Download Introduction to the Oscilloscope - Physics and Physical Oceanography

Memorial University of Newfoundland
Department of Physics and Physical Oceanography
Physics 2055 Laboratory
Introduction to the Oscilloscope
Introduction
An oscilloscope (or CRO) is an instrument for observing electrical signals, and is used by
everyone from television repair technicians to physicists. It is an essential tool for anyone
designing or repairing electronic equipment. 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
phase angle.
Oscilloscope Familiarisation
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 intensity will burn the phosphor in the screen and leave a “dead spot”.
The important controls on the oscilloscope are listed below:
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
completely clockwise.
2. VOLTS/DIV, or Vertical Sensitivity: 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.
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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.
AC Measurements
Adjust the timebase and vertical sensitivity controls so that about 2 or 3 complete cycles are
displayed on the oscilloscope screen. Ensure that the timebase control is in the “calibrate”
position. 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.
1. Amplitude: The amplitude of an alternating waveform is simply the voltage difference
between the maximum value and the zero reference line. Two other ways of describing
amplitude include Peak to peak and Root Mean Square (RMS):
2. Peak to peak amplitude: It is often easier to measure the vertical displacement between 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 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.
3. 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, determine the amplitude and multiply
by 0.7071.
4. Frequency:
• Measure the horizontal displacement (in squares) for one or more complete cycles,
and and hence determine the average displacement of one complete wave.
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• Calculate the period, t, of the wave, given by the number of horizontal divisions
multiplied by the time base setting, and hence calculate frequency from f = 1/t.
Triggering
1. The oscilloscope needs to know when to begin a trace. Automatic triggering is accomplished 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. Trigger LEVEL Control: PULL (-) SLOPE: This 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).
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.
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