Memorial University of Newfoundland
Department of Physics and Physical Oceanography
Physics
2055 Laboratory
Memorial
University
of Newfoundland
Department of Physics and Physical Oceanography
Introduction
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Oscilloscope
Physics 2055
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Oscilloscope
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Introduction
Introduction
An oscilloscope (or CRO) is an instrument for observing electrical signals and is essential
An oscilloscope (or CRO) is an instrument for observing electrical signals and is an essential
for anyone designing or repairing electronic equipment. The primary advantage of using an
tool for anyone designing or repairing electronic equipment. The primary advantage of using
oscillosope
insteadinstead
of a digital
voltmeter
is that
can be
directly
on aonscreen,
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voltmeter
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AC
characteristics
such
as
frequency,
peak
to
peak
screen, facilitating direct measurement of AC characteristics such as frequency, peak to peak(p-p)
voltage
phase
angle.
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voltage
and
phase angle.
Figure1:1: Oscilloscope
Oscilloscope used
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thelaboratory.
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A photograph of the oscilloscope used in this
1 laboratory is shown in Fig 1. It is capable
1
A photograph of the oscilloscopes used in this laboratory is shown in Fig 1. They are
capable of displaying two distinct external signals in two channels, labelled CH1 and CH2.
of displaying two distinct external signals in two channels, labelled CH1 and CH2. We use
We use a waveform generator as an ac voltage source, shown in Fig 2. Both items are used
a waveform generator as the ac source, shown in Fig 2. Turn on both units and allow them
extensively in ac circuits. Turn on both units and allow them to warm up for a few minutes
to warm up for a few minutes before proceeding.
before proceeding.
Figure 2: Waveform
Waveform generator
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Display aa signal
signal on
on the
the Oscilloscope
Display
Oscilloscope
Connect aa BNC
BNC cable
cable from
from the
1.1. Connect
the OUTPUT
OUTPUT terminal
terminal ofof the
theAC
ACsignal
signalgenerator
generatortoto
channel CH1.
CH1. Set
Setthe
the signal
signal generator
generator to
channel
to output
output aa sine
sinewave
wavewith
withaafrequency
frequencyofofa afew
few
hundred
Hz.
hundred Hz.
2. Press the AUTO key. The oscilloscope automatically sets the vertical, horizontal, and
2. Press the AUTO key. The oscilloscope automatically sets the vertical, horizontal, and
trigger controls so that about three complete cycles of the waveform are displayed.
trigger controls so that about three complete cycles of the waveform are displayed.
You can adjust any of these manually using the Vertical and Horizontal controls if you
You can adjust any of these manually using the Vertical and Horizontal controls if you
need to optimize the display.
need to optimize the display.
• POSITION: Moves the trace for a particular channel vertically or horizontally
• POSITION: Moves the trace for a particular channel vertically or horizontally
on the screen.
on the screen.
• Vertical Sensitivity: this control sets the voltage required to deflect the beam
• Vertical Sensitivity: this control sets the voltage required to deflect the beam
through one vertical division. The range for this CRO is from 2 mV/div up to
through one vertical division. The range for this CRO is from 2 mV/div up to 5
V/div. The sensitivity is displayed2 at the bottom left corner of the screen.
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• Time Base: The time base setting sets the time required for the beam to sweep
across one large horizontal division. The vertical sensitivity and time are displayed
at the bottom of the screen.
Try adjusting these and explain in your own words how they affect the appearance of
the waveform on the oscilloscope.
Triggering
1. The oscilloscope needs to know when to begin a trace. Triggering is accomplished
automatically by this oscilloscope. When an input signal is applied, the sweep automatically adjusts itself to trigger at the mean level of the input waveform.
2. Press the Trigger MENU key. Check that ‘Source’ is set to the input signal (CH1) and
that ‘Mode’ is set to AUTO. Then press the ‘Slope’ softkey to determine whether the
triggering is done on a positive-going waveform or a negative-going one.
3. Explain briefly how triggering works, using a triangular wave output from the waveform
generator. What does the ‘level’ control do?
AC Measurements
Amplitude: The amplitude of an alternating waveform is the voltage difference between
the maximum value and the zero reference line.
Peak to peak amplitude: This is the vertical displacement between the maximum and minimum peaks (for example, 3.5 divisions × 1 mV/div = 3:5 mV p-p). For a sine wave on your
oscilloscope screen, what is the p-p amplitude?
Root Mean Square (rms) Amplitude: The rms value of an alternating voltage or current is
√
given by 1/ 2 or 0.7071 times its maximum value. The root mean square current is the
value of a sine-wave current which will produce the same heating effect in a resistor as a dc
current of the same magnitude. Most AC multimeters measure rms values. What is the rms
amplitude of your oscilloscope trace?
Frequency: Measure the horizontal displacement (in squares) for one or more complete
cycles and hence determine the average displacement of one complete wave. Calculate the
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period, t, of the wave, given by the number of horizontal divisions multiplied by the time
base setting. The frequency of the waveform is f = 1/t.
Using CURSOR
The CURSOR key provides two vertical or horizontal lines which help you identify a peak
or other features of a waveform.
To measure peak-peak amplitude:
• Press CURSOR to display the on-screen menu
• Press the ‘Type’ softkey and select ‘Voltage’
• Press the Y1/Y2 softkey until Y1 is illuminated
• Use the ‘Adjust’ dial to move the cursor to the maximum peak position
• Press the Y1/Y2 softkey until Y2 is illuminated
• Move the cursor to the Minimum peak position. The voltage difference between the
two cursors is displayed at the right hand side of the screen.
To measure a time difference:
• Press the ‘Type’ softkey and select ‘Time’
• Press the X1/X2 softkey until X1 is illuminated
• Use the ‘Adjust’ dial to move the cursor to the start time
• Press the X1/X2 softkey until X2 is illuminated
• Move the cursor to the end time position. The time difference (∆x) between the two
cursors is displayed at the right hand side of the screen. If you have measured the time
for a complete waveform, 1/∆x will correspond to frequency.
Automatic Amplitude and Frequency measurement of an AC Signal
1. Press the MEASURE key to display the MEASURE menu.
2. Press the Voltage softkey to display the VOLTAGE menu.
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3. Press the peak-peak softkey to measure the p-p amplitude (or RMS to measure the
RMS amplitude). The amplitude value will be displayed at the bottom of the screen.
4. Press the MEASURE key again.
5. Press the Time softkey to display the TIME menu.
6. Press the Frequency softkey. The frequency value will be displayed at the bottom of
the screen to the right of the voltage value. Check that the frequency is the same as
the value displayed on the waveform generator.
Effect of Output Frequency on DMM Readings
1. Set the signal generator to output a ∼5 v p-p sine wave at about 100 Hz. Use the oscillosope to determine the frequency, pk-pk amplitude and rms amplitude of the displayed
waveform. For comparison, measure the rms voltage using a digital multimeter.
2. Increase the frequency in suitable large steps so that you cover the full range of signal
generator frequencies, up to about 5 MHz, and record the amplitude, frequency and
rms voltage each time. Summarize these in a table and explain why you should be
careful when using a digital multimeter at high frequencies.
Measuring DC signals
Connect a BNC cable from the DC power supply to either channel CH1 or CH2 of the
oscilloscope. Set the ‘COUPLING’ softkey to ‘DC’. DC coupling passes both AC and DC
components of the input signal. A pure DC signal will appear as a horizontal straight line.
As you increase the power supply voltage the line will move vertically upwards. The distance
moved will depend on the vertical sensitivity.
You can also display signals which contain both AC and DC components. Reconnect
the waveform generator to the oscilloscope and move the DC OFFSET control clockwise to
apply a positive voltage, and counterclockwise to apply a negative voltage. (Check the DC
OFFSET switch first).
Observing two signals together
Connect a second BNC cable from the source to channel CH2 and press the CH2 button.
The MATH button allows you to add, subtract or multiply the two signals. For two signals
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of different frequency you will need to set the trigger to alternate between the two. Press
Trigger MENU and Source ‘Alternating’. Try this using a second waveform generator, if
available.
A note on grounding
The oscilloscope is grounded, i.e., it is connected to earth through the third prong on the
mains plug. Consequently, all potentials are measured relative to this as a reference point.
When measuring two signals simultaneously with a CRO, you should first check where the
ground wires are connected, otherwise you may create a short circuit and not see a signal at
all.
To demonstrate this, construct the circuit shown in Fig 3 using a waveform generator
and two resistors in series. Use R ∼ 200 Ω and r ∼ 100 Ω. Use a digital meter to confirm
that the two (rms) voltages measured across each resistor add (approximately) to the source
voltage.
VR
R
r
Vr
Figure 3: Connect two resistors in series with the power supply
Now use the oscilloscope to measure the p-p voltage across each resistor. Explain what
happens when you try to measure the voltage across R. Reconfigure the circuit so that you
can measure VR using the oscilloscope.
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Charge and Discharge of a Capacitor
Replace the circuit above with a ∼ 1000 Ω resistor and ∼ 1 µ F capacitor connected in
series. Set the waveform generator to output a square wave and display the signal across
the capacitor. Adjust the voltage and time base controls so that you can see one or two
complete cycles for charge and discharge. Sketch both traces and, in particular, describe the
shape of the capacitor trace when the input signal changes from
(a) minimum to maximum
(b) maximum to minimum
Summary
Summarize the ways that an oscilloscope can be used in the laboratory as a measuring
device, and discuss its ability to measure AC and DC signals when compared to a digital
multimeter.
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