Exercise 4 - THE OSCILLOSCOPE

Transcription

1 Exercise 4 - THE OSCILLOSCOPE INTRODUCTION You have been exposed to analogue oscilloscopes in the first year lab. As you are probably aware, the complexity of the instruments, along with their importance in electrical measurements, has led us to insert the oscilloscope as one of the second year exercises. This exercise gives you the opportunity to perfect your mastery of the use of the instrument and, as we will be mainly having you do the exercise on a digital oscilloscope, to learn a very different model 'scope. The following is a series of exercises designed to lead you through the systematics of the 'scope and the use of most of its functions. There are very many controls on an oscilloscope and it will take you many times using the instrument before you can take advantage of all of them. Today you will be going on a "tour" of the main oscilloscope functions. These instructions do not really tell you how to use the oscilloscopes, just what to do with them. If you are unable to make something happen on the 'scope after consulting either the Wolf and Smith reference or the Tektronix reference, consult your demonstrator. The successful performance of these exercises will depend on your questions to your demonstrator. On all oscilloscopes, the controls can be grouped from the point of view of three basic functions: Controls governing vertical (y) motion of the beam; (vertical position, vertical sensitivity, CH1CH2 beam selection, DC-AC-Ground input coupling switches.) Controls governing horizontal (x) motion of the beam; (horizontal sweep speed, horizontal position and x sensitivity when in the x-y mode.) Controls governing the time base circuits which internally feed the x deflection of the beam; (all the trigger controls.) Recognizing these 3 aspects makes understanding the multiplicity of controls easier. The layout of the Tektronix TDS210 involves 7 turnable knobs and a number of buttons, all grouped according to the above systematics. With the knobs you dial-up vertical sensitivities, sweep speeds, vertical and horizontal positions, and trigger levels. Each of the main buttons calls-up a menu on the right of the screen and the buttons beside the menu allow you to select the various functions. EQUIPMENT USED The voltages used in all but exercise G are provided by one `box' which supplies various variations of 60 Hz A.C. and D.C.. A circuit diagram of the contents of the `box' is shown in Figure 1. The circuit diagram is for your information, but your use of this `box' is in no way dependent on your understanding of its circuit. This `box' provides the following voltages: 1

2 Fig 1. - Circuit Diagram of ``The Box'' VDC - about 11 V D.C. [between the DC and the COMMON terminals]. VACDC - a small A.C. (1 V peak-to-peak) superimposed on about 11 V D.C. [between the DC AND AC and the COMMON terminals]. VREF - about 8 V RMS (11 V amplitude) A.C. [between the AC REFERENCE and the COMMON terminals]. VPH - about 8 V RMS A.C. with phase relative to the VREF voltage adjustable by means of a phase control knob (which is totally uncalibrated). [This voltage appears between the AC PHASE and the COMMON terminals]. (VREF - VDC) - about 8 V RMS A.C. superimposed on about 11 V [between the AC REFERENCE and the DC terminals]. These five outputs will be referred to in the following instructions by the names VDC, VACDC, VREF, VPH, (VREF-VDC). THE EXPERIMENT EXERCISE A - Plotting One Voltage on a Y Input as a Function of Time Here you use the oscilloscope in the time-base mode, with only one beam turned on. You also will observe the functions of the y sensitivity and input coupling switches. Notice that when you first turn on the oscilloscope you will be starting with the settings that the person who used it before you last used. You might wish to push `AUTOSET' which sets the instrument to the setting IT thinks would be most 2

3 appropriate for you. (Caution: what it thinks and what you want may differ.) From the TRIGGER menu select MODE as AUTO and select SOURCE as AC LINE. Make sure you have selected to look only at the display of Channel 1 (CH1) by pushing the CH2 button sufficient number of times so that the CH2 trace disappears, and then pushing the CH1 button till the CH1 trace appears. (Note that the zero on the vertical scale is indicated by the position of the small arrow on the left side of the screen, accompanied by the channel number ("1").) With the Channel 1 signal input connected to the `box' as described and the CH1 menu available on the screen, note and understand the displays for each of the following cases: Set the beam at vertical centre using the vertical position knob with the COUPLING set to GROUND. With the COUPLING set at DC, observe VDC With the COUPLING set at AC, observe VDC With the COUPLING set at DC, observe VREF With the COUPLING set at AC, observe VREF Observe VACDC changing the COUPLING back and forth between AC and DC. While doing this, increase the y sensitivity (i.e. volts/div) to observe the A.C. component in greater detail, and note the function of the input coupling switch. Comment: Note that the input coupling switches are labelled DC, AC and GROUND. These stand for DIRECT COUPLING, ALTERNATING-CURRENT COUPLING, and A ZERO VALUE OF VOLTAGE. Normally you should use the oscilloscope in the DC setting, as this gives a display on the screen of the actual voltage. The other two switch settings provide special useful functions. Work out what the switch does. Under what conditions would you use it on the AC position? (The answer is not ``when you are observing A.C. signals''!) Hint: - in the AC position, the oscilloscope lies to you as the frequency components from 30 Hz down to DC are removed from the trace which appears on the screen.) EXERCISE B - Using the Oscilloscope to Plot Two Different Voltages as a Function of Time - Noting Different Ways of Triggering If you wish to understand what triggering is, start by using the oscilloscope in an untriggered condition. Connect the Channel 1 input lead to VREF as you did in exercise "A" and in the TRIGGER menu select SOURCE as EXT and MODE as AUTO. Notice the unusable trace you get on the screen. The sweeping of the display has no synchronization to the timing of the arrival of the input signal. Triggering instructs the oscilloscope to start the sweep of the trace across the screen according to specification you give as to when in the signal the scope should start sweeping. STRATEGIES FOR USING THE TRIGGERING CONTROLS - You will notice that the last four options on the TRIGGER menu are: 3

4 SLOPE (rising, falling) SOURCE (ch1, ch2, ext, ac line) MODE (auto, normal, single) COUPLING (ac, dc, noise reject, hf reject, lf reject) By setting these switches appropriately, you instruct the 'scope as to what should be the conditions for it to trigger (where triggering means the spot on the screen starting to sweep when certain set conditions are met). TRIGGER SOURCE decides which signal is the one which will determine the triggering, TRIGGER SLOPE decides whether triggering takes place on the rise or the fall of the signal, TRIGGER MODE decides whether triggering takes place strictly according to a voltage level of the signal ("normal mode") or whether the oscilloscope improvises its triggering by free-running when no signal is present ("auto mode"), TRIGGER COUPLING decides the filtering the oscilloscope gives to the trigger signal to make it respond only to some A.C. or D.C. voltage level of the input signal or other noise gets filtered out. Moreover there is a TRIGGER LEVEL knob - this decides at what voltage of the signal the triggering will take place (this control is quite ineffective in the AUTO triggering mode). The following two exercises help you to learn what each of these triggering controls does. In exercise B you use the oscilloscope in the time-base mode with both beams (channel displays) turned on with VREF into the channel 2 input and VPH into the channel 1 input. In all these cases, position the traces horizontally so that the trigger moment (indicated by the arrow at the top of the screen) is near the horizontal centre of the screen. Set the SEC/DIV sweep speed control so that between one and two cycles of the signal appear across the screen. With AUTO trigger mode and the phase shifter set for about 60 degree phase difference, observe the trace changes as you change the trigger source from CH1 to CH2 to EXT, to AC LINE. While on external trigger, try connecting the oscilloscope external trigger input to the VREF connection. With NORMAL trigger mode and CH1 as the trigger source, explore what happens when you change the trigger LEVEL and when you change the trigger SLOPE. Also explore the effects of changing the horizontal position knob. Here you are really only interested in the effects on the channel 1 trace. From the displacement you see of the two traces on the screen, find and record the phase control dial settings which produce phase differences of 0 degrees, 45 degrees and 90 degrees. Use your own choice of appropriate trigger modes and settings to enable you to make these observations. [A comment on limitations of the connections to the oscilloscope when feeding two or more signals into it: The three input connectors to the oscilloscope, although feeding signals into three separate places, are not completely independent. The outer ring of the BNC connector which is connected to the black banana plug on the cable you use, is connected to a "common" or "ground" in the oscilloscope. This ground is connected to the round pin of the power plug which is connected to a water pipe in the basement of the building. This applies to all the BNC input connectors. Thus, if you use two input leads on your oscilloscope, the black lead of one is connected to the black lead of the other. This implies that your circuit connection strategies must result in all the black leads being connected together with the voltages being measured being those of the red leads relative to the black leads.] 4

5 EXERCISE C - Frequency Measurement Using whatever trigger settings you consider appropriate, measure the period and frequency of the VREF signal from the display on the screen. (It is most appropriate to be viewing only the channel 1 trace on the screen.) EXERCISE D - The "Measure" Function Using the same setup as in Exercise C, find out how the scope will do measurements for you. Push the MEASURE button and select SOURCE as CH1. Now find out what it tells you for TYPE set for each of FREQ, PERIOD, MEAN, PK-PK, CYC RMS. You should figure out what each of the measurements means. Do these values agree with what you observe from the trace on the screen? EXERCISE E - Oscilloscope Calibration Signal The PROBE COMP terminal provides a 1 khz, approximately 5V (p-p) voltage between that terminal and the 'scope ground that has a very square wave shape. This is useful in checking out calibrations and in particular in checking out the response to a square wave (a check particularly important when using a probe on the input). Observe the `height' and frequency and shape of the signal from this terminal. EXERCISE F - Using the Oscilloscope as a Two Dimensional Voltmeter Up to now you have used the oscilloscope to plot one or two voltages as a function of time. This application will enable you to plot one voltage as a function of another. Here you use the oscilloscope in the x - y mode, so that the spot position is the vector sum of displacements in two perpendicular directions proportional to the two applied voltages. To get into this mode, in the DISPLAY menu select FORMAT, XY. (The FORMAT was previously set to YT.) To do this part, make sure that the COUPLING for both CH1 and CH2 are both set to DC. In this exercise you will view the Lissajous figures obtained by plotting sinusoidal displacements of the same frequency but different phases in two perpendicular directions. Apply VREF to the channel 1 input and VPH to the channel 2 input, and observe the patterns on the screen for the three phase control settings used in third part of EXERCISE B. Do you understand the shapes of these figures? EXERCISE G - Triggering the Oscilloscope to Observe Pulses Which Arrive Irregularly This exercise will be a good test in how well you have understood all the above exercises. Often physical equipment produces pulses which are similar in shape, but vary considerably in time of arrival. To make these pulses easily visible on the oscilloscope screen, it is necessary to trigger the 'scope in a way 5

6 appropriate to the pulses. In this case you will be looking at the pulses produced by a Geiger-Muller (GM) tube when beta particles and gamma-rays pass through the tube. (We do not expect that you understand the GM tube and associated equipment in this section. We merely are asking you to look at the randomly arriving pulses produced by the equipment.) Fig 2. - Typical Pulse from G-M Tube Connect the GM tube to the DETECTOR INPUT connector on the Picker Scaler, set the Picker scaler high voltage to the range indicated on the GM tube, and turn on the power and high voltage switches on the picker scaler. Do not place any radioactive source in front of the GM tube. The Picker Scaler unit here is used to provide the GM tube with the appropriate operating voltages and conditions. The output pulse from the GM tube which you want to observe on your oscilloscope is found on the BNC connector labelled PULSE INPUT. Hook the oscilloscope input to this connector and use the skills you have already obtained to obtain a pulse of the form of Figure 2. This task may not be an easy one as pulse arrivals are infrequent. (If the Picker Scaler is set to count the pulses, every time a pulse arrives the counting tubes indicate an increase in one count.) We suggest that you try changing the TRIGGER MODE from AUTO to NORMAL to SINGLE to see what results you get. (The RUN/STOP button can serve to reset the trace when in the SINGLE TRIGGER MODE.) Also try changing the horizontal SEC/DIV knob and the horizontal POSITION knob. Also try using the RUN/STOP function button. Ask your demonstrator for help if you are not having success. MAKE SURE YOU ARE CONNECTED TO THE CORRECT PLACE. HAVE YOUR DEMONSTRATOR CHECK YOUR CONNECTION. THERE ARE HIGH VOLTAGES ON THE PICKER SCALER, AND CONNECTING THE OSCILLOSCOPE TO THESE VOLTAGES WILL DAMAGE YOUR SCOPE. [In exercise G you will find it difficult at first to obtain reliable triggering. The following will lead you through a series of steps that will show you the logic of using various functions of the oscilloscope to get a usable trace under such awkward signal conditions. 1. The first thing you must do is figure out what kind of pulse you have (Is it positive or negative going, is it fast or slow, is it large or small in voltage?) In the TRIGGER menu, select MODE AUTO. AUTO enables you to have a trace on your screen at all times, even when you haven't achieved triggering synchronization.) Now vary the horizontal sweep speed (SEC/DIV) to see if you can see anything deviating from the horizontal straight line. In this case, a moderate speed around of 50 ms/div is probably a good choice. If you do see any pulse anywhere on the screen (as indicated by an apparently vertical line) stop the instrument (with the RUN/STOP button). If you haven't captured that pulse on the screen, push the button again to start the instrument, and then try again. Once you have some pulse stopped on the screen, expand it across the screen with the SEC/DIV and the horizontal POSITION knobs. You can now see if you have a positive or a negative going pulse, its approximate length (ms) and its approximate height (volts). 2. The next thing you must do is set the trigger specifications to match the pulse you have seen. Set the CH1 VOLTS/DIV knob to make the trace big enough (vertically) on the screen. Set TRIGGER SLOPE to RISING or FALLING to match what you know about the pulse, and set the TRIGGER LEVEL knob to a level about half way up the pulse. Now start the instrument (RUN/STOP button) and set the TRIGGER MODE to NORMAL. 6

7 3. You are probably very close to satisfactory triggering, but not quite. To get a reliably triggered trace play with the following controls in any order: a) vary the TRIGGER LEVEL control by a small amount; b) try out to see if one type of TRIGGER COUPLING works better than another and select the most successful! you have a choice of AC, DC, NOISE REJECT, HF REJECT, LF REJECT; c) set the CH1 VOLTS/DIV and the HORIZONTAL SEC/DIV to best show the details of the pulses.] EXERCISE H - Analogue Oscilloscopes There are several Phillips PM3217 analogue oscilloscopes around the lab. If you have time, either now or at a later date, try repeating Exercise B on this scope and note the differences between the devices. In real life you will be using both types of instruments and you will find that the superiority of one over the other depends on what you are doing. NOTE: Be sure you understand what your oscilloscope is doing at every step in these exercises. If, at any time, you do not understand, consult a reference or your demonstrator. REFERENCES S. Wolf and R.F.M. Smith, Student Reference Manual for Electronic Instrumentation Laboratories, Chapter 6 (chapter on oscilloscopes) TEKTRONIX web page, XYZs of Analog and Digital Oscilloscopes, the URL is (A copy of this booklet is available in room 229.) jbv 1985, 1987, 1992,