THE HONG KONG POLYTECHNIC UNIVERSITY
Department of Electronic and Information Engineering
ENG237-E01/1
ENG237-E01: Cathode Ray Oscilloscope (CRO)
Objective
To introduce the control facilities of CRO, and to use CRO to measure DC and AC
voltages.
Apparatus
1. Oscilloscope
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
block diagram of CRO can be simplified as:
Channel 1
Input
(Y1)
Vertical
Amplifier
Channel 2
Input
(Y2)
Vertical
Amplifier
Cathode-ray-tube
screen
Y
Trigger
X
Time base
Horizontal Amplifier
INT(internal)
EXT(exterenal)
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Department of Electronic and Information Engineering
ENG237-E01/2
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:
BRILLIANCE: 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.
VARIABLE -TIME/DIV: enables sweep speed to be set at intermediate speeds
between the TIME/DIV settings. When it is turned to CAL
(Calibrated), the sweep speed is indicated by the
TIME/DIV knob.
VOLT/DIV: provides steps of attenuation of each channel's input signal.
VARIABLE -VOLT/DIV: enables voltage scale to be set at intermediate levels. When
it is turned to CAL, the screen is calibrated as shown by
the VOLT/DIV knob.
AC-GND-DC Selector: switches the input to AC coupled, DC coupled or ground.
STABILITY: enables a stationary trace on the screen.
TRIGGERING LEVEL: determines the starting level of the stationary trace. In the
“AUTO” trigger mode, this adjustment is not required.
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THE HONG KONG POLYTECHNIC UNIVERSITY
Department of Electronic and Information Engineering
ENG237-E01/3
Procedure
(A) Using the CRO only
1.
Use the power switch to turn the CRO "ON”.
2.
Select Channel 1 and EXT X (external X input or x-y mode).
3.
Increase BRILLIANCE or INTENSITY until one illuminated spots appear. Do not
increase intensity too much. A very bright spot may be harmful to the screen and
human eye.
4.
Adjust the FOCUS to obtain the sharpest spots.
5.
Turn the “
” position knob, the spot showed move along X axis.
6.
Turn the “
” position knob, the spot should move along Y axis.
7.
Select Channel 1 and Channel 2 (double trace mode). Repeat (A) 2 to (A) 6 on
channel 2 only.
8.
Turn the TIME/DIV (TIME base) knob out of EXT X and to 0.5 sec/DIV. Turn
the “VARIBLE” knob to "CAL” (for calibration) position. The spots should sweep
across the screen slowly. Use the stop watch to check the speed. Record the results.
Repeat with setting at 0.1 sec/DIV. Turn the “VARIABLE” knob out of ”CAL”,
repeat 8.
9.
Setting the time base to 0.5 ms/DIV, each spot will trace a line on the screen. Turn
the knob one step at a time and observe the screen.
(B) DC and AC voltage measurements.
1.
Use Channel 1 only. Connect an input coaxial lead to the Channel 1 input socket.
The "black" color wire is the “ground” wire and the “red” color wire is the input
wire. Clip the two wires together, the input is at zero volt (0 V). By turning the
“
” position knob, the reference 0 V line can be placed anywhere on the screen.
Normally the centre line is chosen as the 0 V line. The input a1so can be connected
to ground by switching the AC-GND-DC selector to GND.
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THE HONG KONG POLYTECHNIC UNIVERSITY
Department of Electronic and Information Engineering
ENG237-E01/4
2. Connect the input wires to the output terminal of the given DC power supply. Switch
the AC-GND-DC selector to DC. Turn the “VARIABLF-VOLT/DIV” knob to
"CAL". 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.
3.
Connect the input wires to the output terminals of the signal generator: the ground
wire to "LO" and the wire to "HI". Switch the AC-GND-DC selector to AC.
4.
Set the signal generator to "sine wave" at frequency of 500 Hz. Adjust the amplitude
to maximum. Set the trigger of the time-base to “INTERNAL TRIGGER” and
“AUTO” mode, and then adjust the stability control to obtain a stationary trace on
the screen. Measure and record period, frequency, maximum voltage and peak to
peak voltage:
One cycle
Max. Volt.
Peak to Peak
Volt.
Fig. 2
5.
Repeat procedure 4 with frequency set at 5 kHz, 50 kHz, 500 kHz etc. Adjust the
TIME/DIV knob to obtain proper displays.
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Department of Electronic and Information Engineering
ENG237-E01/5
Comments
1.
Is the CRO a voltmeter and frequency meter with a high speed display
mechanism? Why?
2.
Before measuring a voltage or frequency, what do you do with the "VARIABLE"
knobs?
3.
Comment on the results obtained from the CRO.
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