Memorial University of Newfoundland
Department of Physics and Physical Oceanography
Physics 2055 Laboratory
Introduction to the Oscilloscope
An oscilloscope (or CRO) is an essential tool for anyone designing or repairing electronic
equipment. It is an instrument for observing electrical signals, and is used by everyone from
television repair technicians to physicists. The primary advantage of using an oscillosope
instead of a digital voltmeter is that the signal can be viewed directly on a screen, facilitating
direct measurement of AC characteristics such as frequency, peak to peak (p-p) voltage and
The oscilloscopes used in this laboratory are capable of displaying two distinct external
signals in two channels, labelled CH1 and CH2. Turn on the oscilloscope and allow it to
warm up for a few minutes before proceeding. Locate the two traces.
Use the minimum intensity necessary to observe the trace: excessive in-
tensity will burn the phosphor in the screen and leave a “dead spot”.
Connect a BNC cable from the MAIN OUT terminal of the AC signal generator to channel
CH1, and set its frequency to a few hundred Hz. The INT-LINE-EXT switch should be
in the INT position. If the trace is not stable, connect a second BNC cable from the AUX
OUT terminal to the TRIG IN terminal of the CRO and set the INT-LINE-EXT switch to
EXT. Adjust the timebase and vertical sensitivity controls so that about 2 or 3 complete
cycles are displayed on the oscilloscope screen.
The important controls on the oscilloscope are listed below. Describe in your own words
what each does.
1. Time Base: The time base (TIME/DIV) setting shows the time required for the beam
to sweep across one large horizontal division. Waveforms of diﬀerent frequency can be
displayed by changing the time base scale. Note that the CAL switch must be turned
2. VOLTS/DIV, or Vertical Sensitivity: this control sets the voltage required to deﬂect
the beam through one vertical division. The range for this CRO is from 5 mV/div up
to 5 V/div. The control knob in the centre should be turned completely to the right.
3. VAR PULL x5 GAIN: Controls the vertical deﬂection sensitivity. Normally it is
turned to the far right. When pulled out, the vertical sensitivity is magniﬁed ﬁve times.
4. POSITION: Moves the trace for a particular channel vertically on the screen.
5. AC-GND-DC: In the DC position the signal is input directly to the ampliﬁer of
the oscilloscope. In the AC position the signal is coupled to the ampliﬁer by a small
capacitor which blocks any DC component of the signal, allowing the AC component
only to reach the ampliﬁer. In the GND setting, the input is grounded.
6. POSITION/PULL x10 MAG: This switch moves both traces horizontally. Pulling
the switch expands the horizontal scale by 10 times.
7. MODE select switch. What is the diﬀerence between the ALT and CHOP settings?
1. Amplitude: The amplitude of an alternating waveform is the voltage diﬀerence be-
tween the maximum value and the zero reference line. Two other ways of describing
amplitude include Peak to peak and Root Mean Square (RMS):
Peak to peak amplitude: It is often easier to measure the vertical displacement be-
tween the maximum and minimum peaks to obtain the peak to peak (p-p) voltage (for
example, 3.5 divisions × 1 mV/div = 3.5 mV p-p. The amplitude is simply one half
of the pk-pk value. You can express the amplitude either way, but you must make it
clear which one you are using.
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. Most AC multimeters measure
rms values. 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. To
determine the rms voltage from the oscilloscope, measure the amplitude and multiply
by 0.7071. What is the amplitude of your waveform right now? Calculate the rms
amplitude, and compare this result with the DMM reading. Remember to select ‘AC’
on the DMM.
• Measure the horizontal displacement (in squares) for one or more complete cycles,
and and hence determine the average displacement of one complete wave.
• Calculate the period, t, of the wave, given by the number of horizontal divisions
multiplied by the time base setting.
• Calculate the frequency, f , of the waveform. Compare your result from the oscil-
loscope screen with a digital frequency meter.
Variation of Output Frequency
1. Set the signal generator to output a 5 v p-p signal at about 100 Hz. Use the oscillosope
to determine the frequency and amplitude of the displayed waveform as accurately as
possible. For comparison, measure the rms voltage using a digital multimeter.
2. Take about ten readings of frequency over the full range of signal generator frequencies
(up to about 1 MHz), and draw a table which shows frequency, amplitude of the
displayed waveform, calculated rms amplitude and multimeter reading in each case.
Explain why you should be careful when using a digital multimeter at high frequencies.
3. Discuss the suitability of the oscilloscope versus a digital meter for measuring the
amplitude and frequency and shape of a.c. signals.
Use the “DC Oﬀset” control on the waveform generator to add a dc component to your ac
signal. Describe and explain what happens to the CRO trace when you switch between “AC”
and “DC”. Replace the oscilloscope with a digital multimeter and explain what happens to
the meter reading when you swap AC and DC inputs.
1. The oscilloscope needs to know when to begin a trace. Automatic triggering is accom-
plished by moving the trigger mode select switch into the AUTO position. When an
input signal is applied, the sweep automatically adjusts itself to trigger at the mean
level of the input waveform. The SOURCE select switch (INT-LINE-EXT) is used to
select the triggering signal source:
• INT — the input signal applied to CH1 or CH2 becomes the triggering signal.
• LINE — the CRO triggers on the 60 Hz, 120 volt AC mains power supply.
• EXT — an external signal applied to TRIG IN becomes the triggering signal.
2. The Trigger LEVEL Control: PULL (-) SLOPE control is used to decide where
the waveform should be started. It also determines whether the triggering is done on
a positive-going waveform (normal position) or a negative-going one (PULL position).
3. Explain how triggering works, using a triangular wave output from the waveform gen-
60 Cycle Hum
In North America all AC power is generated at a frequency of 60 Hz. The alternating
currents produce an associated magnetic ﬁeld, so that the space around is full of 60 Hz
electromagnetic radiation. This is generally not a problem, although the long term eﬀects
of such exposure have yet to be studied.
To observe this radiation, simply connect an open-ended BNC cable to CH1 of the
oscilloscope. Set the timebase to about 10 ms/div and the amplitude to 50 mv/div. Place the
cable near an AC power cord, and move the cable around until the signal is maximized. The
cable is now acting as a simple antenna. Measure the frequency and amplitude of the signal
induced in the cable. Notice that the induced signal has other noise on it, corresponding
to electromagnetic pickup from non-60 Hz sources. What is the maximum amplitude you
observe? Attempt to sketch the resulting CRO trace.
Connect a signal generator to CH2 and set the output frequency to 60 Hz. Try to
superimpose the two traces to convince yourself that the induced frequency is 60 Hz as
expected. Whenever you use the oscilloscope, you must ensure that pickup of this 60 Hz
signal is reduced as much as possible, so that the signal of interest is not obscured by it.
A note on grounding
The oscilloscope is grounded, i.e., 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 following circuit using a waveform generator and two
resistors in series. Use R ∼ 100 Ω and r ∼ 200 Ω. Use a DMM to conﬁrm that the two (rms)
voltages measured across each resistor add (approximately) to the voltage measured across
R r Vr
Figure 1: Connect two resistors in series with the power supply.
Now use the oscilloscope to measure the pk-pk voltage across the resistors. What hap-
pens when you try to measure the voltage across R? Reconﬁgure the circuit so that you
can measure VR . Conﬁrm that the two pk-pk voltages measured across the resistors add
(approximately) to the pk-pk voltage measured across the source.
Summarize the ways that an oscilloscope can be used in the laboratory as a measuring device,
and assess its ability to measure AC and DC signals when compared to a digital multimeter.