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Evaluating Oscilloscope Fundamentals
Application Note
This application note provides an overview of oscilloscope
fundamentals. You will learn what an oscilloscope is and how
it operates. We will discuss oscilloscope applications and give
you an overview of basic measurements and performance
characteristics. We will also look at the different types of probes
and discuss their advantages and disadvantages.
Introduction
Electronic technology permeates our lives. Millions Table of Contents
of people use electronic devices such as cell phones,
televisions, and computers on a daily basis. As Introduction . . . . . . . . . . . . . . . . . . 2
electronic technology has advanced, the speeds Electronic Signals . . . . . . . . . . . . . 3
at which these devices operate have accelerated. Wave properties . . . . . . . . . . . . . . . . 3
Today, most devices use high-speed digital Waveforms . . . . . . . . . . . . . . . . . . . . 4
technologies. Analog versus digital signals . . . . . . 5
What is an Oscilloscope and
Engineers need the ability to accurately design Why Do You Need One? . . . . . . . . 6
and test the components in their high-speed Signal integrity . . . . . . . . . . . . . . . . . 6
digital devices. The instrumentation engineers What an oscilloscope looks like . . . . 7
An oscilloscope's purpose . . . . . . . . 8
use to design and test their components must be
Types of oscilloscopes . . . . . . . . . . . 9
particularly well-suited to deal with high speeds and
Where oscilloscopes are used . . . . 12
high frequencies. An oscilloscope is an example of
just such an instrument. Basic Oscilloscope Controls and
Measurements . . . . . . . . . . . . . . . 12
Basic front-panel controls . . . . . . . 12
Oscilloscopes are powerful tools that are useful
Softkeys . . . . . . . . . . . . . . . . . . . . . 15
for designing and testing electronic devices. They
Basic measurements . . . . . . . . . . . 16
are vital in determining which components of Basic mathematical functions . . . . 17
a system are behaving correctly and which are
Important Oscilloscope
malfunctioning. They can also help you determine
Performance Characteristics . . . . 18
whether or not a newly designed component
Bandwidth . . . . . . . . . . . . . . . . . . . 18
behaves the way you intended. Oscilloscopes are far Channels . . . . . . . . . . . . . . . . . . . . . 18
more powerful than multimeters because they allow Sample rate . . . . . . . . . . . . . . . . . . 19
you to see what the electronic signals actually look Memory depth . . . . . . . . . . . . . . . . 20
like. Update rate . . . . . . . . . . . . . . . . . . 21
Oscilloscope connectivity . . . . . . . . 21
Oscilloscopes are used in a wide range of fields, Oscilloscope Probes . . . . . . . . . . . 22
from the automotive industry to university research Loading . . . . . . . . . . . . . . . . . . . . . . 22
laboratories, to the aerospace-defense industry. Passive probes . . . . . . . . . . . . . . . . 22
Companies rely on oscilloscopes to help them Active probes . . . . . . . . . . . . . . . . . 22
uncover defects and produce fully-functional Current probes . . . . . . . . . . . . . . . . 23
products. Probe accessories . . . . . . . . . . . . . 23
Conclusion . . . . . . . . . . . . . . . . . . 23
03 | Keysight | Evaluating Oscilloscope Fundamentals - Application Note
Electronic Signals
The main purpose of an oscilloscope is Wave properties tions. The first is often referred to as
to display electronic signals. By viewing the peak amplitude and is defined
signals displayed on an oscilloscope you Electronic signals are waves or pulses. as the magnitude of the maximum
can determine whether a component of Basic properties of waves include: displacement of a disturbance. The
an electronic system is behaving properly. second is called the root-mean-square
So, to understand how an oscilloscope Amplitude (RMS) amplitude. To calculate the
operates, it is important to understand RMS voltage of a waveform, square
Two main definitions for amplitude are
basic signal theory. the waveform, find its average voltage
commonly used in engineering applica-
and take the square root.
For a sine wave, the RMS amplitude is
equal to 0.707 times the peak amplitude.
Phase shift
peak amplitude Phase shift refers to the amount
RMS amplitude
of horizontal translation between
two otherwise identical waves. It is
measured in degrees or radians. For
a sine wave, one cycle is represented
by 360 degrees. Therefore, if two sine
waves differ by half of a cycle, their
relative phase shift is 180 degrees.
Period
Figure 1. Peak amplitude and RMS amplitude for a sine wave
The period of a wave is simply the
amount of time it takes for a wave to
repeat itself. It is measured in units of
seconds.
Frequency
Every periodic wave has a frequency.
The frequency is simply the number
of times a wave repeats itself within
one second (if you are working in units
of Hertz). The frequency is also the
reciprocal of the period.
period
Figure 2. The period of a triangular wave
04 | Keysight | Evaluating Oscilloscope Fundamentals - Application Note
Electronic Signals (continued)
Waveforms Sine waves Square/rectangular waves
A waveform is the shape or representa- Sine waves are typically associated with A square waveform periodically jumps
tion of a wave. Waveforms can provide you alternating current (AC) sources such as between two different values such that the
with a great deal of information about your an electrical outlet in your house. A sine lengths of the high and low segments are
signal. For example, it can tell you if the wave does not always have a constant equivalent. A rectangular waveform differs
voltage changes suddenly, varies linearly, peak amplitude. If the peak amplitude in that the lengths of the high and low
or remains constant. There are many continually decreases as time progresses, segments are not equal.
standard waveforms, but this section will we call the waveform a damped sine wave.
cover the ones you will encounter most
frequently.
Figure 3. A sine wave
Figure 4. A square wave
05 | Keysight | Evaluating Oscilloscope Fundamentals - Application Note
Electronic Signals (continued)
Triangular/sawtooth waves Analog versus digital signals simply tells you the hour and the minute.
It is, therefore, discretized into minute in-
In a triangular wave, the voltage varies Analog signals are able to take on any tervals. One second it might be 11:54 and
linearly with time. The edges are called value within some range. It is useful to then it jumps to 11:55 suddenly. Digital
ramps because the waveform is either think of an analog clock. The clock hands signals are likewise discrete and quan-
ramping up or ramping down to certain spin around the clock face every twelve tized. Typically, discrete signals have two
voltages. A sawtooth wave looks similar hours. During this time, the clock hands possible values (high or low, 1 or 0, etc.).
in that either the front or back edge has a move continuously. There are no jumps or The signals, therefore, jump back and
linear voltage response with time. How- discreteness in the reading. Now, com- forth between these two possibilities.
ever, the opposite edge has an almost pare this to a digital clock. A digital clock
immediate drop.
Pulses
A pulse is a sudden single disturbance in
an otherwise constant voltage. Imagine
flipping the switch to turn the lights on in
a room and then quickly turning them off.
A series of pulses is called a pulse train.
To continue our analogy, this would be like
quickly turning the lights on and off over Figure 5. A triangular wave
and over again.
Pulses are the common waveform of
glitches or errors in your signal. A pulse
might also be the waveform if the signal is
carrying a single piece of information.
Complex waves
Waves can also be mixtures of the above
waveforms. They do not necessarily need Figure 6. A sawtooth wave
to be periodic and can take on very com-
plex wave shapes.
Figure 7. A pulse
06 | Keysight | Evaluating Oscilloscope Fundamentals - Application Note
What Is an Oscilloscope and Why Do You Need One?
Signal integrity shape or characteristics as the true signal.
It is, however, important to remember
The main purpose of an oscilloscope is to that the waveform on an oscilloscope
give an accurate visual representation of will never be an exact representation of
electrical signals. For this reason, signal the true signal, no matter how good the
integrity is very important. Signal integ- oscilloscope is. This is because when you
rity refers to the oscilloscope's ability to connect an oscilloscope to a circuit, the
reconstruct the waveform so that it is an oscilloscope becomes part of the circuit.
accurate representation of the original In other words, there are some loading
signal. An oscilloscope with low signal effects. Instrument makers strive to mini-
integrity is useless because it is pointless mize loading effects, but they always exist
to perform a test when the waveform on to some degree.
the oscilloscope does not have the same
07 | Keysight | Evaluating Oscilloscope Fundamentals - Application Note
What Is an Oscilloscope and Why Do You Need One? (continued)
What an oscilloscope looks like Display Horizontal control section
In general, modern digitizing oscilloscopes
look similar to the one seen in Figure 8.
However, there are a wide variety of oscil-
loscope types, and yours may look very Trigger
different. Despite this, there are some ba- control
sic features that most oscilloscopes have. section
The front panel of most oscilloscopes can
be divided into several basic sections: the
Vertical
channel inputs, the display, the horizontal control
controls, the vertical controls, and the section
trigger controls. If your oscilloscope does
not have a Microsoft Windows-based op-
erating system, it will probably have a set
of softkeys to control on-screen menus. Softkeys Channel inputs
You send your signals into the oscilloscope Figure 8. Front panel on the Keysight InfiniiVision 2000 X-Series oscilloscope
via the channel inputs, which are con-
nectors for plugging in your probes. The
display is simply the screen where these
signals are displayed. The horizontal and
vertical control sections have knobs and
buttons that control the horizontal axis
(which typically represents time) and verti-
cal axis (which represents voltage) of the
signals on the screen display. The trigger
controls allow you to tell the oscilloscope
under what conditions you want the time-
base to start an acquisition.
An example of what the back panel of an
oscilloscope looks like is seen in Figure 9.
As you can see, many oscilloscopes have Figure 9. Rear panel on the Keysight Infiniium 9000 Series oscilloscope
the connectivity features found on person-
al computers. Examples include CD-ROM
drives, CD-RW drives, DVD-RW drives,
USB ports, serial ports, and external
monitor, mouse, and keyboard inputs.
08 | Keysight | Evaluating Oscilloscope Fundamentals - Application Note
What Is an Oscilloscope and Why Do You Need One? (continued)
An oscilloscope's purpose
An oscilloscope is a measurement and
testing instrument used to display a
certain variable as a function of another.
For example, it can plot on its display
a graph of voltage (y-axis) versus time
(x-axis). Figure 10 shows an example of
such a plot. This is useful if you want
to test a certain electronic component
to see if it is behaving properly. If you
know what the waveform of the signal
should be after exiting the component,
you can use an oscilloscope to see if
the component is indeed outputting the
correct signal. Notice also that the x Figure 10. An oscilloscope's voltage versus time display of a square wave
and y-axes are broken into divisions by
a graticule. The graticule enables you to
make measurements by visual estimation,
although with modern oscilloscopes, most
of these measurements can be made
automatically and more accurately by the
oscilloscope itself.
180 degrees; 1:1 ratio 90 degrees; 1:1 ratio 90 degrees; 1:2 ratio 30 de
An oscilloscope can also do more than
plot voltage versus time. An oscilloscope
has multiple inputs, called channels, and
each one of these acts independently.
Therefore, you could connect channel
1 to a certain device and channel
180 degrees; 1:1 ratio 90 degrees; 1:1 ratio 90 degrees; 1:2 ratio 30 degrees; 1:3 ratio
2 to another. The oscilloscope could then
plot the voltage measured by channel
1 versus the voltage measured by channel
2. This mode is called the XY-mode of an Figure 11. Lissajous patterns
oscilloscope. It is useful when graphing
I-V plots or Lissajous patterns where
the shape of these patterns tells you the
phase difference and the frequency ratio
between the two signals. Figure 11 shows
examples of Lissajous patterns and the
phase difference/frequency ratio they
represent.
09 | Keysight | Evaluating Oscilloscope Fundamentals - Application Note
What Is an Oscilloscope and Why Do You Need One? (continued)
Types of oscilloscopes Digital storage oscilloscopes
Analog oscilloscopes (DSOs)
Digital storage oscilloscopes (often
The first oscilloscopes were analog oscil-
referred to as DSOs) were invented to
loscopes, which use cathode-ray tubes
remedy many of the negative aspects
to display a waveform. Photoluminescent
of analog oscilloscopes. DSOs input a
phosphor on the screen illuminates when
signal and then digitize it through the
an electron hits it, and as successive bits
use of an analog-to-digital converter.
of phosphor light up, you can see a repre-
Figure 12 shows an example of one
sentation of the signal. A trigger is needed
DSO architecture used by Keysight
to make the displayed waveform look
Technologies, Inc. digital oscilloscopes.
stable. When one whole trace of the dis-
play is completed, the oscilloscope waits
The attenuator scales the waveform.
until a specific event occurs (for example,
The vertical amplifier provides additional
a rising edge that crosses a certain volt-
scaling while passing the waveform to the
age) and then starts the trace again. An
analog-to-digital converter (ADC). The
untriggered display is unusable because
ADC samples and digitizes the incoming
the waveform is not shown as a stable
signal. It then stores this data in memory.
waveform on the display (this is true for
The trigger looks for trigger events while
DSO and MSO oscilloscopes, which will
the time-base adjusts the time display
be discussed below, as well.)
for the oscilloscope. The microprocessor
system performs any additional
Analog oscilloscopes are useful be-
postprocessing you have specified before
cause the illuminated phosphor does
the signal is finally displayed on the
not disappear immediately. You can see
oscilloscope.
several traces of the oscilloscope over-
lapping each other, which allows you to
Having the data in digital form enables
see glitches or irregularities in the signal.
the oscilloscope to perform a variety of
Since the display of the waveform occurs
measurements on the waveform. Signals
when an electron strikes the screen, the
can also be stored indefinitely in memory.
intensity of the displayed signal correlates
The data can be printed or transferred
to the intensity of the actual signal. This
to a computer via a flash drive, LAN,
makes the display act as a three-dimen-
USB, or DVD-RW. In fact, software now
sional plot (in other words, x-axis is time,
allows you to control and monitor your
y-axis is voltage, and z-axis is intensity).
oscilloscope from a computer using a
virtual front panel.
The downside of an analog oscilloscope
is that it cannot "freeze" the display and
keep the waveform for an extended period
of time. Once the phosphorus substance Channel
deluminates, that part of the signal is lost. memory
Also, you cannot perform measurements
on the waveform automatically. Instead
you have to make measurements usu- Channel Vertical Micro-
Attenuater ADC MegaZoom Display
ally using the grid on the display. Analog Input amplifier processor
oscilloscopes are also very limited in the
types of signals they can display because
there is an upper limit to how fast the
Trigger Time-base
horizontal and vertical sweeping of the
electron beam can occur. While ana-
log oscilloscopes are still used by many
Figure 12. Digitizing oscilloscope architecture
people today, they are not sold very often.
Instead, digital oscilloscopes are the
modern tool of choice.
10 | Keysight | Evaluating Oscilloscope Fundamentals - Application Note
What Is an Oscilloscope and Why Do You Need One? (continued)
Mixed signal oscilloscopes
(MSOs)
In a DSO, the input signal is analog and
the digital-to-analog converter digitizes it.
However, as digital electronic technology
expanded, it became increasingly neces- 8 digital
sary to monitor analog and digital signals channels 4 analog channels
simultaneously. As a result, oscilloscope
vendors began producing mixed signal os-
cilloscopes that can trigger on and display Figure 13. Front panel inputs for the four analog channels and eight or sixteen digital channels on a mixed-
both analog and digital signals. Typically signal oscilloscope
there are a small number of analog chan-
nels (2 or 4) and a larger number of digital
channels (see Figure 13).
Mixed signal oscilloscopes have the
advantage of being able to trigger on a
combination of analog and digital signals
and display them all, correlated on the
same time base.
Portable/handheld
oscilloscopes
As its name implies, a portable oscillo-
scope is one that is small enough to carry
around. If you need to move your oscil-
loscope around to many locations or from
bench to bench in your lab, then a por-
table oscilloscope may be perfect for you.
Figure 14 shows an example of a portable
instrument, the Keysight InfiniiVision 2000 Figure 14. Keysight InfiniiVision 2000 X-Series portable oscilloscope
X-Series oscilloscope.
The advantages of portable oscilloscopes
are that they are lightweight and portable,
they turn on and off quickly, and they
are easy to use. They tend to not have as
much performance power as larger oscil-
loscopes, but scopes like the Keysight
InfiniiVision 2000 and 3000 X-Series are
changing that. These oscilloscopes of-
fer all the portability and ease typically
found in portable oscilloscopes, but are
also powerful enough to handle most of
today's debugging needs up to 1 GHz
bandwidth.
11 | Keysight | Evaluating Oscilloscope Fundamentals - Application Note
What Is an Oscilloscope and Why Do You Need One? (continued)
Types of oscilloscopes High-performance oscilloscopes Where oscilloscopes are used
Economy oscilloscopes High-performance oscilloscopes provide If a company is testing or using elec-
the best performance capabilities avail- tronic signals, it is highly likely they
Economy oscilloscopes are reasonably able. They are used by people who require have an oscilloscope. For this reason,
priced, but they do not have as much per- high bandwidth, fast sampling and update oscilloscopes are prevalent in a wide
formance capability as high-performance rates, large memory depth, and a vast ar- variety of fields:
oscilloscopes. These oscilloscopes are ray of measurement capabilities. Figure 15
typically found in university laboratories. shows an example of a high-performance