THE CATHODE RAY OSCILLOSCOPE

Transcription

1 The Department of Engineering SS1.2 THE CATHODE RAY OSCILLOSCOPE Objectives The objective of this laboratory is for you to familiarise yourself with the operation of a cathode ray oscilloscope (CRO). Once you have completed this laboratory you should be able to use an oscilloscope to achieve the following: Equipment 1. Check the calibration of the oscilloscope 2. Measure the rise time of a signal using an oscilloscope 3. Measure the amplitude of a signal 4. Measure the frequency of a signal 5. Measure the phase difference between two sine waves 6. Use Lissajous figures to measure frequency and phase difference The following equipment is to be used to perform the laboratory: 1. Oscilloscope Hameg HM Oscillators 3. AVOmeter 4. RC Network Box Experimental Procedure Please note the following two symbols have been used throughout this laboratory script: This is a question, and requires an answer to be written up in your daybook. This is an instruction and should be followed in order to achieve the aims of the laboratory. 1. Setting Up the Oscilloscope For a labelled diagram naming all the controls of the oscilloscope refer to Appendix 1. In order to make sure the various controls of the oscilloscope are set into the appropriate position the following procedure must be followed before switching on: 1. All push buttons should be in the out position i.e. released. 2. Rotate the variable controls with arrows (e.g. TIME/DIV, CH1, CH2 and HOLD OFF) to their Calibrated position. 3. Set all controls with marker lines to their midrange position (marker lines pointing vertically). ADB 1

2 4. The TV SEP. Lever switch and the TRIG. Selector lever switch in the X-field should be set to their uppermost position. 5. Both GD input coupling pushbutton switches for CH1 and CH2 in the Y-field should be set to the GD position. The oscilloscope can now be switched on by depressing the red POWER pushbutton. An LED will illuminate to indicate POWER is now present. The trace, displaying one baseline, should be visible after a short period of 10 seconds. Adjust Y-POS 1 and X-POS controls to centre the baseline. Adjust INTENS. (intensity) and FOCUS controls for medium brightness and optimum sharpness of the trace. The oscilloscope is now ready for use. CAUTION If only a spot appears on the oscilloscope screen reduce the intensity immediately and check that the X-Y pushbutton is in the released (out) position. If the trace is not visible, check the position of all the controls (particularly AT/NORM Button is in the out position). To obtain the maximum lifetime from the cathode-ray tube the minimum intensity setting necessary for the measurement being taken and the ambient light conditions should be used. 2. Calibration of the Y Amplifier It is important to check the calibration of the oscilloscope before relying upon its accuracy. To assist with this the oscilloscope has two built-in square wave test signals (0.2 V and 2 V), that are labelled CAL. Centralise the trace position using the X POS and Y POS controls. Set the VOLTS/DIV control on each channel to 0.5 volts and the FINE GAIN controls to the CAL position. Set the TIME/DIV switch to 1 millisecond and the X MAG and the central TIME/DIV controls to the CAL position. Release the GD buttons on both CH1 and CH2. Connect the Y1 channel input to the 2 V CAL connection using a BNC to 4 mm plugs lead. A square wave should now be displayed on the screen of the oscilloscope as in Fig. 1. To obtain a stationary trace, set the AT/NORM control to auto (AT button out), the EXT trigger control to internal (EXT button out). The waveform should occupy 4 divisions peak-to-peak with the VOLTS/DIV switch set to 0.5. Check the effect of the Y-POS and X-POS controls. Check the calibration of this range. Repeat the calibration check for the 1 volt range Repeat the calibration check once more for the 2 volt range. ADB 2

3 Figure 1 Waveform Display on Oscilloscope Quote any error as a percentage in your daybook. Repeat all of the calibration checks on the Y2 amplifier. Note and explain the effect of the AC/DC switch on the Y amplifiers, and the +/- switch on the time base triggering. 3. The Time Base The TIME/DIV control determines the horizontal scale (the time scale) of the graph which appears on the oscilloscope screen. This is achieved by a sweep voltage that is applied to the X plates of the oscilloscope. It is so called because it is used to sweep the electron beam horizontally across the screen. Think about the form an ideal sweep voltage waveform would take and draw it. With a suitable setting of the TIME/DIV switch, note the effect of varying the X- MAG and the variable TIME/DIV control. 4. Rise Time When dealing with square or pulse waveforms, the critical feature is the rise time of the voltage. The rise time of a signal is defined as the time required for it s leading edge to rise from 10 percent of it s final amplitude to 90 percent of it s final amplitude. Using the 2 volt CAL signal as the test input, measure the rise time of this signal. Adjust the gain of the Y amplifier as necessary using the variable gain centre knob on the VOLTS/DIV switch to make use of the scale markings on the screen (the 10% and 90% levels are marked with dotted lines). The X-MAG button may help when calculating the rise time, discuss it s effect. What happens to the waveform in the regions outside the rise time measurement (i.e. 0 10% and %)? Can you offer suggestions as to why the rise time is defined between two points such as 10% up and 10% down? ADB 3

4 5. Effect of a Circuit When dealing with certain electronic circuits it is useful to know the effect of that circuit upon a known test signal. Using an oscillator, apply a square wave of 1 KHz to the RC circuit as shown in Fig. 2. Adjust the CRO so that one cycle of the square wave is displayed together with one cycle of the output waveform. Sketch the waveforms, and label the sketches. In order to achieve a stationary display of a waveform, the CRO has to start the display at the same point of the waveform each time. This is accomplished by a technique referred to as triggering, and is the synchronisation of the timebase to the input waveform. Press the TRIG I/II button in, so that triggering now occurs from the Y2 channel. Using the LEVEL control, show how it is possible to start triggering at almost any point on the waveform. Where, on the waveform, is the greatest difficulty experienced in triggering, and why? Figure 2 RC Circuit 6. Amplitude and Frequency Measurement Generally, amplitudes of alternating voltages are quoted as RMS (root-mean-squared) values. However, when measuring the magnitude of such a signal using an oscilloscope, the peak to peak voltage (Vpp) value is used. It is also important to note that the amplitude of one half of the cycle is known as the peak voltage (Vp). The relationship of all three voltage levels can be seen in Fig. 3. The peak to peak voltage and the peak voltage can be converted to RMS as follows: V RMS VPP VP = = = VPP = VP ADB 4

5 Figure 3 Voltage Sine wave If a signal repeats, it has a frequency. This frequency is measured in Hertz (Hz) and equals the number of times the signal is repeated in one second (cycles per second). The signal also has a period, which is the time taken for one complete cycle to occur before it starts to repeat itself. The oscilloscope can be used to measure the frequency of a repetitive signal. This is achieved by measuring the period of one complete cycle on the screen of the oscilloscope. Since period and frequency are reciprocals of each other (i.e. 1/period equals the frequency, and 1/frequency equals the period), the time taken for one cycle (the period) can be used to calculate the frequency. The example shown in Fig. 4 demonstrates this principle. Figure 4 Frequency Example Set the oscillator to produce a sine wave with a frequency of 1KHz with an amplitude of 10 V. Now appraise the accuracy of the oscillator by measuring the amplitude and frequency of the signal using the oscilloscope. Repeat this task with the oscillator set to a frequency of 100 KHz. What can you say about the accuracy of this higher frequency signal? 7. Phase Measurement Since sine waves are based on circular motion they illustrate phase difference very well. One complete cycle of a sine wave relates to one complete circle and therefore to 360. This means that the phase angle of a sine wave can be represented using degrees. Figure 5 shows how a complete sine wave cycle relates directly to 360. ADB 5

6 Figure 5 - A Complete Sine wave Phase shift describes the timing difference between two otherwise similar signals. The example in figure 6 shows two similar sine waves of the same frequency. T denotes the period of one complete cycle (10 cm on screen), and t signifies the time between the zero transition point of both signals (3 cm on screen). Figure 6 Phase Shift Example The phase difference in degrees is calculated from ϕ o = T t 3 10 o o 360 = 360 = 108 o Re-connect the RC circuit box as in Fig. 2, except this time set the oscillator to a sine wave output of 1 KHz with an amplitude of 5 V. Set up the CRO, as before, so that one complete cycle can be seen on screen of both the input to the RC circuit and the output. Measure the time between the zero transition points of both signals, and calculate the phase difference in degrees. What is the phase relationship between the input and the output? Think about what is causing the phase change. ADB 6

7 8. Lissajous Figures When signals are applied to both the X and Y plates of a cathode ray oscilloscope, Lissajous Figures can be displayed on the screen. These figures can be used for phase and frequency comparisons between two signals. With a sine wave signal of known frequency applied to either the X or Y plates of the CRO, a comparison can be made with a sine wave signal of unknown frequency applied to the remaining set of plates. The Lissajous figures produced as a result of this comparison indicate the frequency relationship between them. With a known signal of 1 KHz applied to the X plates, and the unknown signal applied to the Y plates a pattern will be displayed on the CRO screen. This pattern would take the form of a figure of eight if the unknown signal frequency were twice the frequency of the known signal i.e. 2 KHz. This is also known as a bow-tie pattern. Figure 6 below shows the resultant Lissajous figures for two sine waves of the same amplitude with different phase angles. Figure 7 - Lissajous Figures Showing Phase Difference The phase angle or phase shift between X and Y input voltages (after measuring the distances a and b on the screen) is achieved by following the following formula. a sin ϕ =, b a cosϕ = 1, b 2 a ϕ = sin 1 b For X-Y operation, press the X-Y button in. Now apply two separate sine waves from separate oscillators, one to CH1 and the other to CH2 (in the X-Y mode of operation the CH2 input drives the horizontal deflection). Use the same oscillator for CH1 as was used in the amplitude and frequency exercise earlier as this oscillators accuracy has already been measured at a frequency of 1 KHz. Set the oscillator to 1 KHz. Adjust the other oscillator to approximately the same frequency. What is the resultant display? What happens when the frequency of the second oscillator is changed? What happens when the frequency of the first oscillator is changed? ADB 7

8 How can these changes be used if the frequency of one signal is known accurately? Repeat the phase measurement section of this laboratory, but this time use Lissajous figures to measure the phase difference. Results Write the answers to the questions posed throughout this Laboratory sheet in your daybook and submit them to a Laboratory Demonstrator for checking. References The following books are available from the Brynmor Jones Library, and deal with oscilloscopes as a measuring instrument. J.J. Carr Elements of Electronic Instrumentation and Measurement Prentice Hall 1986 TK7870C3 B.A. Gregory An Introduction to Electrical Instrumentation Macmillan Press Ltd., 1981 TK7870G8 E.O. Doebelin Measurement Systems Mcgraw Hill 1983 T50D6 C.N. Herrick Instruments and Measurements for Electronics Mcgraw Hill 1972 TK7870H5 ADB 8

9 Appendix 1 The Oscilloscope Controls Figure A1-1 Oscilloscope controls ADB 9

10 ADB 10