Class:
E
Name:
18h
(
)
Date:
18h
Use of CRO (Supplementary)
Apparatus
You will learn how to use the cathode ray oscilloscope (CRO) to
measure d.c. and a.c. voltage and frequency.
• 1 CRO
• 1 low voltage
power supply
• 1 battery box
Don’t hesitate to set this
as a student experiment.
The CRO, though
expensive and
sophisticated, is a
‘student-proof’ equipment
which cannot easily be
damaged through misuse.
Overloading will damage a
moving-coil meter but will
not do any harm to the
electron beam of the
CRO.
As an introduction,
demonstrate the function
of the time base using
1 Hz signal from a signal
generator. First, switch off
the time base and the light
spot will be seen
oscillating up and down.
Next, switch off the signal
and switch on the time
base to a low sweep
speed. The spot will
slowly sweep across the
screen. Finally, switch on
both the time base and
the signal generator. The
spot, subject to a
horizontal sweep and a
vertical oscillation, will
trace a wave on the
screen.
Procedure
Setting up the CRO
(a) Switch on the CRO (Fig 18h-1).
1
(b) Wait until a light spot appears on the screen.
(c) Adjust the brightness of the spot.
(d) Use the X-shift and the Y-shift to bring the spot to the middle of
the screen.
Y-shift
X-shift
Intensity
Power
Focus
Variable
Variable
Cal
Cal
AC
GND
time base
DC
Ext
Trig
Input
Y-gain
Fig 18h-1
Measuring d.c. voltage
(a) Connect the CRO to a battery box.
2
(b) Observe what happens to the light spot.
(c) Switch on and adjust the time base to turn the light spot into
a steady horizontal line (Fig 18h-2).
(d) Calculate the d.c. voltage from the gain control setting and the
shift of the line.
As we connect the CRO to a battery box, the spot
=
0.5
Shift
=
3
⇒
Y-gain setting
© Oxford University Press 2006
d.c. voltage =
1.5
shifts up
.
V/cm
cm
V
New Physics at Work (Second Edition)
53
18h
Class:
Before the measurements
of voltage and frequency,
ensure the variable knobs
of the Y-gain and time
base are fully turned
clockwise respectively.
Only when the voltage
variable knob is fully
clockwise (i.e. at the CAL
position, where CAL
means calibrated), the
sensitivity indicated on the
Y-gain can be used to
calculate the unknown
input voltage. The same
applied to the variable
knob of the time base.
When the variable knob is
at any other position, the
CRO is merely used to
display the waveform. The
variable knob is there for
the flexibility of attenuating
the amplitude of the
displayed waveform when
needed.
Name:
(
)
Date:
battery box
Fig 18h-2
Measuring a.c. voltage
3
Connect the CRO to an a.c. power supply. Adjust the time base and
Y-gain to display a proper waveform (Fig 18h-3).
a.c. supply
Fig 18h-3
4
waveform displayed
(a) Switch off the time base and measure the length of the vertical
trace (Fig 18h-4).
peak voltage
Fig 18h-4
54
New Physics at Work (Second Edition)
time base switched off
© Oxford University Press 2006
Class:
Name:
(
)
Date:
18h
(b) Half the length gives the peak voltage of the a.c.
(c) Calculate the peak voltage from the gain control setting of the
CRO.
Y-gain setting
=
0.5
V/cm
Length of vertical trace
=
5.6
cm
Half length of vertical trace =
2.8
cm
⇒
=
0.5
V/cm ×
=
1.4
V
Peak voltage
2.8
cm
Measuring frequency
(a) Switch on the time base again to display the waveform of the
a.c. voltage.
5
(b) Measure the width of several complete cycles of the wave.
(c) Calculate the frequency.
=
5
2.5
cycles =
10
cm
Width of 1 cycle
=
4
cm
T=
5
m s/cm ×
=
20
ms=
Time base setting
Width of
⇒
⇒
Period
Frequency
f=
1
=
T
m s/cm
50
4
0.02
cm
s
Hz
Discussion
What would happen to the waveform in step 3 if the Y-gain setting
is changed from 0.5 V/cm to 1 V/cm? Sketch the waveform in
Figure 18h-5.
The amplitude (peak voltage) of the displayed waveform is halved, but the width of
each cycle of the wave remains unchanged.
Fig 18h-5
© Oxford University Press 2006
New Physics at Work (Second Edition)
55
18h
Class:
Name:
(
)
Date:
What would happen to the waveform in step 3 if the time base setting
is changed from 5 m s/cm to 2 m s/cm? Sketch the waveform in
Figure 18h-6.
The amplitude (peak voltage) of the displayed waveform remains unchanged, but the
width of each cycle of the wave increases.
Fig 18h-6
The CRO is basically a voltmeter. It can be used to measure
voltage
the
and
waveform
frequency
and to display
of the input voltage.
Related questions in HKCEE
1999 Paper 2 Q32
The above diagram shows the trace of a signal on a CRO with the time
base set at 10 ms cm–1 and Y-gain at 1 V cm–1. If the time base is changed
to 20 ms cm–1 and Y-gain changed to 0.5 V cm–1, which of the following
shows the trace of the signal on the CRO?
A
B
D
E
C
A
56
New Physics at Work (Second Edition)
© Oxford University Press 2006
Class:
Name:
(
)
Date:
18h
2001 Paper 2 Q33
Y-gain
Time- Y-shift
base
Figure (a)
Y-gain
Time- Y-shift
base
Figure (b)
Figure (a) shows the trace of a signal on a CRO. In order to change the
trace of the signal into the form shown in Figure (b), which of the
following settings of the CRO should be adjusted?
© Oxford University Press 2006
A
Y-gain only.
B
Y-shift only.
C
time base only.
D
Y-gain and Y-shift.
E
Y-gain and time base.
D
New Physics at Work (Second Edition)
57