THE OSCILLOSCOPE
This orientation exercise will introduce you to some of the basic functions of the cathode-ray
oscilloscope. If you are already well experienced in the use of an oscilloscope, then with the permission
of your instructor you may skip it.
Equipment
Philips model 3050 oscilloscope
sine/square signal generator
wire leads
Basic Functions
The oscilloscope displays information by translating an input signal voltage into the position at
which an electron beam strikes a fluorescent screen. This is done by deflecting the beam in an electric
field which is proportional to the
input voltage.
The idea is illustrated in Fig. 1.
Each input signal (generally
time-dependent) is applied to a pair
of deflection plates. An electron
passing between these plates is
acted on by a force along the field
direction, and is deflected. Since,
at any point on its trajectory, the
force on the electron is directly proportional to the applied potential
difference, the deflection of the
beam is proportional to the input
signal voltage V.
In an oscilloscope, one set of
plates produces a vertical deflection of the electron beam, and
another a horizontal deflection.
Figure 1
Schematic diagram of an oscilloscope
The spot at which the beam strikes the fluorescent screen therefore "maps" the instantaneous
values of the two input signals to a point on the screen.
The most fundamental mode of operation of the oscilloscope is to display the time dependence of a
single signal. In this mode, an internal circuit generates a signal, something like that shown in Fig. 2,
which is applied to the X (horizontal) input. This signal voltage increases linearly with time over a fixed
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THE OSCILLOSCOPE
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interval (A to B) , after which it resets very quickly (B to
C) to its initial value, and repeats. The spot at which the
electron beam hits the screen thus sweeps at a uniform
speed across the screen, resets, and sweeps across
again and again. The oscilloscope display thus becomes a graph whose horizontal axis corresponds to
time, and whose vertical axis to the voltage applied at
the Y input. If the input signal is periodic, moreover, the
repetition rate of the sweep can be synchronized with it,
so that the identical display is traced out over and over.)
Figure 2
A sweep signal
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THE OSCILLOSCOPE
Procedure — Philips M/3050 Oscilloscope
NOTE: A diagram of the oscilloscope front panel controls appears below. The reference numbers on
the connectors and controls in Fig. 3 do not appear on the actual instrument!
(1) Connect the oscilloscope to the AC line. Push in the POWER switch so that it stays in the IN
position.
(2) The display tube (CRT) controls — INTENSITY and FOCUS, etc. — are in the cluster at the far left
of the instrument face. The oscilloscope will come on in a sweep mode (1 ms/cm), so you will see a
horizontal line across the screen. Change to the x-y deflection mode by pushing the X DEFL switch
(19). You should now see a bright spot near the center of the screen — the position of the
undeflected electron beam.
What are the effects of the INTENSITY (2) and FOCUS (4) controls on the beam spot? Set these
so that the spot is sharply focused and just bright enough to see clearly.
NOTE: NEVER leave a bright spot stationary on the screen for any extended period of time, as this
can damage the screen.
(3) What are the effects of the X POS (34) and Y POS (27) controls? Use them to position the beam
THE OSCILLOSCOPE
spot at the center of the screen. Note that there are two vertical deflection channels, A and B. As
they are identical, only the A controls are described. The oscilloscope works by applying voltages
to the X and Y deflection plates, causing deflections of the spot on the screen. In the following,
we connect the vertical deflection to the A input (36), and the horizontal deflection to the B input
(36).
(4) Make sure that the VAR controls (26, 28) are set to the full clockwise (CAL) setting. Use the
VOLTS/CM controls (6, 7) to set the vertical deflection sensitivity on both channels A and B to 0.5
V/cm. Pushing the left-hand side of this switch decreases the sensitivity, the right-hand side
increases it. The setting is shown on the LCD readout panel just to the left of the switch. Use the
AC/DC input switches (11, 16) to set the input modes to DC. Re-center the beam spot if
necessary. Now use the A/B (12) switch to set the Y-deflection source to A, and use the TRIG or
X SOURCE (17) switch to set the X-deflection source to B.
(5) Now connect a dry cell to input A (36), using a coaxial cable. (The outer shield of the cable
connector is automatically connected to ground at the oscilloscope input connector, and a
separate ground connection is not necessary.)
(6) When you made the connection, what did the beam spot do? The battery voltage is around 1.5 V,
and you've set the oscilloscope to translate each volt at the Y input into 0.5 cm vertical deflection.
How far did the beam spot move? Repeat with the sensitivity setting at 1 V/cm, and again with it
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THE OSCILLOSCOPE
as 2 V/cm, and record your results.
(7) Interchange the connections at the battery terminals, and repeat step (5). What difference did
this make?
(8) Move the battery connection from the A to the B (X-deflection) input. Repeat steps (6) and (7).
(9) Try the same thing (one sensitivity setting is enough) with both X and Y deflection connected to
the A input, and explain what happens.
(10) Disconnect the battery. Push the X DEFL switch (19) to toggle back to sweep mode, set the
TRIG or X SOURCE (20) switch to A and the TB TRIG MODE (17) switch to AUTO. Adjust the
sweep rate to 0.5 (sec/cm) using control 8. You should see the beam spot sweeping horizontally
across the screen; if not, adjust the INTENSITY control until it is visible. Notice that the spot
moves horizontally from left to right at a constant speed, then reappears at the left, pretty much
instantaneously, and starts again (see Figure 2).
(11) Adjust the POS controls so that the starting point is (-5 cm, 0). Use your watch to determine the
time it takes for the spot to travel 10 cm horizontally, and record the result. Does it agree with the
switch setting of 0.5 sec/cm? Repeat this determination with the sweep rate set at 0.2 sec/cm.
What is the effect of varying the horizontal VAR control (30) on the rate at which the spot sweeps
across the screen? With this control set fully counterclockwise, change the sweep rate control to
1 m (sec/cm). You can see the path of the spot across the screen, but you can no longer observe
it moving. Why not?
(12) Now set the VOLTS/CM control
to 2 V/cm and the SEC/DIV switch to
2 msec/cm. Set the TB TRIG MODE
(17) switch to TRIG, the triggering
slope switch (21) for positive slope,
and the triggering LEVEL (33)
control at or just above the middle of
its range. Turn on the signal
generator, set its frequency to 1 kHz,
sine wave output, and the amplitude
control to about the "1 o'clock"
position. Connect a coaxial cable
Figure 3
Sinusoidal signal
from the HI output of the signal
display
generator to the A input of the
oscilloscope. You should see a sinusoidal waveform, looking something like Fig. 4, displayed on
the screen; if not, small adjustments of the LEVEL control should bring it in. If you have trouble
with this, call your instructor.
(13) Change the sweep rate switch setting (8); what effect does this have on the pattern? Can you
THE OSCILLOSCOPE
restore its original appearance by changing the signal generator frequency? Continue making
such changes until the relation between signal frequency, sweep rate, and display scale is clear to
you, and record your conclusions. In the same way, observe and record the relation between signal amplitude, volts/cm setting, and the vertical scale of the display.
(14) Reset everything as for (12) above. Vary the triggering LEVEL control. What happens to the
display? What do you think this control does?
(15) Make changes in the V/cm, msec/cm, etc., controls until the display is very different from Figure
3— much too small, much too large, whatever. Now push AUTO SET (9). What happened?
This is one of the nicer features of this oscilloscope model.
Oscilloscope.doc
last updated 6/02
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