THE OSCILLOSCOPE AND NOISE. Objectives:

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1 -26- Preparaory Quesions. Go o he Web page hp:// and read a leas he firs four subsecions of he secion on Trigger Conrols (which iself is a subsecion of he secion The Conrols ) and answer he following quesions. 1. Wha is he difference beween Auo and Normal rigger modes? 2. Wha does he Trigger Slope Conrol do? Objecives: o develop faciliy in using a digial oscilloscope o recognize noise signals around us, where hey come from and how hey affec observaion Inroducion: We live in an environmen of many elecrical signals of various frequencies. The elecric power ha runs hrough he wires around us is a a frequency of 60 Hz. The radio broadcass ha we pick up are abou 1 MHz (AM) and 100 MHz (FM). Compuer moniors show signals in he ens of khz. In he lab, much insrumenaion is elecronic, and he presence of hese signals becomes "noise" in he process of experimenaion. (Noise is any observed effec which is no he effec we are rying o observe. Signal is he effec we are rying o observe.) In order o make reliable measuremens, we wan o maximize he signal-o-noise raio. In his experimen you will be using an oscilloscope which is a sensiive volmeer ha graphs he volage on is inpu as a funcion of ime. The oscilloscope you use will le you look a volages as low as 1 mv (10-3 Vols) and currens as low as 1 na (10-9 Amperes) and look a waveforms a frequencies up o 100 MHz. For he early sages of he experimen, we sugges seings for he oscilloscope conrols. Once you ge he hang of hem, you will find ha i is ofen quicker o play around wih he seings ill you arrive a he display ha you wan. Also noe ha many of he measuremens you make will be approximae, so use your judgemen on wheher error esimaes are appropriae or no.

2 -27- The Oscilloscope: On all oscilloscopes, he conrols can be grouped according o hree basic funcions: Conrols governing verical (y) moion of he race on he screen; (verical posiion, verical sensiiviy [vols/div], CH1CH2 beam selecion, DC-AC-Ground inpu coupling). Conrols governing horizonal (x) moion of he race; (horizonal sweep speed [sec/div], horizonal posiion). Conrols governing he ime base circuis which inernally feed he x deflecion of he race; (rigger level, rigger slope, rigger source, rigger mode). The ime base is he circuiry ha makes he x deflecion ino an acual ime axis on he screen and synchronizes ha ime scale so ha he race's posiion maches he ime of arrival of he signals being measured. The Tekronix TDS210 has 7 urnable knobs and a number of buons, all grouped according o he above sysemaics. Wih he knobs you dial-up verical sensiiviies, sweep speeds, verical and horizonal posiions, and rigger levels. You can se he parameers of each of CH1, CH2, Horizonal and Trigger by pressing he appropriae Menu buon, and using he unlabelled buons nex o he differen seings ha appear on he righ hand side of he screen. The inpu o he oscilloscope is via a coaxial cable, consising of wo wires, one being he cenre wire and he oher being he surrounding shielding wire braid; he braid is covered wih he black ouer coaing of he cable. (Look a he sample of coaxial cable available.) The ouer braid is conneced o he frame of he oscilloscope and from here o he round pin of he power plug which connecs o a waer pipe in he basemen of he building. I is called he ground wire. The cenre wire is conneced o he elecronics inside he oscilloscope. The oscilloscope measures he volage beween he cenre wire and he ouer braid. For convenience, you are provided wih leads ha have a "BNC" connecor on he end ha goes o he oscilloscope, and, a he oher end, wo plugs which are he connecions you use, he red one conneced o he cenre wire and he black one conneced o he grounded ouer braid. In his experimen, you will mosly be using only one inpu of he oscilloscope, and will have only one race on he screen. To do his, channel 2 should be urned off and he inpu lead should be conneced o channel 1. Push he "CH1 MENU" and "CH2 MENU" o urn on and off a channel. The small arrow on he lef side of he screen shows he channel number and he posiion of zero vols.

3 -28- The Experimen: Things o remember: a) When he oscilloscope is urned on, i sars off configured jus as i was before i had been urned off. Push "AUTOSET" o give he defaul seings, ofen a good place o sar. b) The "RUN/STOP" is a convenien buon ha allows you o freeze he display a any ime. 1. Low frequency elecrosaic fields. To sar, use he following oscilloscope seings: CH1 menu and knobs: Coupling=DC, BW Limi=ON(20MHz), VOLTS/DIV=(change as required) TRIGGER menu: Source=AC Line, Mode=Auo, Coupling=DC HORIZONTAL: SEC/DIV=beween 1ms and 25ms - change o ge he paern you wan Hold he red lead in one hand and repor wha you observe. Now bring i close o a power wire plugged ino he elecric source, and again repor wha you observe. To measure he frequency and volage of he signals you observe, press "Cursor" wih "Source=CH1" and roae he wo "Verical Posiion" knobs o move he cursors. The Volage can be read off when he seing is Type=Volage and he frequency can be read off wih he Type=Time seing. 2. Higher frequency elecrosaic fields (above 60 Hz bu below 20 MHz). To sar, use he following oscilloscope seings: CH1 menu and knobs: Coupling=DC, BW Limi=ON(20MHz), VOLTS/DIV=2.00mV TRIGGER menu and knob: Source=CH1, Mode=Auo, Coupling=DC, TRIGGER LEVEL=0.00V HORIZONTAL: SEC/DIV=beween 5ns and 1ms, Posiion=0.000s Here you have changed he rigger source and level and have changed he sweep speed. Bring he red lead near he oscilloscope screen. Skech he wave shape produced by he refresh circuiry, and measure is frequency. Repea near a compuer screen o view he scan of he elecron beam and also he much slower refresh rae. Then look a he paerns from a digial mulimeer and a hand calculaor. Does he relaive magniude of hese frequencies make sense, given he funcion of he differen elecronic insrumens you are observing? Noe ha changing he "rigger level" can someimes make a saionary paern more visible. 3. Elecrosaic Charge pickup. Noe: Don spend much longer ha 10 minues on each of pars 3 and 4 since par 5 requires more ime. Look a elecrosaic charge pickup from a syrofoam cup. Choose iniial seings of: CH1 menu and knobs: Coupling=DC, BW Limi=ON(20MHz), VOLTS/DIV=20.0mV TRIGGER menu and knob: Source=CH1, Mode=Normal, Coupling=DC, TRIGGER LEVEL=0.00V HORIZONTAL: SEC/DIV=50mS

4 -29- The coaxial cable acs as a cylindrical capacior wih he red lead a he cenre. Rub he cup on your sweaer and hen pass i by he end of he red lead. Explain he shape of he pulse and deduce he sign of he charge on he cup. Adjus he Trigger Level o obain a saionary picure of he pulse. 4. Sound Signals. A microphone is provided o examine sound signals (or noise). Saring wih seings of: CH1 menu and knobs: Coupling=DC, BW Limi=ON(20MHz), VOLTS/DIV=2.00mV TRIGGER menu: Source=CH1, Mode=Normal, Coupling=DC HORIZONTAL: SEC/DIV=10mS o 100µS invesigae he effec of various seings of "TRIGGER LEVEL" on your abiliy o see he races. Invesigae he range of frequencies you can cover wih a (quie!!) whisle. There is high frequency and low frequency "noise". Which noise is from sound pickup and which is from elecrical pickup? Noe ha pressing Force Trigger always makes a race appear. Sing a single noe and measure is frequency. Sing a noe an ocave higher (DO, RE, ME, FA, SO, LA, TI, DO) and measure is frequency. Wha change in frequency is an ocave? 5. Charge and Discharge of a Capacior. A capacior is a device for soring charge; in is simples form i consiss of wo parallel conducing plaes separaed by a hin layer of insulaor. The greaer he charge sored, Q, he greaer is he volage, V, across he capacior plaes. The relaionship beween he wo is Q=CV, where C, a consan ha depends on he area and separaion of he capacior plaes, is called he capaciance of he capacior. If a charged capacior is conneced o a resisor, R (measured in ohms), he charge will leak away a a rae deermined by wha is called he ime consan, 2. The expression for he rae of decay of he volage across he capacior from is iniial value V 0 is given by V() V o e 2 V o e Similarly, when a capacior is charged up hrough a resisance he volage builds up o is maximum volage V 0 according o he equaion V() V o (1 e RC 2 ) V o (1 e For a discharging charging capacior, he ime consan 2 is he ime for he volage o fall 1/e = of is iniial value For a charging capacior, he ime consan 2 is he ime for he volage o rise o (1-1/e) = of is final volage (or i s final volage minus iniial volage if he iniial volage is no zero). Noe ha since we are dealing wih exponenial funcions, i doesn maer wha volage we choose as he iniial volage. RC )

5 -30- I is convenien o charge and discharge he capacior using a volage ha urns on and off in a periodic way. Such a volage is produced by a signal generaor producing a square wave. The capacior will hen charge up and discharge every period, and is volage can be displayed on he oscilloscope. The DaaSudio 750 inerface produces he required signal. Open DaaSudio, and drag he Signal Oupu icon on o he oupus on he picure of he 750 inerface. Push he Auo buon, on he Signal Generaor window and he On - Off swiches will become acive. Choose he Square Wave Funcion; he frequency and ampliude of he signals can hen be chosen as required. For his experimen a frequency of around 100 Hz is reasonable place o sar. Use he circui shown; firs connec he larges capacior and he larges resisor on he "breadboard" in series o he oupu signal generaor of he 750. Then connec he CH1 oscilloscope leads across each side of he capacior; make sure ha he ground (black) lead is on he same side of he circui as he ground oupu from he signal generaor. Now use your experise wih he oscilloscope o obain a saionary paern on he 'scope. Then adjus he frequency of he signal ill he capacior is fully charged up by he end of he posiive square wave cycle (and also, of course, fully discharged by he end of he negaive square wave cycle.) Using he cursors, measure he ime consan 2 of your circui, by finding he poin a which he charging capacior reaches (1-e -1 ) of is asympoic value (or, alernaively, he poin a which he discharging capacior falls o e -1 of is iniial value). Noe ha since he 750 inerface produces a square wave oupu, he capacior is alernaely being charged posiively and negaively and he volage is no jus going beween 0 and some fixed value. However, since we are dealing wih exponenial funcions, he absolue values of he volages are no imporan bu wha is imporan is he difference beween he iniial and asympoic values of he volages. Thus, 0" is arbirary and can be se a he boom of he display. Repea for he oher hree R-C combinaions available o you on he breadboard, and compare o expecaion (Noe: he quoed value of he capaciance is good o abou 10%; he approximae values of he resisors can be read off using he colour code - see he relevan secion in he Laboraory Manual - or, more accuraely, measured direcly using a mulimeer). jbv,jp,awk