Table of Contents

Chapter 4: Electricity and Magnetism Experiments

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

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. 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 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.


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.

(3) What are the effects of the X POS (34) and Y POS (27) controls? Use them to position the beam 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 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 from the HI output of the signal 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 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.


Procedure -- Tektronix Model T921 15 MHz Oscilloscope

(1) Connect the oscilloscope to the AC line. Push in the POWER switch (3) so that it stays in the IN position.

(2) Set the INTENSITY (1) and FOCUS (2) controls (top left of scope face) at about mid-range, and the TRIGGERING SOURCE (4) switch to X-Y. After a few seconds a small spot should appear on the screen. What are the effects of the INTENSITY and FOCUS controls? Set these so that the spot is sharply focused and just bright enough to see clearly.

(3) What is the effect of the X (11) and Y (15) POSITION controls? Use them to place the spot at the center of the screen.

(4) Set the VOLTS/DIV switch (13) to the 0.5 (volt/cm) position. The setting of this switch is read in the X1 PROBE window. Rotate the red VAR (14) control to its maximum clockwise position. Set the INPUT MODE switch (16) to DC. Re-center the spot if necessary.

(5) Connect a dry cell to the oscilloscope Y input (12). (When a coaxial -- shielded -- cable is used, the outer shield of the cable connector is automatically connected to ground at the oscilloscope input, so a separate ground connection is unnecessary.) When you make the connection, what does the beam spot do? The battery voltage is around 1.5 V, and you've set the oscilloscope to translate each 0.5 V at the Y input into 1 cm vertical deflection. How far did the beam spot move? Repeat with the VOLTS/DIV switch at 1 V/cm, and again with it as 2 V/cm, and record your results.

(6) Interchange the connections at the battery terminals, and repeat step (5). What difference did this make?

(7) Disconnect the Y input, and use the X and Y position controls to reposition the spot at (-5 cm, 0). Set the red X1-X10 control (10) in its extreme counterclockwise position (X1). Connect the dry cell to the X input (7). Note the deflection of the spot. Now remove the lead from the X-input, turn the X1-X10 control about 1/4 turn clockwise, reposition the spot at (-5 cm, 0). Now reconnect the X-input lead, and note the deflection of the spot. What is different? Continue in this way, changing the X1-X10 control, until you can explain clearly what this control does. Record your conclusions.

(8) Disconnect the X input. Set the TRIGGERING SOURCE (4) switch to INT and the TRIGGERING MODE (5) switch to AUTO. Set the SEC/DIV switch (9) to 0.5 (sec/cm) and the X1-X10 control (10) fully counterclockwise. 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 more or less instantaneously at the left, and starts again; compare Figure 2.

(9) Adjust the POSITION controls (11), (15) 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 SEC/DIV switch set at 0.2 sec/cm. Keep trying different settings of this switch until you can explain clearly what it does. Record your conclusions.  Also determine what effect varying the X1-X10 control has on the rate at which the spot sweeps across the screen.

With the X1-X10 control set fully counterclockwise, turn the SEC/DIV switch 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?

(10) Now set the VOLTS/DIV switch to 2 V/cm and the SEC/DIV switch to 2 msec/cm. Set the TRIGGERING MODE switch to NORM, the SLOPE switch (6) out (+), and the LEVEL control (8) at or just above the middle of its range. Turn on the signal generator; set its frequency to 1 x 1 Khz, the shape control to the sine wave, and the amplitude control to about "1 o'clock" position. Connect a coaxial cable from the HI output of the Wavetek to the Y-input of the oscilloscope. You should see a sinusoidal waveform displayed on the screen, looking something like Fig. 5. If you do not, small adjustments of the signal display and TRIGGERING LEVEL controls should bring it in.

Change the SEC/DIV switch setting; what effect does this have on the pattern? Can you 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/DIV setting, and the scale of the display.

DC Circuits -- Ohm's and Kirchhoff's Laws

last updated 6/96