Oscilloscope Basics Guide
A guide to oscilloscope operation, architecture and
common measurements.
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©2013 Tektronix, Inc.
This document may be reprinted, modified and distributed in whole or in part for the limited purpose of
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Overview of an Oscilloscope
Introduction
An oscilloscope is an electronic test instrument that displays electrical signals graphically, usually as a
voltage (vertical or Y axis) versus time (horizontal or X axis) as shown in figure 1. The intensity or
brightness of a waveform is sometimes considered the Z axis. There are some applications where other
vertical axes such as current may be used, and other horizontal axes such as frequency or another
voltage may be used.
Oscilloscopes are also used to measure electrical signals in response to physical stimuli, such as
sound, mechanical stress, pressure, light, or heat. For example, a television technician can use an
oscilloscope to measure signals from a television circuit board while a medical researcher can use an
oscilloscope to measure brain waves.
Oscilloscopes are commonly used for
measurement applications such as:
• observing the wave shape of a signal
• measuring the amplitude of a signal
• measuring the frequency of a signal
• measuring the time between two events
• observing whether the signal is direct
current (DC) or alternating current (AC)
• observing noise on a signal
An oscilloscope contains various controls that assist in the analysis of waveforms displayed on a
graphical grid called a graticule. The graticule, as shown in figure 1, is divided into divisions along both
the horizontal and vertical axes. These divisions make it easier to determine key parameters about the
waveform. In the case of the TBS1000 Series oscilloscope, there are 10 divisions horizontally and 8
divisions vertically.
A digital oscilloscope acquires a waveform by conditioning the input signal in the analog vertical
amplifier, sampling the analog input signal, converting the samples to a digital representation with an
analog-to-digital converter (ADC or A/D), storing the sampled digital data in its memory, and then
reconstructing the waveform for viewing on the display.
Figure 1: Typical Oscilloscope Display
Figure 2: Typical Digital Oscilloscope Block Diagram
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Performance Terms and Considerations
There are many ways to specify digital oscilloscope performance, but the most important are bandwidth,
rise time, sample rate, and record length.
Bandwidth
Bandwidth is the first specification to consider. Bandwidth is the frequency range of the oscilloscope,
usually measured in Megahertz (MHz). It is the frequency at which the amplitude of the displayed sine
wave is attenuated to 70.7% of the original signal amplitude.
When measuring high-frequency or fast rise-time signals, oscilloscope bandwidth is especially critical.
Without adequate bandwidth, an oscilloscope will not be able to display and measure high-frequency
changes. It is generally recommended that the oscilloscope’s bandwidth be at least 5 times the highest
frequency that needs to be measured. This “5-times rule” allows for the display of the 5th harmonic of the
signal and assures that measurement errors due to bandwidth are minimized.
th
5≥
Example: If the signal of interest is 100 MHz, the oscilloscope would need a bandwidth of 500 MHz.
Rise Time
The edge speed (rise time) of a digital signal can carry more high-frequency content than its repetition
rate might imply. An oscilloscope and probe must have a sufficiently fast rise time to capture the higher
frequency components, and therefore show signal transitions accurately. Rise time is the time taken by
a step or a pulse to rise from 10% to 90% of its amplitude level. There is another “5-times rule” that
recommends that the oscilloscope’s rise time be at least 5 times faster than the rise time of the signal
that needs to be measured.
timerisepeoscillosco ≤
signalofharmonicbandwidthpeoscillosco
timerisesignal
Example: If the signal of interest has a rise time of 5 µsec, then the oscilloscope rise time should be
faster than 1 µsec.
Sample Rate
Digital oscilloscopes sample the input signals at a frequency called the sample rate, measured in
samples / second (S/sec). To properly reconstruct the signals, Nyquist sampling requires that the
sample rate be at least twice the highest frequency being measured. That’s the theoretical minimum. In
practice, sampling at least 5 times as fast is generally desirable.
fratesample ∗≥ 5
Highest
Example: The correct sample rate for a 450 MHz signal would be ≥ 2.25 GS/sec.
Record Length
Digital oscilloscopes capture a specific number of samples or data points, known as the record length,
for each acquired waveform. The record length, measured in points or samples, divided by the sample
rate (in Samples/second) specifies the total time (in seconds) that is acquired.
timeacquired =
lengthrecord
ratesample
Example: With a record length of 1 Mpoints and a sample rate of 250 MS/sec, the oscilloscope will
capture a signal 4 msec in length.
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Initial Setup and Screen Explanation
Creating a Stable Display
1. The following steps will describe how to automatically create a stable oscilloscope display using a
1 kHz, 5 V
a. Power up the oscilloscope by pushing the power button.
b. Push the front-panel Default Setup button to set the oscilloscope to a known starting point.
c. Connect a passive probe to the channel 1 input. To connect a probe that uses a BNC
connector, push and turn the probe connector until it slides on the oscilloscope channel input
connector. Then, turn the probe locking ring clockwise to lock the probe connector in place.
d. Attach the probe’s alligator style ground lead to the ground connector next to the oscilloscope
display.
e. Attach the probe tip to the Probe Comp connector just above the ground lead connector. This
connector provides a 1 kHz square wave that this lab will use to demonstrate the operation of
an oscilloscope.
f. Push the front-panel AutoSet button to cause the oscilloscope to automatically set the vertical,
horizontal and trigger settings for a stable display of the probe compensation 1 kHz square
wave.
PLEASE NOTE: All screenshots in this Guide were taken in ink-saver mode to allow for
better visibility of signal changes in a printed document.
square wave.
pk-pk
Key Points to Remember
1. To return the oscilloscope to a known state, push the Default Setup button.
2. The AutoSet button adjusts the vertical, horizontal and trigger settings such that four or five cycles
of the waveform are displayed with the trigger near the middle of the screen.
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Screen Explanation
1. The following is a review of the oscilloscope display.
Explanation of the oscilloscope display:
a. The channel 1 vertical axis button is yellow and most of the elements on the screen that relate
to the channel 1 signal are yellow in color.
b. On the display, the following items are yellow to indicate they are associated with channel 1:
• waveform
• waveform ground level indicator (center left of screen)
• vertical scale readout (bottom left of screen 2.00 V)
c. The channel 2 vertical axis button is blue. The display uses the color coding of this channel just
as it does for the yellow of channel 1.
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d. As can be seen on the oscilloscope screen, the square wave extends up about 2 ½ divisions on
the display graticule from the ground level indicator. Since the vertical scale factor is 2
Volts/div, this indicates the signal’s positive peak is at about +5 V.
e. One cycle of the waveform is about 4 divisions wide. The time per horizontal division is
indicated by the horizontal scale readout which in this case is 250 µsec/div (bottom center of
the display). At 250 µsec/div, the period of the signal is about 1 msec and the frequency is
about 1 kHz.
f. Finally, the trigger frequency readout indicates the channel 1 signal has a frequency of about
1 kHz as shown in the bottom right corner of the display.
Key Points to Remember
1. The input channels are color coded. Onscreen channel information is in that channel’s color,
including the waveform, ground indicator, and vertical scale factor (Volts/div).
2. The amplitude of the signal can be determined by multiplying the number of vertical divisions the
waveform spans times the vertical scale factor.
3. The signal period can be determined by multiplying the number of horizontal divisions times the
horizontal scale factor.
4. Signal frequency is calculated by dividing 1 by the signal period.
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Instrument Controls
The controls of a typical oscilloscope can be grouped into three major categories: vertical, horizontal, and
trigger. These are the three main functions that are used to set up an oscilloscope. The use of these
controls is described in the following sections of this lab.
Here are a few hints that will make using the oscilloscope controls easier:
• Decide if the task is related to oscilloscope’s vertical axis (typically voltage), horizontal axis
(typically time), trigger, or some other function. This will make it easier to find the correct control
or menu.
• Pushing a front-panel button will usually display a first-level menu at the right side of the display.
The menu items are logically prioritized from top-to-bottom. If they are selected in that order, the
setup should be straightforward.
• If the LED next to the multipurpose control is lit, it indicates the front-panel multipurpose control
may be used to change the highlighted menu selection.
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