Appendix IX. CRO-Cathode Ray Oscilloscope
Revised November 21, 2003
A. Introduction
A cathode ray oscilloscope, CRO or
scope, is a versatile electronic instrument
used in many fields of basic and applied
research to measure time-dependent voltage
signals. A CRO consists of a cathode ray
tube, CRT (similar to a television picture
tube), and associated circuits. Because an
oscilloscope has very high resistance inputs
(like a voltmeter), it draws very little current
and thus usually does not disturb the circuit
being studied. The oscilloscope is an essential part of several experiments in our introductory physics courses. Our scopes are not
simplified for teaching but are versatile
models suitable for use in research physics,
engineering and medical laboratories.
4.
5.
B. Components
6.
Figure 1 shows the basic components of
the cathode ray tube. The principal parts of
the tube are:
7.
Figure 1: Components of a cathode ray tube.
1. Filament–heats the cathode with a current of a few amperes.
2. Cathode–a metal surface coated with a
metallic oxide which emits electrons
when it is heated; emission currents of a
few mA are typical.
3. Control Grid–controls the electron current and consequently the brightness of
1
the image. The potential applied to it can
be varied by adjusting the INTENSITY
control. By the time the electron beam
reaches the screen, it is reduced to a few
µA Do not make the spot brighter than
necessary as this may damage the screen.
If the spot has a halo around it, turn
down the intensity; the intensity required
for a spot is less than that required for a
line.
Focusing Anode–permits sharpening the
image. Focus is controlled by the
FOCUS knob.
Acceleration Anode–accelerates the electrons toward the screen so that they strike
it with enough energy, several kV, to
give off light. There is no external control to adjust this voltage.
Vertical Deflection Plates–two horizontal plates parallel to the beam. The beam
can be deflected by a potential difference
between these plates, usually derived
from the amplified signal being studied.
The VERTICAL POSITION control
applies a DC voltage to offset or center
the beam vertically.
Horizontal Deflection Plates–analogous
to the vertical deflection plates. A sweep
voltage is usually applied to these plates
to cause the electron beam to sweep
across the screen at a controlled rate, and
then rapidly return to its starting position.
The sweep rate is controlled by the
TIME/DIV switch. A >DIV= is
generally a cm marking on the screen.
The TIME/DIV often varies from 1 sec
down to 1 µs. The HORIZONTAL
POSITION control is used to offset the
beam horizontally, usually so that the
sweep starts at the left hand edge of the
scale.
Appendix IX. Cathode Ray Oscilloscope
8. Fluorescent Screen–the screen has a
phosphor coating which produces light
when struck by a charged particle. A grid
is marked on the outside of the screen,
usually with cm spacing and small marks
every 2 mm.
The operation of the tube depends on
the fact that charged particles such as electrons can be deflected, accelerated, and
focused by suitably applied electric fields.
Because the electron has little inertia, it can
be deflected quickly, making possible the
study of high frequency and transient effects.
The filament (1) heats the cathode (2)
which emits electrons; these are focused (4)
into a beam by an electric field; the beam
strikes a fluorescent screen at the end of the
tube and causes the screen to emit light.
Before it hits the screen, the beam is
deflected by the electric fields on the plates
(6, 7). This deflection causes the beam to
move vertically and/or horizontally across
the face of the screen. (Television tubes use
magnetic deflection. This permits greater
deflection in a shorter distance, thus larger
screens, but gives up response time.)
return rapidly to its initial position on the
left. A blanking circuit cuts off the beam
during this part of the cycle so that no visible
retrace is seen.
This sequence of events repeats regularly and automatically at frequencies determined by the setting of the TIME/DIV
switch. The face of the cathode tube is
clearly marked in centimeter divisions; a
setting of l msec/cm means that it takes
0.001 s for the spot on the screen to traverse
l cm.
In practice, you seldom leave a scope
free-running as suggested by Figure 2.
Rather you set it up so that it will start each
sweep only when a trigger input allows it.
This trigger control makes it possible to start
each sweep on the same part of a repetitive
signal so that each successive sweep overlaps the previous sweep; otherwise a sine
wave, for example, would start at random
phases each time. On the screen you might
see a jumble of 100 sine waves all shifted
randomly with respect to each other. However, if the signal is triggered, they all start
with the same phase and you see one sine
wave.
D. Horizontal Sweep
E.
Built into the oscilloscope is a sweep
generator which generates a time-varying
sawtooth voltage something like the signal
illustrated in Figure 2. The sweep circuit
applies this voltage to the horizontal deflection plates. As the applied voltage increases,
the electron beam moves from left to right
(as viewed on the screen). The sudden drop
of the potential to zero causes the beam to
Refer to the picture in Figure 3. The
function of each switch, if it’s not obvious,
has been described earlier or is described
below.
1. Power switch
2. Power lamp
3. Focus control
4. Trace rotation control - although the
CRT is magnetically shielded, it is still
possible for magnetic fields to distort the
beam. This control lets you level the
trace.
5. Intensity control
6. Channel 1 Input (In X-Y mode, this
becomes the X-axis input.) These are
BNC-type connectors, which are a type
C. Principle of Operation
Figure 2 Sawtooth Sweep Voltage
Appendix IX. Cathode Ray Oscilloscope
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The Controls
Figure 3: CRO controls
of coaxial connector. The signal is at the
center pin while the outer metal casing is
a ground (for this scope).
7. Channel 2 Input (In X-Y mode, this
becomes the Y-axis input. )
8,9. Input coupling switches (AC-GNDDC) AC blocks any DC component,
using a capacitor, so that only changes in
the signal may be seen. GND shorts out
the input to help you determine where
the trace is with zero input. DC shows
you everything.
10,11.
Volts/Div selector
12,13.
VAR (PULL x5 GAIN) Fine
adjustment of vertical input gain.
14. Position control
15. POSITION (PULL INVERT) Inverts the
trace of Channel 1 when pulled out.
16. Mode select switch - CH1, CH2, ALT,
CHOP, ADD. This switch selects which
signal you can view on the CRT. ALT
and CHOP let you see both. ALT completes one sweep of channel 1 and then
one sweep of channel 2, etc. CHOP
moves quickly back and forth between
the two channels many times on each
sweep. This happens too fast to see and
is better when viewing slow signals that
don’t complete a sweep in less than 100
msec. ADD is generally used with
INVERT to see the difference between
two signals.
17. DC Bal attenuator adjustments (Do not
adjust these.)
18. Time/Div select switch
19. Sweep Variable control
20. POSITION (PULL x10 MAG)
21. Trigger SOURCE select switch. This
determines what is used for triggering the
scope. If SOURCE is set on INT, it triggers with a change in the vertical input
voltage. If it is set on LINE, it triggers
with the AC line frequency (60 hertz).
The EXT setting is for an external trigger
signal. You will generally use an internal
trigger, derived from the signal itself.
22. Internal Trigger. Selects the internal triggering signal source, channel 1, etc.
23. External Trigger (or X-IN connector
Input for external trigger).
24. Level Control Sets the voltage level at
which the sweep starts or triggers. When
it is set to +, the ’scope triggers on the
rising edge (positive slope) of the
waveform; when set to -, it triggers on
the falling edge.
25. Trigger Mode select switch. You will
generally use AUTO, which lets the
scope free-run if it is not triggered. On
MANUAL, the scope will just go blank
if it=s not triggered.
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Appendix IX. Cathode Ray Oscilloscope
26. Calibrate 0.5V (~1 kHz at 0.5V.)
27. Ground
F.
adjust the TIME/DIV (18) so that only 1-2
periods appear on the screen.
F.2. Voltage Measurement
Adjust the Volts/Div switch so that
your waveform nearly fills the screen vertically. Make sure the Var Gain (12,13) is
locked. Measure the amplitude of the signal
in screen divisions and multiply by the setting of the Volts/Div switch to obtain the
signal amplitude in volts.
F.3. Lissajous Figures and Dual Trace
The oscilloscope is often used to compare the time dependence and amplitudes of
two signals. For this purpose the >scope has
two input connectors and circuits and can
display two signals at the same time with a
common time axis and a trigger derived from
either source.
An alternative way to compare two signals is to plot one signal on the horizontal
axis (x) and the other on the on the vertical
axis (y). This will produce a Lissajous figure
on the screen. Connect one signal to the xaxis (CH1) and the other to the y-axis (CH2).
Select XY mode using the TIME/DIV knob
(18). Adjust the input sensitivities until the
signal nearly fills the screen.
Brief Instructions
Follow these instructions to measure
the period and amplitude of a periodic signal.
(Numbers in parenthesis refer to the controls
in Fig. 3.)
a. Plug in the scope and turn on the power
(1).
b. Connect the input signal to the Channel 1
input (6).
c. Set the trigger SOURCE (21) selection
switch to INT (for internal triggering).
d. Set MODE(25) to AUTO.
e. Set the INT TRIG(22) selection switch to
CH1.
f. Adjust the level control to obtain a stable
image.
F.1. Measuring a Period
Adjust the Time/Div switch until you
see a sequence of a few oscillations on the
screen. Make sure the Sweep Variable control (19) is locked (inner knob fully clockwise). Otherwise, any measurements will be
meaningless. Measure as large an image as
possible to obtain the highest precision in
your time measurements. If there are 4 full
periods on the screen, measure the time for
the 4 periods and divide by 4. Alternately,
Appendix IX. Cathode Ray Oscilloscope
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