THE HONG KONG POLYTECHNIC UNIVERSITY
Department of Electronic and Information Engineering
ENG237-E01r1
ENG237-E01: Cathode Ray Oscilloscope (CRO)
Objectives
To introduce the control facilities of CRO, and to use CRO to measure DC and AC
voltages.
Apparatus
1.
Oscilloscope: GOS-620, GOS-652G, SS-7802A or SS-7804
2.
DC power supply
3.
Signal generator
4.
Stop watch
Introduction
The CRO is said to be the eyes of electronic engineers, because it is a very useful and
versatile instrument to detect and measure signals in electronic circuits. Basically, the
CRO is a voltmeter and frequency meter with a high speed display mechanism. The
simplified block diagram of CRO is shown in Fig. 1.
Fig. 1
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THE HONG KONG POLYTECHNIC UNIVERSITY
Department of Electronic and Information Engineering
ENG237-E01r1
It can be seen from the diagram that the CRO will display Y inputs on the vertical axis and
time-base or X input on the horizontal axis on the screen. The commonly used control
facilities of CRO are as follows:
INTENSITY:
varies the intensity of the display.
FOCUS:
controls the definition of the display.
POSITION:
displaces the trace in the Y axis or X axis manually.
TIME/DIV:
selects sweep speeds and EXT X.
VOLT/DIV:
provides steps of attenuation of each channel's input signal.
AC-GND-DC Selector:
switches the input to AC coupled, DC coupled or ground.
TRIGGERING LEVEL: determines the starting level of the stationary trace.
Procedure
(A) Initial setup
1. GOS-620: make sure all VAR knobs are locked at CAL position. The “x10 MAG”
button should be “up”.
2. GOS-652G: make sure all VAR knobs are locked at CAL position. The “x10
MAG” and “LOCK” buttons should be “up”.
3. Do not choose GOS-643 and GOS-652.
(B) Using the CRO only
1. Use the power switch to turn the CRO “ON”.
2. Select Channel 1 only.
(SS-7802A, SS-7804: toggles buttons “CH1” and “CH2” such that “1:” is
displayed on the bottom of screen whereas “2:” is not displayed.)
Set time-base to 500ms/DIV.
Select GND input for channel 1.
(SS-7802A, SS-7804: toggles button “GND” to see a “ground” symbol on the
display.)
Select ALT mode.
3. Adjust the intensity (“INTEN” knob)
4. Adjust the FOCUS.
(SS-7802A, SS-7804: adjust it to see a clear display of text.)
(GOS-620, GOS-652G: adjust it to obtain the sharpest spots.)
5. Turn the horizontal position knob. The spot shall move along X axis.
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THE HONG KONG POLYTECHNIC UNIVERSITY
Department of Electronic and Information Engineering
ENG237-E01r1
6. Turn the vertical position knob. The spot shall move along Y axis. Leave the spot
moves in the upper portion of screen.
7. Select Channel 2 only and set its input to GND. Repeat (B)5 and (B)6 but leave the
spot moves in the lower portion of screen.
8. Select both Channel 1 and Channel 2 (DUAL mode on GOS-620 & GOS-652G).
For GOS-652G only, set time-base to 2ms/DIV. Notice how the two spots is
sweeping on the screen. Select CHOP mode. Notice the difference.
9. Set time base to 200us/DIV. Try ALT/CHOP modes alternatively to see any
difference. Leave it at ALT mode.
10. Set time base to 500ms/DIV. The spots should sweep across the screen slowly. Use
the stop watch to check the speed. Record the results. Repeat with setting at
100ms/DIV.
11. Set the time base to 500µs/DIV stepwise. Observe the screen at each time base
setting. Each spot shall trace a line on the screen.
(C) DC and AC voltage measurements
1. Select Channel 1 only. Move the trace to the center of screen. Normally the center
line is chosen as the 0 V line.
2. Select DC coupling.
(SS-7802A, SS-7804: toggles buttons “GND” and “AC/DC” such that a “~” is
shown on top of the letter V in the vertical scale annotation.)
3. Connect the input wires to the output terminal of the given DC power supply.
Increase the power supply voltage to 20 V in steps of 2 volt. At each step measure
the voltage on the scope. Change the VOLT/DIV setting when needed. Record the
results. Switch the DC power supply on and off three times and observe the trace
on the screen.
4. Set the signal generator to “sine wave” at frequency of 500 Hz. Adjust the
amplitude to maximum (attenuator = 0, amplitude = max). Set the trigger source of
the time-base to CH1 and “AUTO” mode.
(SS-7802A, SS-7804: cycling the “SOURCE” button to see “CH1” on the top of
screen)
Then adjust the “TRIG LEVEL” knob to obtain a stationary trace on the screen.
Measure and record the period, frequency, maximum (peak) voltage, and
peak-to-peak voltage as shown in Fig. 2.
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THE HONG KONG POLYTECHNIC UNIVERSITY
Department of Electronic and Information Engineering
ENG237-E01r1
5. Repeat procedure 4 with frequency set at 5 kHz, 50 kHz, 500 kHz etc. Adjust the
“TIME/DIV” knob to obtain proper displays.
6. Set the signal generator to “square wave” at frequency of 500 Hz. Observe the
waveform. Now use AC coupling instead of DC coupling. Notice any difference.
Comments:
1. Is the CRO a voltmeter and frequency meter with a high speed display mechanism?
Why?
2. What is the use of AC coupling?
3. There is a “GND” connection for each channel input. Are they connected together
and where do they connect to? Comment on the limitations of the scope due to this
consequence.
4. Comment on the results obtained from the CRO.
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