380 Physics Laboratory Manual Loyd Physics Laboratory Manual Loyd LABORATORY 38 Vertical Deflection Plates Horizontal Deflection Plates Brightness Focus Oscilloscope Measurements ---7--7 Fluorescent Screen Electron Gun OBJECTIVES Investigate the fundamental principles and practical operation of the oscilloscope using signals from a function generator. Measure sine and other waveform signals of varying voltage and frequency, Compare voltage measurements with the oscilloscope to voltage measurements using an alter- nating current voltmeter. EQUIPMENT LIST • Oscilloscope (typical direct current to 20 Mhz), alternating current voltmeter (high frequency capability) • Function generator (sine wave plus additional wave form such as a square wave or triangular wave), appropriate connecting wires (BNC banana plug) Accelerating Electrode Figure 38-1 Cathode-ray tube. We can deflect the electron beam in the horizontal (x) direction to represent a time scale by applying a time-varying sawtooth voltage waveform as shown in Figure 38-2. When a voltage of that waveform and of the appropriate maximum voltage is applied to the horizontal plates, the beam spot will sweep across the fluorescent screen once each time the voltage linearly increases from its minimum up to its maximum. At the end of the sweep of the beam across the screen, the beam returns to the left of the screen. The time this takes will equal the period T of the sawtooth waveform. Because this waveform sweeps the beam across the screen, it is commonly called the sweep generator. If the period of the sweep generator is 1s, the beam will clearly be recognizable as a spot that moves at constant speed across the tube face. If the period is as short as 0.1 s, the beam is no longer recognizable as a spot, but instead appears to be a somewhat pulsating line. This is because of the persistence of the phosphor, which causes the trace to still be glowing from one pass of the beam when another pass of the beam begins. For periods T of 0.01s or less, the beam moving across the screen so often that the persistence of the phosphor makes the trace appear as a steady line. The lloscope is designed so that a series of specific sweep generator perio be applied to horizontal plates by selecting the position of a multiposition switch. The width of oscilloscope screen is fixed, usually 10 cm. Each different choice of period T represents a specific time per length of scale division in the horizontal direction. Typically these are chosen to decrease in a series of scales that are in the ratio 2:1:0.5. For a typical student-type oscilloscope, the time scales would be 19 settings ranging from 0.2 s/cm to 0.2 ms/cm. Because the screen is 10 cm wide, there is a factor of 10 between the period T and the time scale. If the period of the sweep generator is 10 ms, the time scale is 1 ms/cm. Time t=0 is assumed to occur at the left of the screen, and time is assumed to increase to the right. In the vertical direction the screen is typically smaller, usually about 8 cm total. The vertical input is calibrated directly in volts. The input voltage scale is also variable by the choice of a multiposition switch that selects the appropriate amplification of the input voltage over some chosen voltage range. The typical range of possible voltage scales is from 5 V/cm to 5 mV/cm. This choice of voltage scales allows a range of input voltages to be displayed with deflections on the oscilloscope screen that are large enough to be easily visible. For the choices stated, the maximum voltage that can be displayed on the screen is 20V. The voltage can be either positive or negative polarity, so the vertical scale has its zero in the center of the screen to display both positive and negative voltages. THEORY The fundamental working part of an oscilloscope is a device called a cathode-ray tube (CRT). Its com- ponents include a heated filament to emit a beam of electrons, a series of electrodes to accelerate, focus, and control the intensity of the emitted electrons, two pairs of deflection plates that deflect the electron beam when there is a voltage between the plates (one pair for deflection in the horizontal direction and one pair for deflection in the vertical direction), and a fluorescent screen that emits a visible spot of light at the point where the beam of electrons strikes the screen. Together the heated filament and series of electrodes are called an electron gun. The electron gun and deflecting plates are arranged linearly inside an evacuated glass tube, and the fluorescent screen coats the glass tube at the opposite end of the tube from the electron gun as shown in Figure 38-1. When there is no voltage between either pair of deflection plates, the electron beam will travel straight down the evacuated tube and strike the center of the fluorescent screen. When a constant voltage is applied between either the horizontal or vertical deflection plates, the beam will be displaced by a constant amount on the fluorescent screen in either the horizontal (x) or vertical (y) direction. The direction of the displa- cement depends upon the sign of the voltage, and the magnitude of the displacement is proportional to the voltage. If a time-varying voltage is applied to either set of deflecting plates, the displacement of the beam will vary with time as the applied voltage varies with time, and the electron beam spot will move on the screen as a function of time. When the beam strikes the screen the phosphor glow persists for approximately 0.1s. COPYRIGHT © 2008 Thomson Brooks Cole im Time (s) THOMSON 2008 Thomson Brooks Dols , a part of the Thomson Caporefon Thomson the Starogard Books/Cole are trademarks used herein under loss. ALL RIGHTS RESERVED. Noper is work coveredbytte arriere may be reproduced or used in any form or by anymorphic, electronic, mecenica, rudrophotocopying, recording taping wedsbution, tomation BROOKS/COLE storage and real systems, or any other without the written permission of the publisher Figure 38-2 Sawtooth voltage waveform. 379 Laboratory 38 Oscilloscope Measurements 381 382 Physics Laboratory Manual Loyd ම ය 6088 15 GW GOLLOSOS 14 31 6. 2. Set the TIME/DIV control to 1 ms/DIV, the SWP VAR control rotated fully clockwise to the CAL position, the VOLTS/DIV control to 1 V/DIV, and the VAR (PULL 5 GAIN) control rotated fully clockwise to the CAL position. 3. Turn on the power to the function generator and let it come to thermal equilibrium for at least 10 minutes. Select a sine wave voltage, set the frequency f=100 Hz, and connect the output of the function generator to the CH1 INPUT of the oscilloscope. Adjust the amplitude control of the function generator to zero. Adjust the VERTICAL POSITION control of the oscilloscope until the flat trace is exactly on the center line of the vertical display. 4. (a) Adjust the amplitude control of the function generator until the display on the oscilloscope is full- scale positive on the positive part of the cycle and full-scale negative on the negative part of the cycle. In the laboratory report section, carefully draw on the grid labeled 1A what is displayed on the screen. (b) Leaving all other parameters fixed, set the VOLT/DIV control to 2 V/DIV, and draw on the grid labeled 1B what is now displayed on the screen. (c) Leaving all other parameters fixed, set the VOLT/DIV control to 5 V/DIV, and draw on the grid labeled IC what is now displayed on the screen. 3 12 5. (a) Leaving all other parameters fixed, set the VOLT/DIV control to 1 V/DIV, and select f=200 Hz from the function generator. Draw on the grid labeled 2A what is now displayed on the screen. (b) Leaving all Figure 38-3 The Hitachi model V-212 oscilloscope. other parameters fixed, select f= 400 Hz from the function generator, and draw on the grid labeled 2B what is now displayed on the screen. (c) Leaving all other parameters fixed, select f=600 Hz from the function generator, and draw on the grid labeled 2C what is now displayed on the screen. The most common use of the oscilloscope is to use the time scale provided by the sweep generator to (a) Leaving all other parameters fixed, set the VOLT/DIV control to 1 V/DIV, the TIME/DIV control to display the time variation of a voltage signal that is applied to the vertical plates. Usually this is some specific 2 ms/DIV, and select f=100 Hz from the function generator. Note that the trigger slope control is still waveform that is repeated with a fixed frequency. For example, if a simple sine wave voltage is applied to the set at (+). Draw on the grid labeled 3A what is now displayed on the screen. (b) Leaving all other vertical plates, a display of the voltage versus time will be directly displayed on the oscilloscope screen as a parameters fixed, pull out the trigger level control that sets the trigger slope to (-). Draw on the grid sine wave trace of the beam with a maximum amplitude proportional to the maximum voltage of the signal, and with a period on the time scale of the oscilloscope that is equal to the period of the signal. If the voltage labeled 3B what is now displayed on the screen. waveform applied to the vertical plates is a more complex waveform, the resulting trace on the screen will 7. (a) Leaving all other parameters fixed, push in the trigger level control that sets the trigger slope to (+), represent shape of that plex waveform. and the trigger level is still set at zero. Draw on the grid label 4A what is now displayed on the screen. The discussion so far has ignored one important point, which involves the means to coordinate the (b) Leaving all other parameters fixed, slowly turn the trigger level control clockwise, increasing starting time of the sweep generator with the starting point of the voltage signal that is to be displayed. the trigger level. Increase it only so long as the display remains triggered. At the maximum level that the We accomplish this by using some waveform as a "trigger" to start the sweep generator. The triggering display is triggered, draw on the grid labeled 4B what is displayed on the screen. (C) Leaving all other waveform can be the same signal that is input to the vertical plates for analysis, a secondary external signal, parameters fixed, slowly turn the trigger level control counterclockwise, decreasing the trigger level. or the 60 Hz line voltage. When the signal itself is used as the trigger for the sweep generator, the signal Decrease it only so long as the display remains triggered. At the minimum level that the display is is observed on the oscilloscope as a steady display that is constant in time because the sweep generator is triggered, draw on the grid labeled 4C what is displayed on the screen. initiated at the same point on the repetitive vertical signal for each pass of the sweep generator. On most 8. Push the trigger level control in for (+) slope and turn the level back to zero. Set the TIME/DIV to oscilloscopes this is referred to as internal triggering. That is the mode we will use in this laboratory. 2 ms/DIV and set the function generator to a sine wave off=100 Hz. Use the alternating current voltmeter to set the output of the function generator to 1.00 V as read on the voltmeter. Input this sine wave to the oscil-loscope and measure the peak voltage of the sine wave. To measure the peak voltage of the sine wave, you are free to adjust the VOLT/DIV control to give the most accurate measurement EXPERIMENTAL PROCEDURE possible. Generally this means adjusting the scale for as large a deflection as possible. Record the peak The procedure refers to the Hitachi model V-212 in Figure 38-3. It is a typical student oscilloscope. If using voltage of the sine wave as read from the oscilloscope in Data Table 1. Complete all the measurements in another oscilloscope, refer to the instruction manual for the corresponding settings and controls. Data Table 1 from 1.00V to 5.00 V. For each voltage, set the output from the generator using the voltmeter, In several of the instructions below, you are asked to draw what is on the oscilloscope display on the and then read the voltage from the oscilloscope, each time choosing the VOLT/DIV that will allow the grids provided. In each of those cases, assume that the VOLTS/DIV and TIME/DIV are properly calibrated, most accurate reading from the oscilloscope. and fill in the blank given for the values of VOLTS/DIV and TIME/DIV for the exercise associated with 9. Set the function generator to output a triangular wave with f=1000 Hz, and the TIME/DIV on the each set of grids. On the vertical scale OV is labeled. Label the full-scale voltage both positive and negative. oscilloscope to 1 ms/DIV. Use the alternating current voltmeter to set the output of the function The time scale is labeled with Os. Label the value of the full-scale time on the horizontal axis. Do this for generator to 1.00 V as read on the voltmeter. Input this triangular wave to the oscilloscope and measure each grid. the peak voltage of the wave. Proceed as instructed for the sine wave above, this time measuring the 1. Turn on the power to the oscilloscope and let it come to thermal equilibrium for at least 10 minutes, e 397 of 50 voltages between 1.00 V and 5.00 V as read on the voltmeter. Record the results in Data Table 2 Set the oscilloscope mode setting to CH1, the trigger source to INT, the trigger level to zero (center of range), trigger SLOPE to + (level knob pushed in), trigger MODE to AUTO, the INT TRIG to CH1, and CHI to AC COPYRIGHT © 2008 Thomson Brooks, Cole Laboratory 38 Oscilloscope Measurements 383 10. The goal of this laboratory is to introduce students to the oscilloscope. Now simply experiment for yourself with the features of the oscilloscope. Input as many different frequencies and waveforms as time allows and attempt to learn everything you can about the operation of the oscilloscope by simply trying different settings of all of the oscilloscope controls. CALCULATIONS 1. Perform a linear least squares fit to the data in Data Table 1 with the peak voltage read on the oscillo- scope as the horizontal axis and the voltage as read on the voltmeter as the vertical axis. Determine the slope, the intercept, and the correlation coefficient. Record those values in Calculations Table 1. 2. Perform a linear least squares fit to the data in Data Table 2 with the peak voltage read on the oscilloscope as the horizontal axis and the voltage as read on the voltmeter as the vertical axis. Determine the slope, the intercept, and the correlation coefficient. Record those values in Calculations Table 2. This page intentionally left blank COPYRIGHT © 2008 Thomson Brocks. Cole Page 399 of 509 386 Nawe Section Date Physics Laboratory Manual Loyd 5. A typical student oscilloscope on its least sensitive calibrated scale can display a voltage up to a maximum of approximately (a) 1V (b) 5 V (c) 20 V (d) 200 V. 6. A typical student oscilloscope on its most sensitive calibrated scale can display a voltage down to a minimum of approximately (a) 1 mV (b) 5 mV (c) 20 V (d) 200 mV. 7. A sawtooth wave with a period of 100ms is applied to an oscilloscope with screen 10 cm wide. What time is represented by 1 cm on the screen? 38 LABORATORY 38 Oscilloscope Measurements PRE-LABORATORY ASSIGNMENT 1. Describe the components that make up the electron gun in a cathode-ray tube. 2. Describe the voltage waveform that produces a linear time scale when applied to the horizontal plates of a cathode-ray tube. 3. When the electron beam strikes the fluorescent screen, the phosphor glow that results has persistence. Approximately how long does the glow persist? 4. A function generator outputs a sine wave off=200 Hz. It is input to an oscilloscope set at 1 ms/DIV. How many complete cycles of the sine wave are displayed on the oscilloscope? (Hint-The period of the sine wave T is related to the frequency f of the wave by T=1/4, and there are 10 divisions on the time display of the oscilloscope.) COPYRIGHT © 2008 Thomson Brooks Cole 385 388 Physics Laboratory Manual Loyd Name Section Date 4A. TIME/DIV> 4A. VOLTS/DIV= 48. TIME/DIV= 4B. VOLTS/DIVE 4C. TIME/DIV= 4C. VOLTS/DIY = Lab Partners OV OV OV 38 LABORATORY 38 Oscilloscope Measurements OS OS Os Data Table 1 Data Table 2 Voltmeter (V) Oscilloscope (V) Voltmeter (V) Oscilloscope (V) LABORATORY REPORT 1.00 1.00 1A. TIME/DIV 1A. VOLTS/DIV = 1B TIME/DIV= 1B. VOLTS/DIV 1C. TIME/DIV = 1C. VOLTS/DIV= 2.00 2.00 3.00 3.00 4.00 4.00 OV OV OV 5.00 5.00 OS OS OS Calculations Table 1 Calculations Table 2 2A. TIME/DIV 2A. VOLTS/DIV 28. TIME/DIV= 28. VOLTS/DIV 2C. TIME/DIV= 2C. VOLTS/DIV= Intercept Intercept Slope Slope OV OV OV T %3D OS Os OS SAMPLE CALCULATIONS None 3A. TIME/DIV 3A, VOLTS/DIV 3B. TIME/DIV= 3B. VOLTS/DIV = OV OV QUESTIONS 1. In the grid labeled 2A, how many complete cycles are sketched in your figure? From your sketch, what is the period of the wave? Using this period, calculate the frequency of the wave for this sketch. Is it in agreement with the frequency used for this part of the experiment? OS Os COPYRIGHT 2008 Thomson Brooks Cole 387