
The following exercises cover information presented in these sections:
• The Oscilloscope
• Oscilloscope Terminology
Check how well you have absorbed the information in these sections by doing this short self-test. Answers are
given on page 34.
Vocabulary Exercise
Write the number of the definitions in the right column next to the correct words in the left column.
Term
A. The unit of electric potential difference.
2. ___ Analog
B. A performance measurement indicating the precision of an ADC, measured in bits.
3. ___ Bandwidth
C. Term used when referring to degree points of a sine wave.
4. ___ Digital Phosphor
D. The number of times a signal repeats in one second.
5. ___ Frequency
E. The amount of time it takes a wave to complete one cycle.
6. ___ Glitch
F. A stored digital value that represents the voltage of a signal at a specific point in time.
7. ___ Period
G. A common waveform shape that has a rising edge, a width, and a falling edge.
8. ___ Phase
H. A performance measurement indicating the fastest edge a given oscilloscope can
accurately display.
9. ___ Pulse
28
Definition
1. ___ Acquisition
I. Oscilloscope circuitry that controls the timing of the sweep.
10. ___ Waveform Point
J. An intermittent error in a circuit.
11. ___ Rise Time
K. A signal measured by an oscilloscope that only occurs once.
12. ___ Sample Point
L. The oscilloscope’s process of collecting sample points from the ADC, processing
them, and storing them in memory.
13. ___ Digital Storage
M. Something that operates with continuous values.
14. ___ Time Base
N. Digitizing Oscilloscope that captures 3 dimensions of signal information in real- time.
15. ___ Transient
O. Digitizing Oscilloscope with serial processing.
16. ___ ADC Resolution
P. A frequency range.
17. ___ Volt
Q. The raw data from an ADC used to calculate waveform points.
Using Oscilloscopes Exercise
Circle the best answer for each statement. Some statements have more than one right answer.
1.
With an oscilloscope you can:
a. Calculate the frequency of a signal.
b. Find malfunctioning electrical components.
c. Analyze bird calls.
e. All the above.
2.
The difference between analog and digitizing oscilloscopes is:
a. Analog oscilloscopes do not have on-screen menus.
b. Analog oscilloscopes apply a measurement voltage directly to the display system, while digital oscilloscopes first convert the voltage into digital values.
c. Analog oscilloscopes measure analogs, whereas digitizing oscilloscopes measure digits.
d. Analog oscilloscopes do not have an acquisition system.
3.
An oscilloscope’s vertical section does the following:
a. Acquires sample points with an ADC.
b. Starts a horizontal sweep.
c. Lets you adjust the brightness of the display.
d. Attenuates or amplifies the input signal.
4.
The time base control of the oscilloscope does the following:
a. Adjusts the vertical scale.
b. Shows you the current time of day.
c. Sets the amount of time represented by the horizontal width of the screen.
d. Sends a clock pulse to the probe.
5.
On an oscilloscope display:
a. Voltage is on the vertical axis and time is on the horizontal axis.
b. A straight diagonal trace means voltage is changing at a steady rate.
c. A flat horizontal trace means voltage is constant.
d. All the above.
6.
All repeating waves have the following properties:
a. A frequency measured in hertz.
b. A period measured in seconds.
c. A bandwidth measured in hertz.
d. All the above.
7.
If you probe inside a computer with an oscilloscope, you are likely to find the following types of signals:
a. Pulse trains.
b. Ramp waves.
c. Sine waves.
d. All the above.
8.
When evaluating the performance of an analog oscilloscope, some things you might consider are:
a. The bandwidth.
b. The vertical sensitivity.
c. The ADC resolution.
d. The sweep speed.
9.
The difference between digital storage oscilloscopes (DSO) and digital phosphor oscilloscopes (DPO) is:
a. The DSO has a higher bandwidth.
b. The DPO captures three dimensions of waveform information in real-time.
c. The DSO has a color display.
d. The DSO captures more signal details.
29
Part II Exercises
The following exercises cover information presented in these sections:
• Setting Up
• The Controls
• Measurement Techniques
Check how well you have absorbed the information in these sections by doing this short self-test. Answers are
given on page 34.
Vocabulary Exercise
Write the letter of the definitions in the right column next to the correct words in the left column.
Term
1. ___ Averaging Mode
A. The unintentional interaction of the probe and oscilloscope with the circuit being
tested which distorts a signal.
2. ___ Circuit Loading
B. A conductor that connects electrical currents to the Earth.
3. ___ Compensation
C. A sampling mode in which the digital oscilloscope collects as many samples as it can
as the signal occurs, then constructs a display, using interpolation if necessary.
4. ___ Coupling
D. A sampling mode in which the digital oscilloscope constructs a picture of a repetitive
signal by capturing a little bit of information from each repetition.
5. ___ Earth Ground
E. A device that converts a specific physical quantity such as sound, pressure, strain, or
light intensity into an electrical signal.
6. ___ Equivalent-Time
F. A test device for injecting a signal into a circuit input.
7. ___ Graticule
G. A processing technique used by digital oscilloscopes to eliminate noise in a signal.
8. ___ Interpolation
H. The method of connecting two circuits together.
9. ___ Real Time
30
Definition
I. A “connect-the-dots” processing technique to estimate what a fast waveform looks
like based on only a few sampled points.
10. ___ Signal Generator
J. The grid lines on a screen for measuring oscilloscope traces.
11. ___ Single Sweep
K. A trigger mode that triggers the sweep once, must be reset to accept another trigger
event.
12. ___ Transducer
L. A probe adjustment for 10X attenuator probes that balances the electrical properties
of the probe with the electrical properties of the oscilloscope.
Using Oscilloscopes Exercise
Circle the best answer for each statement. Some statements have more than one right answer.
1.
To operate an oscilloscope safely, you should:
a. Ground the oscilloscope with the proper three-pronged power cord.
b. Learn to recognize potentially dangerous electrical components.
c. Avoid touching exposed connections in a circuit being tested even if the power is off.
d. All the above.
2.
Grounding an oscilloscope is necessary:
a. For safety reasons.
b. To provide a reference point for making measurements.
c. To align the trace with the screen’s horizontal axis.
d. All the above.
3.
Circuit loading is caused by:
a. An input signal having too large a voltage.
b. The probe and oscilloscope interacting with the circuit being tested.
c. A 10X attenuator probe being uncompensated.
d. Putting too much weight on a circuit.
4.
Compensating a probe is necessary to:
a. Balance the electrical properties of the 10X attenuator probe with the oscilloscope.
b. Prevent damaging the circuit being tested.
c. Improve the accuracy of your measurements.
d. All the above.
5.
The trace rotation control is useful for:
a. Scaling waveforms on the screen.
b. Detecting sine wave signals.
c. Aligning the waveform trace with the screen’s horizontal axis on an analog oscilloscope.
d. Measuring pulse width.
6.
The volts per division control is used to:
a. Scale a waveform vertically.
b. Position a waveform vertically.
c. Attenuate or amplify an input signal.
d. Set the numbers of volts each division represents.
7.
Setting the vertical input coupling to ground does the following:
a. Disconnects the input signal from the oscilloscope.
b. Causes a horizontal display to appear on the screen.
c. Lets you see where zero volts is on the screen.
d. All the above.
8.
The trigger is necessary to:
a. Stabilize repeating waveforms on the screen.
b. Capture single-shot waveforms.
c. Mark a particular point of an acquisition.
d. All the above.
9.
The difference between auto and normal trigger mode is:
a. In normal mode the oscilloscope only sweeps once and then stops.
b. In normal mode the oscilloscope only sweeps if the input signal reaches the trigger point; otherwise
the screen is blank.
c. Auto mode makes the oscilloscope sweep continuously even without being triggered.
d. All the above.
31
10.
A digital oscilloscope’s acquisition controls let you specify:
a. Whether the oscilloscope uses real-time or equivalent-time sampling to collect sample points.
b. Whether to average a collection of records to form a waveform.
c. How sample points are processed to form waveform points.
d. All the above.
11.
The acquisition mode that best reduces noise in a repeating signal is:
a. Sample mode.
b. Peak detect mode.
c. Envelope mode.
d. Averaging mode.
12.
The two most basic measurements you can make with an oscilloscope are:
a. Time and frequency measurements.
b. Time and voltage measurements.
c. Voltage and pulse width measurements.
d. Pulse width and phase shift measurements.
13.
If the volts/division is set at 0.5, the largest signal that can fit on the screen (assuming an 8 x 10 division
screen) is:
a. 62.5 millivolts peak-to-peak.
b. 8 volts peak-to-peak.
c. 4 volts peak-to-peak.
d. 0.5 volts peak-to-peak.
14.
If the seconds/division is set at 0.1 ms, the amount of time represented by the width of the screen is:
a. 0.1 ms.
b. 1 ms.
c. 1 second.
d. 0.1 kHz.
15.
By convention, pulse width is measured:
a. At 10% of the pulse’s maximum voltage.
b. At 50% of the pulse’s maximum voltage.
c. At 90% of the pulse’s maximum voltage.
d. At 10% and 90% of the pulse’s maximum voltage.
16.
You attach a probe to your test circuit but the screen is blank. You should:
a. Check that the screen intensity is turned up.
b. Check that the oscilloscope is set to display the channel that the probe is connected to.
c. Set the trigger mode to auto since norm mode blanks the screen.
d. Set the vertical input coupling to AC and set the volts/division to its largest value since a large DC
signal may go off the top or bottom of the screen.
e. Check that the probe isn’t shorted and make sure it is properly grounded.
f. Check that the oscilloscope is set to trigger on the input channel you are using.
g. All of the above.
32
Answers to Exercises on Back
33
Answers to Written Exercises
This section provides the answers to all written exercises in the previous sections.
Part I: Vocabulary Exercise Answers
1. L
5. D
9. G
13. O
2. M
6. J
10. F
14. I
3. P
7. E
11. H
15. K
4. N
8. C
12. Q
16. B
17. A
Part I: Oscilloscope Usage Exercise Answers
1. D
3. D
5. D
7. D
2. B, D
4. C
6. A,B
8. A, B, D
9. B
Part II: Vocabulary Exercise Answers
1. G
4. H
7. J
10. F
2. A
5. B
8. I
11. K
3. L
6. D
9. C
12. E
Part II: Oscilloscope Usage Exercise Answers
34
1. D
5. C
2. A, B
6. A, C, D
10. D
9. B,C
14. B
13. C
3. B
7. D
11. D
15. B
4. A, C
8. D
12. B
16. G
Glossary
AC
(Alternating Current) A signal in which the current
and voltage vary in a repeating pattern over time.
ADC
(Analog-to-Digital Converter) A digital electronic
component that converts an electrical signal into
discrete binary values.
Alternate Mode
A display mode of operation in which the oscilloscope completes tracing one channel before beginning to trace another channel.
Amplitude
The magnitude of a quantity or strength of a signal.
In electronics, amplitude usually refers to either
voltage or power.
Analog Oscilloscope
One of three prevalent oscilloscope architectures
(the other two are DSOs and DPOs – see definitions
below). An instrument that creates a waveform
display by applying the input signal (conditioned
and amplified) to an electron beam moving across a
CRT screen. A chemical phosphor coating on the
CRT creates a glowing trace wherever the beam
hits.
Attenuation
A decrease in signal voltage during its transmission
from one point to another.
Averaging
A processing technique used by digital oscilloscopes to eliminate noise in a signal.
Bandwidth
A frequency range.
CRT
(Cathode-Ray Tube) An electron-beam tube in
which the beam can be focused on a luminescent
screen and varied in both position and intensity to
produce a visible pattern. A television picture tube
is a CRT.
Chop Mode
A display mode of operation in which small parts
of each channel are traced so that more than one
waveform can appear on the screen simultaneously.
Circuit Loading
The unintentional interaction of the probe and
oscilloscope with the circuit being tested,
distorting the signal.
Compensation
A probe adjustment for 10X probes that balances
the capacitance of the probe with the capacitance
of the oscilloscope.
Coupling
The method of connecting two circuits together.
Circuits connected with a wire are directly
coupled; circuits connected through a capacitor or
a transformer are indirectly (or AC) coupled.
Cursor
An on-screen marker that you can align with a
waveform to take accurate measurements.
DC
(Direct Current) A signal with a constant voltage
and current.
Digital Phosphor Oscilloscope (DPO)
A digitizing oscilloscope that closely models the
display characteristics of an analog oscilloscope
while providing traditional digitizing oscilloscope
benefits (waveform storage, automated measurements, etc.). The DPO uses a parallel processing
architecture to pass the signal to the raster-type
display. This provides intensity-graded viewing
characteristics.
Digital Storage Oscilloscope (DSO)
An oscilloscope that acquires signals via digital
sampling (using an analog-to-digital converter). It
uses a serial architecture that employs a single
processor to control acquisition, user interface, and
the raster display.
Division
Measurement markings on the CRT graticule of the
oscilloscope.
Earth Ground
A conductor that will dissipate large electrical
currents into the Earth.
Envelope
The outline of a signal’s highest and lowest points
acquired over many repetitions.
Equivalent-time Sampling
A sampling mode in which the oscilloscope
constructs a picture of a repetitive signal by
capturing a little bit of information from each repetition.
Focus
The oscilloscope control that adjusts the CRT electron beam to control the sharpness of the display.
Frequency
The number of times a signal repeats in one
second, measured in Hertz (cycles per second). The
frequency equals 1/period.
Gigahertz (GHz)
1,000,000,000 Hertz; a unit of frequency.
Glitch
An intermittent error in a circuit.
Graticule
The grid lines on a screen for measuring oscilloscope traces.
Ground
1. A conducting connection by which an electric
circuit or equipment is connected to the earth to
establish and maintain a reference voltage level.
2. The voltage reference point in a circuit.
Hertz (Hz)
One cycle per second; the unit of frequency.
Kilohertz (kHz)
1000 Hertz; a unit of frequency.
Interpolation
A “connect-the-dots” processing technique to estimate what a fast waveform looks like based on only
a few sampled points.
35
Megahertz (MHz)
1,000,000 Hertz; a unit of frequency.
Megasamples per second (MS/s)
A sample rate unit equal to one million samples
per second.
Microsecond (µs)
A unit of time equivalent to 0.000001 seconds.
Millisecond (ms)
A unit of time equivalent to 0.001 seconds.
Nanosecond (ns)
A unit of time equivalent to 0.000000001 seconds.
Noise
An unwanted voltage or current in an electrical
circuit.
Oscilloscope
An instrument used to make voltage changes
visible over time. The word oscilloscope comes
from “oscillate,” since oscilloscopes are often used
to measure oscillating voltages.
Peak (Vp)
The maximum voltage level measured from a zero
reference point.
Peak-to-peak (Vp-p)
The voltage measured from the maximum point of
a signal to its minimum point, usually twice the Vp
level.
Peak Detection
An acquisition mode for digital oscilloscopes that
lets you see the extremes of a signal.
Period
The amount of time it takes a wave to complete one
cycle. The period equals 1/frequency.
Phase
The amount of time that passes from the beginning
of a cycle to the beginning of the next cycle,
measured in degrees.
Probe
An oscilloscope input device, usually having a
pointed metal tip for making electrical contact with
a circuit element and a flexible cable for transmitting the signal to the oscilloscope.
Pulse
A common waveform shape that has a fast rising
edge, a width, and a fast falling edge.
RMS
Root mean square.
Real-time Sampling
A sampling mode in which the oscilloscope
collects as many samples as it can as the signal
occurs.
Record Length
The number of waveform points used to create a
record of a signal.
Rise Time
The time taken for the leading edge of a pulse to
rise from its minimum to its maximum values
(typically measured from 10% to 90% of these
values).
Sample Point
The raw data from an ADC used to calculate waveform points.
36
Screen
The surface of the CRT upon which the visible
pattern is produced – the display area.
Signal Generator
A test device for injecting a signal into a circuit
input; the circuit’s output is then read by an oscilloscope.
Sine Wave
A common curved wave shape that is mathematically defined.
Single Shot
A signal measured by an oscilloscope that only
occurs once (also called a transient event).
Single Sweep
A trigger mode for displaying one screen full of a
signal and then stopping.
Slope
On a graph or an oscilloscope screen, the ratio of a
vertical distance to a horizontal distance. A positive slope increases from left to right, while a negative slope decreases from left to right.
Square Wave
A common wave shape consisting of repeating
square pulses.
Sweep
One horizontal pass of an oscilloscope’s electron
beam from left to right across the CRT screen.
Sweep Speed
A measurement of how fast the time base “sweeps”
the electron beam across the CRT screen.
Time Base
Oscilloscope circuitry that controls the timing of
the sweep. The time base is set by the seconds/division control.
Trace
The visible shapes drawn on a CRT by the movement of the electron beam.
Transducer
A device that converts a specific physical quantity
such as sound, pressure, strain, or light intensity
into an electrical signal.
Transient
A signal measured by an oscilloscope that only
occurs once (also called a single-shot event).
Trigger
The circuit that initiates a horizontal sweep on an
oscilloscope and determines the beginning point of
the waveform.
Trigger Holdoff
A control that inhibits the trigger circuit from
looking for a trigger level for some specified time
after the end of the waveform.
Trigger Level
The voltage level that a trigger source signal must
reach before the trigger circuit initiates a sweep.
Volt
The unit of electric potential difference.
Voltage
The difference in electric potential, expressed in
volts, between two points.
Waveform
A graphic representation of a voltage varying over
time.
Z-axis
The signal in an oscilloscope that controls electronbeam brightness as the trace is formed.
Waveform Point
A digital value that represents the voltage of a
signal at a specific point in time. Waveform points
are calculated from sample points and stored in
memory.
37
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