Memorial University of Newfoundland Department of Physics and Physical Oceanography. Department of Physics and Physical Oceanography

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1 Memoril University of Newfoundlnd Deprtment of Physics nd Physicl Ocenogrphy Memoril Physics University 2055 Lbortory of Newfoundlnd Deprtment of Physics nd Physicl Ocenogrphy Introduction Physics 2055 to Lbortory Oscilloscope The gol of this lbintroduction is to fmilirize toyou with Oscilloscope oscilloscope, one of most widely-used pieces of lbortory pprtus. It is not lb in regulr sense; The gol of this lb is to fmilirize you with oscilloscope, one of re most is no widely-used Introduction piecesorofresults lbortory to write pprtus. in your It islb notnotebook. lb in You regulr should, however, sense; re tke cre is no to Introduction write downor everything Results tht you should do twrite ech in step your... lb wht electricl notebook. connections You should, do you however, hve? tke... cre whtore write down oscilloscope everything settings tht you (time bse, dovoltge t ech sensitivity, step... whtetc)? electricl... which connections switches do you didhve? your turn?... wht... rewht do yoscilloscope do? You will settings need(time to be bse, blevoltge to usesensitivity, se controls etc)? in... future which controls lbs. did your turn?... wht do y do? Introduction Introduction An oscilloscope (or CRO) is n instrument for observing electricl signls nd is essentil An oscilloscope (or CRO) is n instrument for observing electricl signls nd is n essentil for nyone designing or repiring electronic equipment. The primry dvntge of using n tool for nyone designing or repiring electronic equipment. The primry dvntge of using oscillosope n oscillosope insted insted of digitl of digitl voltmeter voltmeter is tht is tht signl signl cn be cn viewed be viewed directly directly on on screen, fcilitting screen, fcilitting direct mesurement direct mesurement of AC chrcteristics of AC chrcteristics such such s frequency, s frequency, pek pek to to pek pek(p-p) voltge (p-p) nd voltge phse ndngle. phse ngle. Figure Figure 1: 1: Oscilloscope used used in in lbortory. lbortory A photogrph of oscilloscope used in this 1 lbortory is shown in Fig 1. It is cpble 1

2 A photogrph of oscilloscopes used in this lbortory is shown in Fig 1. They re cpble of displying two distinct externl signls in two chnnels, lbelled CH1 nd CH2. of displying two distinct externl signls in two chnnels, lbelled CH1 nd CH2. We use We use wveform genertor s n c voltge source, shown in Fig 2. Both items re used wveform genertor s c source, shown in Fig 2. Turn on both units nd llow m extensively in c circuits. Turn on both units nd llow m to wrm up for few minutes to wrm up for few minutes before proceeding. before proceeding. Figure 2: Wveform genertor usedinin lbortory. Disply signl on Oscilloscope Connect BNC cble from OUTPUT terminl terminlof of AC ACsignl signlgenertor genertortoto chnnel chnnel CH1. CH1. Set Set signl signl genertor genertor to to output output sine sine wve wve with with frequency frequency of of few few hundred hundred Hz. Hz. 2. Press AUTO key. The oscilloscope utomticlly sets verticl, horizontl, nd 2. Press AUTO key. The oscilloscope utomticlly sets verticl, horizontl, nd trigger controls so tht bout three complete cycles of wveform re displyed. trigger controls so tht bout three complete cycles of wveform re displyed. You cn djust ny of se mnully using Verticl nd Horizontl controls if you You cn djust ny of se mnully using Verticl nd Horizontl controls if you need to optimize disply. need to optimize disply. POSITION: Moves trce for prticulr chnnel verticlly or horizontlly POSITION: Moves trce for prticulr chnnel verticlly or horizontlly on screen. on screen. Verticl Sensitivity: this control sets voltge required to deflect bem Verticl Sensitivity: this control sets voltge required to deflect bem through one verticl division. The rnge for this CRO is from 2 mv/div up to through one verticl division. The rnge for this CRO is from 2 mv/div up to 5 V/div. The sensitivity is displyed2 t bottom left corner of screen. 2

3 Time Bse: The time bse setting sets time required for bem to sweep cross one lrge horizontl division. The verticl sensitivity nd time re displyed t bottom of screen. Try djusting se nd explin in your own words how y ffect ppernce of wveform on oscilloscope. Triggering 1. The oscilloscope needs to know when to begin trce. Triggering is ccomplished utomticlly by this oscilloscope. When n input signl is pplied, sweep utomticlly djusts itself to trigger t men level of input wveform. 2. Press Trigger MENU key. Check tht Source is set to input signl (CH1) nd tht Mode is set to AUTO. Then press Slope softkey to determine wher triggering is done on positive-going wveform or negtive-going one. 3. Explin briefly how triggering works, using tringulr wve output from wveform genertor. Wht does level control do? AC Mesurements Amplitude: The mplitude of n lternting wveform is voltge difference between mximum vlue nd zero reference line. Pek to pek mplitude: This is verticl displcement between mximum nd minimum peks (for exmple, 3.5 divisions 1 mv/div = 3:5 mv p-p). For sine wve on your oscilloscope screen, wht is p-p mplitude? Root Men Squre (rms) Amplitude: The rms vlue of n lternting voltge or current is given by 1/ 2 or times its mximum vlue. The root men squre current is vlue of sine-wve current which will produce sme heting effect in resistor s dc current of sme mgnitude. Most AC multimeters mesure rms vlues. Wht is rms mplitude of your oscilloscope trce? Frequency: Mesure horizontl displcement (in squres) for one or more complete cycles nd hence determine verge displcement of one complete wve. Clculte 3

4 period, t, of wve, given by number of horizontl divisions multiplied by time bse setting. The frequency of wveform is f = 1/t. Using CURSOR The CURSOR key provides two verticl or horizontl lines which help you identify pek or or fetures of wveform. To mesure pek-pek mplitude: Press CURSOR to disply on-screen menu Press Type softkey nd select Voltge Press Y1/Y2 softkey until Y1 is illuminted Use Adjust dil to move cursor to mximum pek position Press Y1/Y2 softkey until Y2 is illuminted Move cursor to Minimum pek position. The voltge difference between two cursors is displyed t right hnd side of screen. To mesure time difference: Press Type softkey nd select Time Press X1/X2 softkey until X1 is illuminted Use Adjust dil to move cursor to strt time Press X1/X2 softkey until X2 is illuminted Move cursor to end time position. The time difference ( x) between two cursors is displyed t right hnd side of screen. If you hve mesured time for complete wveform, 1/ x will correspond to frequency. Automtic Amplitude nd Frequency mesurement of n AC Signl 1. Press MEASURE key to disply MEASURE menu. 2. Press Voltge softkey to disply VOLTAGE menu. 4

5 3. Press pek-pek softkey to mesure p-p mplitude (or RMS to mesure RMS mplitude). The mplitude vlue will be displyed t bottom of screen. 4. Press MEASURE key gin. 5. Press Time softkey to disply TIME menu. 6. Press Frequency softkey. The frequency vlue will be displyed t bottom of screen to right of voltge vlue. Check tht frequency is sme s vlue displyed on wveform genertor. Effect of Output Frequency on DMM Redings 1. Set signl genertor to output 5 v p-p sine wve t bout 100 Hz. Use oscillosope to determine frequency, pk-pk mplitude nd rms mplitude of displyed wveform. For comprison, mesure rms voltge using digitl multimeter. 2. Increse frequency in suitble lrge steps so tht you cover full rnge of signl genertor frequencies, up to bout 5 MHz, nd record mplitude, frequency nd rms voltge ech time. Summrize se in tble nd explin why you should be creful when using digitl multimeter t high frequencies. Mesuring DC signls Connect BNC cble from DC power supply to eir chnnel CH1 or CH2 of oscilloscope. Set COUPLING softkey to DC. DC coupling psses both AC nd DC components of input signl. A pure DC signl will pper s horizontl stright line. As you increse power supply voltge line will move verticlly upwrds. The distnce moved will depend on verticl sensitivity. You cn lso disply signls which contin both AC nd DC components. Reconnect wveform genertor to oscilloscope nd move DC OFFSET control clockwise to pply positive voltge, nd counterclockwise to pply negtive voltge. (Check DC OFFSET switch first). Observing two signls toger Connect second BNC cble from source to chnnel CH2 nd press CH2 button. The MATH button llows you to dd, subtrct or multiply two signls. For two signls 5

6 of different frequency you will need to set trigger to lternte between two. Press Trigger MENU nd Source Alternting. Try this using second wveform genertor, if vilble. A note on grounding The oscilloscope is grounded, i.e., it is connected to erth through third prong on mins plug. Consequently, ll potentils re mesured reltive to this s reference point. When mesuring two signls simultneously with CRO, you should first check where ground wires re connected, orwise you my crete short circuit nd not see signl t ll. To demonstrte this, construct circuit shown in Fig 3 using wveform genertor nd two resistors in series. Use R 200 Ω nd r 100 Ω. Use digitl meter to confirm tht two (rms) voltges mesured cross ech resistor dd (pproximtely) to source voltge. V R R r V r Figure 3: Connect two resistors in series with power supply Now use oscilloscope to mesure p-p voltge cross ech resistor. Explin wht hppens when you try to mesure voltge cross R. Reconfigure circuit so tht you cn mesure V R using oscilloscope. 6

7 Chrge nd Dischrge of Cpcitor Replce circuit bove with 1000 Ω resistor nd 1 µ F cpcitor connected in series. Set wveform genertor to output squre wve nd disply signl cross cpcitor. Adjust voltge nd time bse controls so tht you cn see one or two complete cycles for chrge nd dischrge. Sketch both trces nd, in prticulr, describe shpe of cpcitor trce when input signl chnges from () minimum to mximum (b) mximum to minimum Summry Summrize wys tht n oscilloscope cn be used in lbortory s mesuring device, nd discuss its bility to mesure AC nd DC signls when compred to digitl multimeter. 7