NOTES ON OSCILLOSCOPES

Transcription

1 NOTES ON OSCILLOSCOPES

2 NOTES ON... OSCILLOSCOPES... Oscilloscope... Analog and Digial... Analog Oscilloscopes... Cahode Ray Oscilloscope Principles... 5 Elecron Gun... 5 The Deflecion Sysem... 6 Displaying a Volage Waveform... 9 Triggering... Pulse Generaor:... Sweep Generaor... X-Y Operaion... Exernal Triggering... Digial Sorage Oscilloscopes (DSO)... 5 Measuremen Techniques... 6 Phase difference:... 7 Conrols... Display Conrols... Verical Conrols... Posiion and Vols per Division Seings... Horizonal Conrols... Inpu Coupling... X-Y Buon... DUAL Buon... Alernae and Chop Buons... ADD Buon... LEVEL and +/- Buons... 5 Appendix... 7

3 Oscilloscope In many applicaions, observing cerain volage waveforms in a circui plays a crucial role in undersanding he operaion of he circui. For ha purpose several measuremen insrumens are used like volmeer, ammeer, or he oscilloscope. An oscilloscope (someimes abbreviaed as scope ) is a volage sensing elecronic insrumen ha is used o visualize cerain volage waveforms. An oscilloscope can display he variaion of a volage waveform in ime on he oscilloscope s screen Figure. A probe is used o connec he oscilloscope o he circui. Figure shows an oscilloscope and a probe conneced o i. Figure. Figure shows a ypical probe. Oscilloscope shows he poenial difference beween he wo erminals of he probe. The erminal ending wih a hook is usually conneced o he node in he circui whose volage is of ineres. The oher erminal is usually (bu no always) conneced o he ground. The probes are aached o inpu channels of he oscilloscope. Mos oscilloscopes have a leas wo inpu channels and each channel can display a waveform on he screen. Muliple channels are useful for comparing waveforms. For example, one can observe he volage waveforms a he inpu and he oupu erminals of a circui simulaneously, by using a wo channel oscilloscope. + _ Analog and Digial Elecronic equipmens can be divided ino wo ypes: analog and digial. Analog equipmen works wih coninuously variable volages, while digial equipmen works wih binary numbers ( and s) ha may represen volage samples. For example, a convenional cassee player is an analog device; a compac disc player is a digial device. Oscilloscopes also come in analog and digial ypes. An analog oscilloscope works by direcly applying a volage being measured o an elecron beam moving across he oscilloscope screen. The volage deflecs he beam up and down proporionally, racing he waveform on he screen. This gives an immediae picure of he waveform. In conras, a digial oscilloscope samples he waveform and uses an analog-o-digial converer (or ADC) o

4 conver he volage being measured ino digial informaion. I hen uses his digial informaion o reconsruc he waveform on he screen. Figure : Digial and Analog Oscilloscopes Display Waveforms. Analog Oscilloscopes An analog oscilloscope displays he volage waveforms by deflecing an elecron beam generaed by an elecron gun inside a cahode-ray ube on o a fluorescen coaing. Because of he use of he cahode ray ube, analog oscilloscopes are also known as cahode ray oscilloscopes. To undersand how an analog scope displays he volage waveforms, i is necessary o undersand wha is inside he uni. The following secion describes he general principles of he operaion of cahode ray oscilloscopes.

5 Cahode Ray Oscilloscope Principles Figure shows he srucure, and he main componens of a cahode ray ube (CRT). Figure 5 shows he face plane of he CRO screen. Verical Deflecion Plaes Horizonal Deflecion Plaes Fluorescen Coaing CRO Screen Verical Deflecion Plaes Elecron Beam Elecron Gun CRO Screen Figure. Figure 5. Horizonal Deflecion Plaes Elecron beam generaed by he elecron gun firs defleced by he deflecion plaes, and hen direced ono he fluorescen coaing of he CRO screen, which produces a visible ligh spo on he face plane of he oscilloscope screen. A deailed represenaion of a CRT is given in Figure 6. The CRT is composed of wo main pars, Elecron Gun Deflecion Sysem Brighness conrol Cahode -Vgrid +V Focus Adjus kv-kv Verical Deflecion Plaes Horizonal Deflecion Plaes Fluorescen Coaing e- e- e- Elecron Beam Conrol grid Focus anodes and elecrosaic field Screen Elecron Gun Deflecion Sysem Figure 6. Elecron Gun Elecron gun provides a sharply focused elecron beam direced oward he fluorescen-coaed screen. The hermally heaed cahode emis elecrons in many direcions. The conrol grid provides an axial direcion for he elecron beam and conrols he number and speed of elecrons in he beam. The momenum of he elecrons deermines he inensiy, or brighness, of he ligh emied from he fluorescen coaing due o he elecron bombardmen. Because elecrons are 5

6 negaively charged, a repulsion force is creaed by applying a negaive volage o he conrol grid, o adjus heir number and speed. A more negaive volage resuls in less number of elecrons in he beam and hence decreased brighness of he beam spo. Since he elecron beam consiss of many elecrons, he beam ends o diverge. This is because he similar (negaive) charges on he elecrons repulse each oher. To compensae for such repulsion forces, an adjusable elecrosaic field is creaed beween wo cylindrical anodes, called he focusing anodes. The variable posiive volage on he second anode cylinder is herefore used o adjus he focus or sharpness of he brigh spo. The Deflecion Sysem The deflecion sysem consiss of wo pairs of parallel plaes, referred o as he verical and horizonal deflecion plaes. One of he plaes in each se is permanenly conneced o he ground (zero vol), whereas he oher plae of each se is conneced o inpu signals or riggering signal of he CRO. Verical Deflecion Plaes Horizonal Deflecion Plaes Fluorescen Coaing Elecron Beam Elecron Gun Deflecion Sysem Figure 7. CRO Screen As shown in Figure 7, he elecron beam passes hrough he deflecion plaes. In reference o he schemaic diagram in Figure 8, a posiive volage applied o he Y inpu erminal causes he elecron beam o deflec verically upward, due o aracion forces, while a negaive volage applied o he Y inpu erminal causes he elecron beam o deflec verically downward, due o repulsion forces. Similarly, a posiive volage applied o he X inpu erminal will cause he elecron beam o deflec horizonally oward he righ, while a negaive volage applied o he X inpu erminal will cause he elecron beam o deflec horizonally oward he lef of he screen. 6

7 CRO Screen Vy Verical Deflecion Plaes Vx Horizonal Deflecion Plaes Figure 8. The amoun of verical or horizonal deflecion is direcly proporional o he corresponding applied volage. When he elecrons hi he screen, he phosphor emis ligh and a visible ligh spo is seen on he screen. Since he amoun of deflecion is proporional o he applied volage, acually he volages V y and V x deermine he coordinaes of he brigh spo creaed by he elecron beam. Example : Suppose V x = sin(), V y = cos() are applied o he horizonal and verical deflecion plaes respecively. Then he brigh spo would follow a circular pah on he CRO screen. CRO Screen Vy = cos() Vx = sin() Figure 9. 7

8 Example : Vy() Vy() Figure -a. Vs() Vs() Figure -b. Vy() Vy() Vs() Vs() Figure -c. 8

9 In Figure -a, he inpu signal V y () is applied o he verical deflecion plaes, whereas he horizonal deflecion plaes are conneced o ground. I is assumed ha he elecron beam is kep a he exreme lef posiion when he horizonal deflecion plaes are conneced o ground. Under his configuraion, he brigh spo in he CRO screen will follow a verical pah (will go up and down) a he exreme lef posiion of he screen. In Figure -b, he inpu signal V s () is applied o he horizonal deflecion plaes, whereas he verical deflecion plaes are conneced o ground. This ime, he brigh spo will ravel from exreme lef o exreme righ end of he screen and will sop here. In Figure -c, he signals V y () and V s () are applied o he verical and he horizonal deflecion plaes respecively. This ime he brigh spo will follow a sinusoidal pah, resuling a visualizaion of he inpu signal V y () on he CRO screen. Acually he brigh spo mus follow he same pah fas and repeiively (a leas imes in a second) so ha he human eye can perceive he moion of he brigh spo as a coninuous curve. Therefore, in order o display he waveform on he CRO screen for he example in figure -c, he signals V y () and V s () should be applied o he verical and he horizonal deflecion plaes periodically and in synchronizaion. The nex secion discusses he deails of his procedure and depics how CRO handles his problem. Displaying a Volage Waveform In numerous applicaions i will be required o display a periodical volage waveform as a funcion of ime. By applying he volage o be displayed on he CRO, o he verical deflecion plaes (V y ), he verical deflecion of he beam spo will be proporional o he magniude of his volage. I is hen necessary o conver he x axis (horizonal deflecion) ino a ime axis. Noice ha, in he example given in figure -c, he volage waveform V s () (which varies linearly in ime before he brigh spo reaches he exreme righ end of he screen) is used for his purpose and he brigh spo have raveled he pah deermined by V y (). If he signal o be observed is periodic, hen a periodic volage waveform ha varies linearly wih ime, as shown in figure below, is applied o he horizonal deflecion plaes. This ype of waveform is called he sawooh waveform. s() Exreme Righ Exreme Lef Trace ime (Tr) Flyback ime Figure When s() is zero vol, he brigh spo is a he exreme lef-hand posiion, and when s() is maximum, he brigh spo is a he exreme righ posiion. Therefore, he brigh spo ravels from exreme lef o exreme righ in a ime equal o he race ime. During he flyback ime, which is usually very shor compared o race ime, a high negaive volage pulse is applied o he conrol grid of he elecron gun o preven elecron beams reaching he CRO screen. This 9

10 acion is called blanking and prevens any reverse rerace (or shadow) as he beam is going back o he exreme lef-hand posiion. The ime period including he race ime and he flyback ime is called he sweep period. The period of he sawooh waveform plays a crucial role in obaining a seady waveform on he CRO screen. The following secion discusses he requiremens on he period of he sawooh waveform and he need of a synchronizaion beween he sawooh waveform and he inpu waveform. Triggering Vy () CRO SCREEN s() τ Exreme Righ T Exreme Lef Seady Waveform is obained Figure Suppose he inpu V y () and s() shown in Figure are applied o he verical and horizonal deflecion plaes of he CRO respecively. Noe ha, A he beginning of each sweep cycle, (i.e when he brigh spo is a exreme lef) V y () ges exacly he same value (The poins indicaed by red circles). Therefore he brigh spo is following exacly he same pah in each sweep cycle. Thus, we can observe a seady waveform on CRO screen. Noice ha he ime beween he beginning of wo consecuive sweep cycles is a muliple of inpu signal period. (i.e. = n T. T, are shown on he figure, n is a posiive ineger.)

11 Yinpu () CRO SCREEN S() Exreme Righ Exreme Righ Exreme Righ Exreme Lef Exreme Lef Exreme Lef Waveform is no seady! Figure For he given case, he brigh spo is following differen pahs in differen sweep cycles, herefore we can no obain a seady waveform on CRO screen. In order o obain sable and saionary waveform displays, he sawooh signal should be applied o he horizonal deflecion plaes, in synchronism wih he waveform being displayed. CRO handles his synchronizaion problem by using he following srucure. CRO Display Verical Amplifier ~ Yinpu Horizonal Amplifier s() Sweep Generaor p() Pulse Generaor Figure Noice ha, he volage waveform which is o be displayed on he CRO screen (Yinpu in his case) is applied o he verical amplifier. In he amplificaion sage, only he ampliude of he inpu waveform is changed. Afer he amplificaion sage, he oupu of he verical amplifier is applied o he verical deflecion plaes. Then in order o obain a seady waveform on he CRO screen, a sawooh waveform having a period which is an ineger muliple of he period of he inpu volage waveform should be applied o he horizonal deflecion plaes.

12 Yinpu () Level T P() T S() Trace ime (Tr) Figure Pulse Generaor: The main funcion of he pulse generaor (PG) (see figure.) is o produce periodical pulses wih a period of T, which is equal o he period of he inpu signal. For ha purpose, he inpu signal is compared o a cerain volage level ( Level on he figure ). Producing pulses each ime he inpu volage is equal o ha cerain volage level, may seem o resul pulses which are periodic wih period T. Bu his is no he case. Noice ha, he Level inersecs he inpu signal more han once in one period. Therefore, one of he inersecion poins is negleced in each period. The decision on which inersecion poin o neglec is made by inspecing he slope of he signal a he inersecion poin. The pulse generaor produces pulses each ime he inpu volage level is same as he Level, afer checking he slope of he signal a ha ime insan. In he example given in figure, here are wo inersecion poins a each period, and he one wih he negaive slope (blue poins) are negleced. (The seleced slope is posiive for ha case). Acually, he sign of he slope can be seleced by using he +/- buon of he CRO. Also he volage level ha he signal is being compared o, can be adjused by using he level buon on he oscilloscope. Sweep Generaor The main funcion of he sweep generaor is o produce one cycle of a sawooh waveform, when i receives a pulse a is inpu. If he sweep generaor receives a rigger pulse during is sweep cycle (i.e., during he race period Tr), i will simply ignore he pulse and coninue wih he compleion of is sweep cycle. Depending on he seleced level and he slope of he inpu signal, he oupu of he pulse generaor will consis of narrow rigger pulses separaed from each oher by one period T. Each ime he inpu signal crosses a preseleced level (and a preseleced slope), he pulse generaor emis one narrow rigger pulse. The emied pulse riggers he sweep generaor o begin producing one cycle of he sweep waveform; is duraion is he race period Tr. A he

13 end of each sweep cycle, he sweep generaor sops is oupu and awais he arrival of he nex rigger pulse before producing a new sweep cycle. Noice ha if he sweep generaor receives a rigger pulse during is sweep cycle (i.e., during he race period Tr), i will simply ignore he pulse and coninue wih he compleion of is sweep cycle. The rigger pulse received afer he compleion of he race period will iniiae he new sweep cycle. This allows he scope o display more han one cycle, of period T, of he signal conneced o is verical deflecion plaes. The following figure illusraes an example. Yinpu () Level SUMMARY Assume +/- buon of CRO is released. A hese poins, even he inpu is equal o he Level, he pulses are no generaed, because slope is (-)ve Those pulses are ignored by he Sweep generaor, since hey are received during he sweep cycle. P() Tr Noe ha, he Trace ime is deermined by he ime/div buon of he oscilloscope Noe ha, he volage of he saring poin of he plo on he CRO screen is equal o Level. And plo sars wih a (+)ve slope. S() CRO SCREEN Trace ime (Tr) Figure 5. In he given example, firs he volage waveform o be displayed on he CRO screen is compared wih a volage level. The blue and he red poins show he inersecion of he inpu signal wih he level. Assuming he posiive slope is seleced, pulse generaor produces pulses a each ime he inpu signal is equal o level, and is slope is posiive. The pulses generaed by he pulse generaor, rigger he sweep generaor, which produces one cycle of he sawooh waveform. The race ime of he sweep generaor is adjused by he ime/div buon which is available on he fron panel of he oscilloscope. The resuling sawooh waveform is applied o he horizonal deflecion plaes, which leads o a seady display of he inpu signal on he oscilloscope screen. Noice ha, a he beginning of each sweep period (when he brigh spo is a he exreme lef), inpu signal volage is equal o `level` and has a posiive slope. Therefore, he waveform shown on he CRO screen sars wih a posiive slope a he exreme lef and is value is equal o he level. One can change hese seings by varying he level conrol or he +/- buon of he oscilloscope.

14 The whole process is called riggering because, obaining a seady plo on he CRO screen can only be achieved by producing pulses a he inpu of he Sweep Generaor a he correc ime insances. (i.e. riggering he Sweep Generaor a he correc ime insances.) X-Y Operaion When he variaion of one volage waveform, Vy(), as a funcion of anoher, Vx(), eliminaing he parameer ime,, is desired, X-Y mode of operaion is used. In X-Y mode, one signal is applied o he verical deflecion plaes whereas he oher signal is applied o he horizonal deflecion plaes. The XY buon on he fron panel of he oscilloscope disconnecs he riggering signal from he horizonal deflecion sysem, and connecs he second inpu signal insead. This process is done by using a swich shown as X-Y buon on he figure below. CRO Display Verical Amplifier ~ Yinpu Horizonal Amplifier s() Sweep Generaor p() Pulse Generaor ~ Vexernal Xinpu ~ X-Y Buon Ex. Buon Figure 6 Exernal Triggering Raher han he inpu signal iself, an exernal signal can also be used for riggering. For ha purpose muli-posiional swich, which corresponds o Ex. Buon of he CRO, should be se o posiion, as shown in Figure 6. The exernal signal should saisfy cerain condiions in order o obain a seady waveform on he CRO screen. Keeping in my ha he period of he sawooh waveform, s(), should be an ineger muliple of he period of he inpu signal, can you find he condiions needed on he frequency of he exernal riggering signal? See also: hp://

15 Digial Sorage Oscilloscopes (DSO) The concep behind he digial oscilloscope is somewha differen o an analogue scope. Raher han processing he signals in an analogue fashion, he DSO convers hem ino a digial forma using an analogue o digial converer (ADC), hen i sores he digial daa in he memory, and hen processes he signals digially, finally i convers he resuling signal in a picure forma o be displayed on he screen of he scope. Since he waveform is sored in a digial forma, he daa can be processed eiher wihin he oscilloscope iself, or even by a PC conneced o i. One advanage of using he DSO is ha he sored daa can be used o visualize or process he signal a any ime. The analogue scopes do no have memory herefore he signal can be displayed only insananeously. The ransien pars of he signal (which may vanish even in milliseconds or microseconds) can no be observed using an analogue oscilloscope. The DSO s are widely used in many applicaions in view of heir flexibiliy and performance. 5

16 Measuremen Techniques The major concern in observing a signal on he oscilloscope screen is o make volage and ime measuremens. These measuremens may be helpful in undersanding he behavior of a circui componen, or he circui iself, depending on wha you measure. Excep for he X-Y mode of operaion, he oscilloscope displays he volage value of he waveform as a funcion of ime. The oscilloscope screen is pariioned ino he grids, which divides boh he horizonal axis(volage) and he verical axis(ime) ino divisions which will be helpful in making he measuremens. See Figure 7. Figure 7: Oscilloscope Screen. Obviously one needs o know he ime or he volage values corresponding o each division, in order o make accurae calculaions. These values are deermined by wo variables namely he ime/div and he vol/div boh of which can be adjused from he relevan buons available on he fron panel of he oscilloscope (see Figure -). Also noe ha, he ime/div buon conrols he race ime of he sweep generaor, whereas he vol/div buon conrols he `gain` in he verical amplifiers in he verical deflecion sysem. Typical quaniies, which are of primer ineres when observing a signal wih he scope, are shown in Figure 8. DC Value Ground Peak value Peak o peak value Period Figure 8: Sinusoidal Signal on Oscilloscope Screen. 6

17 For he given figure, suppose ha he variables vol/div and ime/div are se o: vol/div = Vols/div. ime/div = millisecond/div Then he corresponding values shown on he figure are calculaed o be; Peak Value = 6vols Peak o peak value = Vols DC Value (Average Value) = Vols Period = milliseconds Frequency = = Hz. Period Noe ha he signal s(), shown on he oscilloscope screen can be expressed as, ( ) s = V sin( π f) + V peak ( π ) ( π) = 6sin + DC = 6sin Vols. Phase difference: In some applicaions, one may need o monior or compare wo or more signals simulaneously. A ypical example can be he comparison of he inpu volage wih he oupu volage of a wo-por (inpu and oupu pors) circui. If he signals ha are being moniored have he same frequency, a ime delay may occur beween he signals (i.e. one signal may lead he oher or vice versa). Two waves ha have he same frequency, have a phase difference ha is consan (independen of ). When he phase difference (modulo ) is zero, he waves are said o be in phase wih each oher. Oherwise, hey are ou of phase wih each oher. If he phase difference is 8 degrees ( radians), hen he wo signals are said o be in ani-phase. If he peak ampliudes of wo ani-phase waves are equal, heir sum is zero a all values of ime,. Figure 9: In-phase Waves Figure : Ou of phase Waves 7

18 The phase difference is expressed in erms of radians or degrees. In Dual Mode of he oscilloscope he phase difference can be calculaed easily as follows. Given he wo signals having he same frequency, as shown in Figure, T T Phase Difference Figure : Two Signals Displayed in DUAL Mode define, T = horizonal spacing of he peak values (or he zero crossings) of he wo signals. T = horizonal spacing for one period. Then he phase difference, is; T o θ = 6 T in deg rees T θ = π T in radians Noe ha, one has o specify he leading or he lagging signal in order o fully describe he ime difference beween he wo signals. In he figure above, he signal represened wih dashed curve leads he oher. Suppose ha he signal represened by he dashed curve is conneced o Channel I of he oscilloscope, and he oher one is conneced o Channel II. In such a case Channel I is leading he Channel II wih phase difference equal o, and Channel II is lagging he Channel I wih phase difference equal o. Deermining he leading or he lagging signal may be frusraing a firs, bu noe ha he dashed curve reaches is maximum value before he oher does. The phase difference beween he signals can also be deermined in XY mode of he oscilloscope. In he XY mode, he x-axis daa is aken from one channel, y-axis daa is aken from he oher. In ha way, Channel I vs Channel II graph can be obained, so ha he variaion of a signal wih respec o anoher can be observed. Figure shows a ypical graph in XY mode, of wo signals having a consan phase difference. 8

19 A B Figure : Phase Difference Calculaion in XY Mode Phase difference is equal o, θ sin A B. = B One can show his relaion by expressing one signal as, y ( ) = sin( w ± θ ) and he oher C signal as, x( ) = sin( w). Then consider he value of y() when x() is zero vols. I should be noed ha, he cener of he ellipsoidal shape (someimes circular or linear shapes) on he screen should be a he origin of CRO unless any DC componen is added o one of he signals. In XY mode, he leading or he lagging signal can no be deermined. One has o swich o DUAL mode in order o specify he leading signal. Figure shows ypical graphs in XY mode corresponding o differen values of phase difference Figure : The Graphs in XY Mode for Differen Phase Difference Values 9

20 Conrols Display Conrols Display sysems may vary beween analog and digial oscilloscopes. Common conrols include: An inensiy conrol o adjus he brighness of he waveform. As you increase he sweep speed of an analog oscilloscope, you need o increase he inensiy level. A focus conrol o adjus he sharpness of he waveform. Digial oscilloscopes may no have a focus conrol. Oher display conrols may le you adjus he inensiy of lighs and urn on or off any on-screen informaion (such as menus). Verical Conrols Verical conrols are used o posiion and scale he waveform verically. Oscilloscopes also have conrols for seing he inpu coupling and oher signal condiioning, described in his secion. Figure shows he verical conrols of he DSO6A Figure : Verical Conrols of DSO6A. Posiion and Vols per Division Seings The posiion knob moves he waveform verically. The scale knob varies vols per division (usually wrien vols/div), which deermines he volage value corresponding o each verical division on he oscilloscope s screen. As he vol/div value is alered, he size of he waveform on he screen changes. The vols/div seing is a scale facor. For example, If here are en verical divisions on he oscilloscope screen and if he vols/div seing is 5 vols, hen each of he verical divisions represens 5 vols and he enire screen can show 5 vols from boom o op. If he seing is.5 vols/div, he screen can display 5 vols from boom o op, and so on. The maximum

21 volage you can display on he screen is he vols/div seing imes he number of verical divisions. Ofen he vols/div scale has eiher a variable gain or a fine gain conrol for scaling a displayed signal o a cerain number of divisions. Figure 5 shows he verical conrols of he HM-7 CRO. Figure 5: Verical Conrols of HM-7 CRO Horizonal Conrols Horizonal conrols are used o posiion and scale he waveform horizonally. Figure 6 and 7 show ypical fron panel for he horizonal conrols. Figure 6: Horizonal Conrols of DSO6A Figure 7: Horizonal Conrols of HM-7 CRO The horizonal posiion conrol (x-pos.) is used o move he waveform from lef and righ o exacly where you wan i on he screen. The ime per division (ime/div) seing les you selec he rae a which he waveform is drawn across he screen (also known as he ime base seing or sweep speed). This seing is a scale facor. For example, if he seing is ms, each horizonal division represens ms and he oal screen widh represens ms (en divisions). Changing he ime/div seing les you look a longer or shorer ime inervals of he inpu signal. As wih he verical vols/div scale, he horizonal sec/div scale may have variable iming, allowing you o se he horizonal ime scale in beween he discree seings. Also noe ha, he ime/div buon acually conrols he race ime of sawooh waveform in he sweep generaor. When sawooh waveform is zero vol, he brigh spo is a he exreme lef-hand posiion, and when i is maximum, he brigh spo is a he exreme righ posiion. Therefore, he brigh spo ravels from exreme lef o exreme righ in a ime equal o he Trace ime. Assume ha he CRO screen is divided ino N equal horizonal divisions. The brigh spo ravels he N divisions in Tr seconds. Therefore each division corresponds o

22 (Tr/N) seconds. If he Trace ime is changed, he corresponding ime for each division is changed. Time per division conrols can be used o selec he appropriae ime/div (i.e., he Trace ime of he sawooh waveform). Inpu Coupling Coupling means he mehod used o connec an elecrical signal from one circui o anoher. In his case, he inpu coupling is he connecion from your circui o he oscilloscope. The coupling can be se o DC, AC, or ground (GND). By seing he coupling conrol o AC, he DC offse volage is removed form he inpu waveform, so ha you see he waveform cenered a zero vols. When DC coupling is seleced, boh AC and DC componens of he inpu waveform are passed o he oscilloscope. Figure 8 illusraes he difference. The signal in Figure 8 is y ( ) = + sin( w) where Vols is DC componen and sin(w) is AC componen. By selecing AC coupling, DC componen is eliminaed and only he signal of sin(w) is shown on he screen (Figure 8-b). The AC coupling seing is useful when he enire signal (alernaing plus consan componens) is oo large for he vols/div seing. Figure 8-a: V peak o peak sinusoidal wih Vols offse, shown in DC mode. Figure 8-b: V peak o peak sinusoidal wih Vols offse, shown in AC mode. The ground seing disconnecs he inpu signal from he verical sysem, which les you see where zero vols is on he screen. Wih grounded inpu coupling and auo rigger mode, you see a horizonal line on he screen ha represens zero vols. Swiching from DC o ground and back again is a handy way of measuring signal volage levels wih respec o ground. X-Y Buon Mos oscilloscopes have he capabiliy of displaying a second channel signal along he X-axis (insead of ime). This is called XY mode. Pressing he X-Y buon he oscilloscope is used in XY mode. See Also (Noes on CRO) DUAL Buon The oscilloscopes have he capabiliy of displaying boh channel signals on he screen a he same ime. This is called he Dual Mode. This mode is usually used o measure phase difference beween wo signals which is explained in Phase difference par on page 7.

23 Alernae and Chop Buons On analog scopes, muliple channels are displayed using eiher an alernae or chop mode. (Digial oscilloscopes do no normally use chop or alernae mode.) Alernae mode draws each channel alernaely - he oscilloscope complees one sweep on channel, hen one sweep on channel, a second sweep on channel, and so on. Use his mode wih medium- o high-speed signals, when he ime/div scale is se o.5 ms or faser. Alernae mode is available when only DUAL buon is depressed. Chop mode causes he oscilloscope o draw small pars of each signal by swiching back and forh beween hem. The swiching rae is oo fas for you o noice, so he waveform looks whole. You ypically use his mode wih slow signals requiring sweep speeds of ms per division or less. Chop mode is available when boh DUAL and ADD buon are depressed. Figure 9 shows he difference beween he wo modes. I is ofen useful o view he signal boh ways, o make sure you have he bes view. Figure 9: ALT and CHOP modes ADD Buon When ADD buon is depressed, he signals of boh channels are algebraically added and he resul is displayed on he screen. Vol/div scales of wo channels should be he same in order o appropriaely see he summaion of he signals. When he vol/div scales of he channels are no he same, he signals are summed up as hey are displayed on he screen (i.e. graphically). Assume a signal sin( w ) is conneced o Channel I and a signal sin( w ) is conneced o channel II. CH I is se o vols/div (Figure -a) and CH II is se o vol/div (Figure -b). When he ADD buon is depressed, he resuling signal on he screen is shown in Figure.

24 Figure -a: The firs signal seen on he oscilloscope wih vol/div scale. Figure -b: The second signal seen on he oscilloscope wih vol/div scale.

25 Figure : The sum of wo signals in Figure -a and -b when ADD buon is depressed. INVERT Buon When he INVERT buon of a channel is depressed, negaive of he signal is displayed on he CRO screen. EXT Buon When he EXT buon is depressed, he oscilloscope is used in exernal riggering mode. Exernal riggering is explained a Secion Exernal Triggering a page. AT/NORM Buon Using he AT/NORM buon you can swich beween auomaic rigger level selecion (AT) and manual rigger level selecion (NORM). When he AT/NORM buon is released, he auomaic rigger level is seleced as zero vols, so ha he value of he signal on he exreme lef of he screen is equal o zero. When he AT/NORM buon is depressed, he user can deermine he rigger volage level (he volage on he exreme lef) manually by adjusing LEVEL knob. LEVEL and +/- Buons The rigger level can be se using he LEVEL knob when he AT/NORM buon is depressed. Using he LEVEL knob, he rigger volage level can be se o values differen han zero. However, if he rigger level is se o a volage value ha is higher/lower han he posiive/negaive peak of he signal, he signal can no be riggered and herefore can no be displayed on he CRO screen (Figure -d). The +/- buon is used o deermine wheher an increasing signal passing from rigger volage, sars he sawooh waveform (+/- buon released) or viceversa. To be familiar wih hese buons, he signals seen on he oscilloscope wih various buon configuraions for he signal in Figure -b (.5sin(w)) are given in Figure. 5

26 Figure -a: The signal when AT/NORM buon is released. (LEVEL is auomaically se o vol.) Figure -b: The signal when AT/NORM buon is depressed, LEVEL is se o Vol and +/- buon is released. Figure -c: The signal when AT/NORM buon is depressed, LEVEL is se o Vol and +/- buon is depressed. Figure -d: The signal when AT/NORM buon is depressed, LEVEL is se o Vol. 6

27 Appendix The fron panel of he oscilloscopes DSO6A and he HM-7 CRO are shown in he following figures respecively. Figure A: Fron Panel of he HM-7 Cahode Ray Oscilloscope. Figure A: Fron Panel of he DSO6A. 7

28 Figure A: Schemaic for he Fron Panel of he DSO6A. 8