POLYTECHNIC UNIVERSITY Electrical Engineering Department. EE SOPHOMORE LABORATORY Experiment 3 The Oscilloscope

Transcription

1 POLYTECHNIC UNIVERSITY Electrical Engineering Department EE SOPHOMORE LABORATORY Experiment 3 The Oscilloscope Modified for Physics 18, Brooklyn College I. Overview of the Experiment The main objective of this experiment is to become acquainted with the operation of the oscilloscope. This goal will be reached by performing a series of measurements designed to illustrate the basic function of the major oscilloscope controls. The oscilloscope is a voltmeter which is able to display the measured values in graphical form. In an oscilloscope, the image is displayed by means of an electron beam which traces out a pattern on a fluorescent screen (see Wolf). Since there are no mechanical moving parts involved in this display mechanism, the oscilloscope is capable of measuring very high frequency signals. The oscilloscope is an extremely useful laboratory instrument, and it will be used extensively in all future laboratory courses. II. Equipment Required 1 Oscilloscope 1 Signal Generator 1 Isolated, 60Hz, Vrms ± 20% source 1 Silicon Diode KΩ Resistor, 5%, 1/4 W 1 each 10KΩ and 18kΩ resistors III. The Differential Mode of the Oscilloscope There are many applications where the differential mode may make the difference in being able to make a measurement. We will explore two such instances. Figure 6-8b on page 151 of the text shows the configuration of a true differential input oscilloscope. If a true differential input scope is not available, one can use an alternate circuit 15

2 (for dual input scopes) to make differential mode measurements. The following example will serve to illustrate this. Suppose that we wish to measure V 2 in Figure 3-1 below, and we have at our disposal a dual input scope which is not equipped with a true differential mode. If we try to use only channel A to measure V 2 it will not work (why?). We can, however, measure V 2 by connecting channels A and B as shown below, and adding the two channels with B inverted. The oscilloscope will display A-B, which in this case corresponds to ( V 2 + V 3 ) - ( V 3 ) = V 2. Channel A Channel B This technique, (the A-B) mode, allows us to make differential mode measurements even though a true differential scope is not available. Function Generator V 1 10k 18k V 2 V 3 Channel B Channel A Fig. 3-1 One interesting application of the differential mode is the ability to measure a small signal in the presence of a much larger undesired signal. Figure 3-2 shows a situation where we wish to measure a small signal, but interference is present. In certain situations the nature of the interference is known and can be isolated. In Figure 3-2 it is assumed that we can access the signal + interference via terminal 1, and we can access the interference alone via terminal 2. To measure the signal without the interference we can use the differential mode (A - B) as shown below: ( Signal + Interference ) - ( Interference ) = Signal Channel A Channel B 16

3 1 2 Signal + Interference Interference Channel B Channel A Scope Set to A - B Mode Fig. 3-2 IV. Experimental Procedure To familiarize yourself with the instrument, draw a large rectangle representing the front panel of the oscilloscope, and locate the controls mentioned on pages of your text. Review pages Make sure you understand the operation and purpose of each control. (Of course, you will obtain a fuller understanding of these controls upon completion of the experiment.) Note: Not all scopes have an identical set of controls; your scope may not have all of the controls mentioned on pages Oscilloscope Basics We will now connect a function generator directly to the oscilloscope to obtain an image on the scope. The following settings will be used on your instruments: Function Generator: 1000 Hz sine wave; 2V rms output (use DMM on a-c mode); d-c offset set to zero. Oscilloscope: vertical sensitivity 1 V/div (calibrated) time/division 0.2 msec/div (cal.) trigger source INT input coupling DC level adjust to see image focus and intensity mid-range a. Using a coaxial cable, connect the function generator to the oscilloscope, channel A. You are now using the oscilloscope to plot voltage as a function of time. b. How many cycles of the sine wave should the oscilloscope display? Does it? 17

4 c. Change the vertical sensitivity and time/division settings, and observe what happens to the image. d. Depress the control labeled "GROUND" or "GND". What is the purpose of this control? Center the horizontal line by using the vertical position control. e. Measure the peak to peak voltage of the image and convert to rms using V rms = V pp (This equation is true only for sinusoids.) Does this value agree with the DMM reading? f. Set the function generator to 100 khz and change the scope sensitivity to display two cycles. g. Repeat part e. Is there any difference now? 2. The Trigger Group (See pps , 169 bottom and 170) The "sweep" signal is a voltage which is applied to the horizontal deflection plates, causing the electron beam to sweep horizontally across the screen. The sweep signal is made to be proportional to time, so that when one applies a signal v(t) to the vertical deflection plates one obtains a display of v(t) vs. t. To obtain a steady image on the scope, the sweep signal must be initiated (triggered) periodically in synchronism with the periodic input signal v(t). In the oscilloscope there are three sources which can trigger the sweep: INT LINE EXT triggers the sweep based on the input signal to the oscilloscope. triggers the sweep based on the 60 Hz line voltage. triggers the sweep from an external source. In most cases we will use the INT mode. Later on you will learn that there are some applications which require the use of the EXT mode. We now want to take a closer look at the LINE triggering mode. This setting triggers the sweep every 1/60 of a second ±0.1%. If the frequency f ss of the input signal we are measuring is related to the frequency f t of the triggering signal by 18

5 fs = Nft where N is either an integer or a fraction, such as 1 1 2, 1 3, 4, etc., then we will obtain a steady image on the scope. Since in this case f ttt is is 30, 60, 90, 120 Hz etc. CALIBRATION OF SIGNAL GENERATOR 60 Hz, we should obtain steady images whenever the input signal We now wish to test the accuracy of the dial settings of the signal generator. a. Set trigger source to LINE. b. Set generator output to 60 Hz sine wave. c. Set time/div control so that one or two cycles are displayed. d. Adjust the generator dial for minimum or no motion of the display. What can you conclude? e. Some of the frequencies for which one can obtain a stationary image include: 30, 60, 90, 120, 180, Prepare a short table of "actual frequency" vs. dial setting. This represents a frequency calibration for the signal generator. 3. Input Coupling Controls (pps ) To test your understanding of the DC and AC coupling modes perform the following experiment: a. Using the same oscilloscope and function generator settings as in Part 1, display a sine wave on the oscilloscope. b. Vary the dc offset on the function generator. Does the position of the image change? c. Repeat a) and b) using the AC coupling mode. d. Can you think of any reason why it might be more desirable to use the AC coupling mode as opposed to the DC coupling mode? The following set of measurements will help you understand why it may not always be desirable to use the AC coupling mode. 19

6 Figure 6-7 in your text shows the input circuitry corresponding to the AC and DC coupling modes. Copy the figure into your lab notebook. a. Use the same settings as in Part 1. Change the generator output to a square wave. Make sure dc offset is set to 0 (use DMM to check). b. Adjust the vertical sensitivity (uncalibrated) so the square wave is one division from the top and bottom boundaries. c. Decrease the frequency to 250, 100, 50, 25 and 15 Hz. At each frequency compare the AC and DC input coupled images. Adjust the time/div so that approximately one cycle is seen. d. For a frequency of 15 Hz record the values of V 1 and V 2 (in terms of divisions), and t 1, t 2, as shown in Figure 3-3. Calculate the time constant of the exponential decay. Knowing that the input resistance to the oscilloscope is 1 MΩ, calculate the series AC coupling capacitance. V V 1 V 2 t Fig. 3-3: Square wave, distorted by ac input coupling circuit. 4. The x-y Mode of the Oscilloscope Up to now we have been using the oscilloscope to display plots of voltage vs. time. In the x-y mode the oscilloscope can display the variation of one voltage vs. another. Since almost any physical quantity can be represented by a voltage, the scope can be used to display the variation of many useful quantities. In this part of the experiment we will use the oscilloscope to obtain the i-v characteristic of an important electronic circuit element -- the semiconductor diode. Set up the circuit shown in Figure 3-4. (See instructions below figure.) As you increase the a-c voltage from 0 V to 10V, note how the appearance of the pattern on the 20

7 scope changes. For your report include a statement which explains the relationship between voltage and current for a diode Why is it necessary to use an isolated source instead of the function generator? a v s i v b 60 Hz Isolated Source ( Do not use Function Generator ) v R c R Fig. 3-4: Circuit to display i-v characteristic of diode. a. Set oscilloscope to the x-y mode. (channel A corresponds to the x-axis, and channel B to the y-axis.) b. Connect channel A '+' to point a, and channel A '-' to point b. c. Connect channel B '+' to point c and channel B '-' to point b. Note that the grounds of both channels A and B must be connected to the same point (in this case point b). Why? d. Press B INV so that we will plot v R vs. v. Since v R is proportional to i, we are actually plotting i vs. v which is what we desire. 5. The Differential Mode A. Measurement of Voltage between Two Ungrounded Points a. Assemble the circuit in Figure 3-1. For V 1 use a 1000Hz sine wave from the signal generator. Set the output of the generator to 5 Vrms (use DMM in AC mode). b. Measure V 1, V 2, and V 3 with the following meters. i) Electronic a-c voltmeter ii) DMM 21

8 iii) Oscilloscope using channel A only. Convert the peak-to-peak reading to rms for comparison to the other meters. c. Convert your oscilloscope to the differential mode as indicated in the above introduction, and measure V 1, V 2, and V 3. Note: Channel B and channel A must be set to the same sensitivity whenever using the A-B mode. For each of the measurements in parts b) and c), does V 1 = V 2 + V 3? results in the report. Explain your V. The Report For each of the 5 parts of the experiment, briefly discuss the questions that were raised in the manual. Make sure to include all relevant measured values (e.g., the series capacitance of AC coupling mode), and graphs (i-v characteristic of diode). 22