Electronic Instrumentation

Transcription

1 December- 0 Third Semeser B.E. Degree Examinaion Elecronic Insrumenaion Time: 3 hrs. Max. Marks: 00 Noe:. Answer any FIE full quesions, selecing a leas TWO quesions from each par. PART - A. (a) Explain he following in brief: (08 Marks) (i) Gross errors : (ii) Relaive errors : (iii) Accuracy ; (iv) Resoluion. (i) Gross errors : Gross errors are hose ha are generaed by human facor involved in measuremens. These include incorrec adjusmens, improper applicaions and misreading of insrumens. These Gross errors can be conrolled by aking proper care while reading, calculaing and recording a quaniy under measuremen. Repeaed readings of he same quaniy (a leas 3-4 imes) should be aken o minimize he error. (ii) Relaive errors : Relaive error is de ned as he raio of he absolue error produced in a given measuremen o he measured value. For example; When a resisor having a known % of error i is called relaive error for example R = 00 ± 0%. (iii) Accuracy : Accuracy refers o he degree of closeness wih which any insrumen reading approaches he rue value of he variable or quaniy being measured. For example, consider he rue value is vols and he measured value is 0.5 vols. This indicaes he exacness of he meer. (iv) Resoluion : Resoluion is de ned as he smalles change in he measured value o which he insrumen responds. Consider a volmeer ha can read volages from 0 o ol. If he meer has his range divided ino 0 equal pars, hen each par will represen 0. ( = 0) ol. Then we say ha he revoluion of he meer is 0.. I can be noed ha his meer can no be used o measure a volage ha is less han 0.. (b) Explain he working of a rue RMS volmeer wih he help of a Block diagram. (0 Marks) Figure () Any complex waveforms can be accuraely measured using rue rms - volmeer. These insrumens sense 4 Elecronic Insru. December 00 December 0.indd 4 8//03 :55:5

2 Elecronic Insrumenaion December 0 Dec - waveforms heaing power and produces meer indicaion proporional o square or rms value of volage. This heaing power is ampli ed & fed o hermo couple and hen he measured oupu volage is proporional o square of rms - value. To measure rue rms volage, wo hermocouples conneced forming a bridge nework is used as shown in gure. The inpu volage o be measured is applied o he heaer elemen of he measuring hermocouple. The heaing effec of heaer is measured by hermocouple and generaes corresponding volage called. The inpu volage is ampli ed and hen given o he heaer elemen of measuring hermocouple o produce enough heaing so ha =. These wo hermocouples balancing and measuring form a bridge n/w and when = he bridge is balanced, oupu volage A( ) = K irms, 0 = sine A is gain of dc ampli er () =. 0 irms = K, = = K. = irms 0 = rms value of he inpu = 0 irms (c) De ne sensiiviy. Deermine he value of he muliplier resisance on he 50 range of a dc volmeer ha uses a 50 A meer movemen wih an inernal resisance of 00. (0 Marks) Sensiiviy of a volmeer is de ned as he resisance of he meer for an applied volage of. In mahemaical erm, full scale de ecion I fsd. and a large resisance of order of k is presen in he mere circui hen for differen volages applied o he mere, he mere wih I fsd produces a volage wih sensiiviy S = I fsd Given daa : 50 Range dc volmeer I fsd = 50 A = 00 R m S = 50 μ A = 4 k /v R = S range = R m R = 40 k /v 500 v 00 = = R = 99.9kΩ. (a) Explain he ramp ype digial volmeer wih he help of a block diagram. (0 Marks) December 0.indd 5 8//03 :55:56

3 Dec - December 0 Elecronic Insrumenaion Figure (a) Ramp ype DM Fig b. Ramp echnique The ramp ype of DM basically consiss of inpu aenuaor and inpu comparaor, ground comparaor and a gae, couner arranged as shown in gure. (a). The basic principle of measuremen is based on he measuremen of he ime aken by a linear ramp o raise from 0 vol o he level of inpu volage i. The ime is measured wih a couner and displayed in numeric form. The linear ramp consiss of ramp generaor generaing ve going ramp ranging from. To sar he measuremen he ramp volage is iniiaed wih inpu volage i and compared coninuously wih i, when hese wo volages are no same he inpu comparaor oupu generaes a sar pulse, where oscillaions are allowed o ener ino he couner. These are illusraed in Fib b. The ramp is coninuously driven o zero vol when he ground comparaor inpus become equal and gae generaes a sop - pulse. The magniude of he coun is proporional o ime inerval beween sar and sop pulse. December 0.indd 6 8//03 :55:56

4 Elecronic Insrumenaion December 0 Dec - The magniude of he coun indicaes he magniude of O which is displayed by he display uni. (b) Wih block diagram, explain he principle and operaion of digial frequency meer. (0 Marks) Digial frequency meer Figure 3(a) Block diagram of digial frequency meer Frequency of an unknown signal is deermined by convering he signal o rigger pulses and applied o an AND gae, a pulse of sec is applied o he oher inpu of he AND gae. The number of pulses couned during sec indicaes he unknown frequency. The signal whose frequency is o be measured is convered ino a rain of pulses, pulse for each cycle of inpu signal. Depending upon he number of pulses occurring in a de nie ime inerval is hen couned by a couner, he number of couns is direcly proporional o frequency of unknown signal. The block - diagram of he digial frequency meer is as shown in he gure.3(a) which consiss of ampli er and Schmi - rigger. The ampli er is used o amplify he inpu signal and Schmi rigger is used o conver inpu sine wave o square wave. As a resul, he oupu from he schmi rigger is a rain of pulses, one pulse for each cycle of inpu signal. The oupu pulse from schmi Trigger is fed o he gae and when gae is enabled he pulses reach couner, which couns he number of pulses. The number of coun is direcly proporional o frequency.. 3. (a) Explain he CRT feaures brie y. (05 Marks) The CRT used in a cahode ray oscilloscope is elecrosaic ype and hey are available in various size and bandwidh o sui individual requiremens. The main feaures of his kind of cahode ray ube are explained here.. Size: I refers o he screen diameer like he T speci caion. CRTs are available from o 7 inches in various sizes, however 3 inch CRT is commonly used in porable insrumens. Recangular ype CRTs are mosly used nowadays raher han he round ype.. Phosphor: This refers o he uorescen maerial used o coa he screen, which decides he color and persisence of he race. These are indicaed by he phosphor. The race colors available for elecrosaic CRTs are blue, green and blue green. Persisence is expressed as shor, medium and long, which refers o he lengh of ime he race remains on he screen afer he signal is removed. The phosphor of CRT is speci ed like P (green medium), P( blue green medium), P (Blue shor) ec. Some ime he speci caions are combined in he ube ype number. For example 5GP indicaes ha i is a 5 inch ube wih medium persisence green race. 3. Operaing olage: Typical operaing volages a differen poins of CRT are lised below, which may vary depend on he model of CRT. a) The heaer volage of he CRT is 6.3 ac/dc, 600 ma. b) Negaive grid conrol volage is 4 o 00. c) Focusing anode volage is 00 o 00. d) Acceleraing anode volage is 600 o 6k 4. De ecion olages: De ecion of beam is achieved hrough eiher ac or dc volage. The de ecion sensiiviy of he ube is usually speci ed as he dc volage required for each cm of de ecion of he spo on he screen. 5. iewing Screen: The viewing screen is he glass face plae of which he inside wall is coaed wih phosphor. I is recangular in shape wih graicule marked on i in cenimeer scale for he purpose of aking measuremens; each cenimeer corresponds o one division. The sandard size of he screen is 8 cm 0 cm. December 0.indd 7 8//03 :55:57

5 Dec - December 0 Elecronic Insrumenaion (b) Draw he basic block diagram of an oscilloscope. Explain he funcions of each block (0 Marks) The block diagram of a basic CRO is shown in gure. The funcion of he various blocks are as follows. Cahode Ray ube (CRT): The funcion of cahode ray ube is o emi elecrons ha srikes he phosphorous screen o provide a visual display of he signals.. erical Ampli er: The inpu signals o be measured are applied o verical ampli er sage o amplify and pass faihfully he enire band of frequencies, and herefore he ampli er used is wide band. 3. Delay line: Delay line basically consiss of RC/LC circui o delay he signal for someime in he verical secion, if delay line is no used hen a par of he oupu signal is los. 4. Time Base generaor: The ime base generaor is used o generae a sawooh volage, which is required o de ec he elecion beam in he horizonal direcion. 5. Horizonal ampli er: The saw ooh volage generaed by he ime base generaor may no be of suf cien srengh o drive he horizonal de ecion plaes, i is ampli ed using a high volage gain ampli er. 6. Trigger circui: The horizonal de ecion of elecion beam should sar a he same poin of he inpu verical signal, and o synchronize horizonal de ecion wih verical de ecion a rigger circui is used. The rigger circui convers he inpu signal ino rigger pules so ha he inpu signal and sweep frequency can be synchronized. 7. Power supply: There are wo power supplies namely, high volage (around -.5k) supply and low volage (around 400) supply wo volages are generaed and passed hrough a bleeder resisor a a few ma. These inermediae volages of bleeder resisor are used for inensiy, focus, horizonal ampli er, verical ampli er. (c) Describe he following modes of operaion available in a dual race oscilloscope: (i) ALTERNATE mode ; (ii) CHOP mode, (05 Marks) In a dual race oscilloscope, a mode conrol swich is used o enable he elecronic swich o display wo signals simulaneously. There are wo modes of operaion ALTERNATE and CHOPE depend on he range of inpu signal frequency (high or low). (i) ALTERNATE MODE: When he swich is in alernae posiion, he elecronic swich alernaely connecs he main verical ampli er o he wo channels, say A and B, and adds differen dc componen o each signal. This dc signal direcs he beam o he upper or lower half of he screen alernaely. The swiching happens a December 0.indd 8 8//03 :55:57

6 Elecronic Insrumenaion December 0 Dec - he sar of each new sweep of he sweep generaor. The swiching rae is synchronized wih he sweep signal so ha he channel A is ge raced on he screen in one sweep and channel B on he succeeding sweep and he cycle coninues as shown in he waveform of Fig. 3(b). Channel A Channel B Channel A Inensiy OFF Fig. 3 (b)time relaion in ALTERNATE MODE (ii) CHOP MODE: In chop mode of CRT, he elecronic swich is free running a rae of KHz, independen of sweep generaor. The swich successively connecs small segmens of A and B channels waveforms o he main verical ampli er a a fas chopping rae of 500KHz as shown in Fig. 3(c). If he chopping rae is slow, he coninuiy of display is los and i is beer o change o ALTERNATE mode. Waveform for chop mode Channel A Channel B Inensiy Off Chop Transiens Blanking Inensiy Brigh Fig. 3 (b)time relaion in CHOP MODE 4. (a) Explain why ime delay is necessary in oscilloscopes. (04 Marks) In a regular CRO, one wihou a ime-delay circui incorporaed ino is sweep circui, he sweep volage will be in synchronism wih he applied inpu volage; hen only we can observe a seady waveform on he screen. The number of he full cycles of he waveform observed on he screen can be seen o be an inegral muliple of he ime-base frequency. However, we can no observe various porions of he applied waveform wih his simple sweep. To observe each and every par of a given waveform in minue deails, we have o use he delayed ime-base. Tha is, o analyze a given waveform, we mus apply he ime base a he appropriae poin on he waveform o be observed. A delayed-ime-base oscilloscope is one in which a variable ime delay is used in he horizonal sweep of a convenional CRO. This allows he sweep o be applied a he end of he delay ime. Figure 4. shows he block diagram of a ypical delayed-ime-base circui. In his case, here are wo ime-base circuis. Time base and a reference volage R are applied o a Schmi-rigger circui. Depending on he value of R, he Schmi rigger oupus a square wave ha drives ime-base circui, as shown. The oupu of ime- December 0.indd 9 8//03 :55:57

7 Dec - December 0 Elecronic Insrumenaion base circui is used o drive he X-X de ecion plaes of he CRO. Figure 4. shows he acion of a ime-delay sweep. Figure 4.(a) shows a sandard sweep and Fig. 4.(b) shows he resuling waveform displayed on he CRT screen. Trigger Time base Schmi Time base rigger CRT R v Fig. 4. Delayed ime-base circui (a) Sandard sweep (b) Resuling wave Time delay (c) Delayed sweep (d) resuling wave Fig. 4. Waveforms associaed wih sandard and delayed sweeps Figure 4.(c) shows a delayed sweep and Fig. 4.(d) shows he resuling delayed waveform. I can be seen ha he delayed sweep sars a he peak of he applied waveform and hence we observe he saring of he display from he peak poin of he applied wave, as shown in Fig. 4.(d). (b) Explain he principle and operaion of sampling oscilloscope wih relevan block diagrams. (08 Marks) Ans: In an ordinary CRO, when we ry o measure high frequency, he image brilliance reduces. I is because, December 0.indd 0 8//03 :55:57

8 Elecronic Insrumenaion December 0 Dec - when he frequency of he verical deflecion signal increases, he wriing speed of elecron beam increases. In order o obain sufficien image brilliance and o mainain normal image brighness he elecron beam mus be acceleraed o a higher velociy so ha more kineic energy is available for ransfer of elecron o he screen. The higher elecron beem velociy can be easily achieved by raising he acceleraing anode volage. A high velociy beam also requires a greaer deflecion poenial o mainain he deflecion sensiiviy. All hese leads o higher demands on he verical amplifier. To make he maer simple, a sampling oscilloscope uses a differen approach alogeher o improve he high frequency performance. In he sampling scope, he inpu waveform is reconsruced from many samples aken during recurren cycles of he inpu wave form and hus over come he frequency limiaions of convenional CRT. This sampling echnique is illusraed in Fig 4(a). From he figure, i can be seen ha he high frequency inpu being sampled by a rain of sampling pulses. The principle involves in aking one sample each from each recurren full cycle of he applied wave a a slighly laer delayed posiion. As illusraed in he figure, sample is aken form he firs cycle, sample from cycle, sample 3 from cycle 3 and so on bu from a slighly advanced posiion or from differen poins on he applied wave, when regrouped hese poins will form a single wave, which is he replica of he applied wave form as shown in he figure 4(a). Reconsruced oupu wave Higher frequency inpu wave Triggering pulse Sampling poins Sampling pulses Sampling pulses Fig: 4(a) Sampling of high frequency signal ino a low frequency signal A simplified block diagram of he sampling circuiry used in he sampling scope is shown in Fig. 4(b). The inpu wave form o be observed is applied o he sampling gae. Sampling pulses momenarily bias he diodes of he balanced sampling gae in he forward direcions briefly connecing he gae inpu capaciance o he es poin. These capaciances are slighly charged owards he volage level of he inpu circui. December 0.indd 8//03 :55:58

9 Dec - December 0 Elecronic Insrumenaion Inpu signal Sampling gae erical Amplifier To verical deflecion plae Trigger Inpu Blocking oscillaor Ramp Generaor olage Comparaor Saircase Generaor To horizonal deflecion plae Aenuaor Fig. 4(b) black diagram of he sampling circuiry in a sampling scope The capacior volage is amplified by he verical amplifier and applied o he verical deflecion plaes. Since he sampling mus be synchronized wih he inpu signal frequency, he signal is delayed in verical amplifier, allowing he sweep riggering o be done by he inpu signal. When he rigger pulse is received, he avalanche blocking oscillaor sars an exacly linear ramp volage, which is applied o volage comparaor. The volage comparaor compares he ramp volage o he oupu volage of a saircase generaor. When he wo volages are equal in ampliude, he saircase generaor is allowed o advance one sep and simulaneously a sampling pulse is applied o he sampling gae. A his movemen, a sample of he inpu volage is aken, amplified and applied he verical deflecion plae. (c) Explain he operaion of digial sorage oscilloscope wih he help of a block diagram. Menion he advanages. (08 Marks) Digial sorage oscilloscope: Inpu signal Inpu erical amplifier S/H circui A/D converer Memory D/A converer Trigger circui Conrol logic D/A converer erical deflecion amplifier Horizonal deflecion amplifier CRT deflecion plans In a digial sorage Oscilloscope, he waveform o be displayed and sored is convered ino binary digis (s December 0.indd 8//03 :55:58

10 Elecronic Insrumenaion December 0 Dec - and 0s), sored in a random access memory, and rerieved for display on screen. The sored wave form may be coninuously displayed by repeaedly scanning he sored waveform and, herefore, a convenional oscilloscope ube can be used for he display. The sored daa can be displayed inde niely as long as power is applied o he memory. The digiized waveform can be analyzed by eiher he oscilloscope iself or by using a digial compuer conneced o i. Figure 4.3 shows he block diagram of a digial sorage oscilloscope. The inpu is ampli ed and aenuaed wih inpu ampli ers as in any oscilloscope. The digial sorage oscilloscope uses he same ypes of inpu circuiry as a convenional oscilloscope and can operae in a convenional mode, bypassing he digiizing and soring feaures. As shown in he gure, he inpu signal ampli ed by he verical ampli er-aenuaor combinaion is applied o an analog-o-digial converer, which hen drives a random-access memory (RAM). This emporarily sores he digiized inpu daa. A conrol logic circui is used o conrol he operaions of he ADC and he memory. The oupu of he memory is applied o a digial-o-analog (DA) converer, which in urn is used o drive he verical de ecion ampli er and verical de ecion plaes. The conrol logic also drives he horizonal-sweep DAC and he horizonal de ecion ampli er. The combined acion of he de ecion plaes, as in he convenional oscilloscope produces display on he screen. Advanages of digial sorage oscilloscope. They can be used o observe fas and slow phenomena alike. Can be used o analyze minue deails in any ype of waveform.. Daa can be sored permanenly for viewing a convenience. 3. Real-ime analysis is also possible. 4. Can be direcly conneced o digial compuers for analysis of waveforms. 5. Waveforms appearing only once (like a ransien) can be observed and analyzed easily. 6. Wih large memory capaciy, a lo of daa can be sored in a DSO. 7. Several channels are possible in modern DSO s which help in analyzing waveform a differen pars of a sysem simulaneously. 8. Can be used o pre-rigger view. 9. Compaible wih modern a-panel displays using TFT s 0. Analyses can be displayed on he screen iself by compuaional abiliy of he scope.. Brigher and bigger display.. Differen colors employed o disinguish muliple races. 3. Much higher resoluions possible. 4. Deecion of peak signal possible. 5. Waveform zooming in and zooming ou is possible for ner examinaions. 6. Allows auomaion. 7. Three dimensional imaging easy. PART - B 5. (a) Wih block diagram, explain convenional sandard signal generaor. Menion he applicaions. (0Marks) December 0.indd 3 8//03 :55:58

11 Dec - December 0 Elecronic Insrumenaion The above gure shown is a +ve feed back sysem. I basically consiss of ampli er wih gain (A) and feedback nework whose feedback fracion is. When he produc of he loop gain A = and oal phase shif around he loop is 360 susained oscillaions are generaed called barkhausen crieria. Figure 5a shows he block diagram of a sandard signal generaor, which some imes is called he sandard AM/FM signal generaor. I consiss of an RF generaor, an AF generaor an ampliude modulaor, a frequency modulaor, an ampli er and an aenuaor. The radio-frequency (RF) oscillaor is used o produce sine waves in he range of I MHz o 00 MHz. Usually, an RF oscillaor of he Harley or Colpi s ype is used o produce he desired radio-frequency sine waves. The oupu frequency of he oscillaor can be varied by varying he ank-circui inducance using ferrie uning. The audio-frequency (AF) oscillaor is of he Wien-bridge ype and is used o produce sine waves in he range of 0 Hz o 00 khz. The ampliude- and frequency- modulaor circuis are, respecively, used o ampliude and frequency modulae he RF waves wih AF waves. These modulaions are aken ou hrough a band swich S o an ampli er-aenuaor combinaion circui. The ampli er-aenuaor combinaion circui is used o conrol he ampliude of he oupu signal. Some imes, a es-signal erminal of -vol, 000-Hz sine wave is also incorporaed on he equipmen s panel board. Sandard AM/FM signal generaor is used o es and measure he performances of radio circuis. They are used as laboraory sandards for esing and repairing RF ransmission sysems. They also help in uning radio receivers for opimum performance in facories and repair shops. In miliary and police signaling schemes, sandard AM/FM signal generaor is one of he highly essenial es equipmens. AM Modulaor AF oscillaor RF oscillaor AM S FM Amplifier Aenuaor Oupu FM Modulaor Fig. 5. a Sandard (AM/FM) signal generaor (b) Explain he operaion of a funcion generaor wih he help of a block diagram. (0 Marks) Consan-curren source Schmi rigger Square Frequency conrol circui I I Inegraor Triangle Consan-curren source Wave-shaper Sine Fig. 5. (a) Block diagram of a ypical funcion generaor December 0.indd 4 8//03 :55:58

12 Elecronic Insrumenaion December 0 Dec - A funcion generaor is an insrumen more versaile han he sine- and square-wave generaor. A funcion generaor, in general, is used o generae sine waves, square waves, riangular waves, and sweep waves. These wave forms are useful for a variey of applicaions in a lab. Figure 5. (a) shows he block schemaics of a funcion generaor. In his circui, wo consan-curren sources driving an inegraor. To produce he required oscillaions he op consan-curren source is used o charge he inegraor capacior a a uniform rae, and his produces a posiive-going ramp. The capacior, afer reaching a finie value, will discharge hrough he second consan-curren source, and a negaive-going ramp resuls in. b + c + d Original Shaped sine wave wave Fig. 5. (b,c,d) Triangular, square, and shaped sine waves The wo acions (i.e., charging and discharging of he capacior) will produce a riangular wave. The riangular wave is applied o schmi rigger circui o produce square wave, as shown in fig. 5. b. The riangular wave is applied Schmi rigger circui o produce square wave, as shown in fig. 5. c. To ge he sinewae oupu, we use a wave-shaping circui using diodes or bipolar juncion ransisors. The wave-shaping circui will curve he edges of he riangular wave o form ino a sine wave, as shown in Fig. 5. d. Similarly, when riangular wave is fed o shaping circui, i produces sinusoidal funcions. The frequency is conrolled by varying he magniude of curren hrough he inegraor. When funcion generaor is urned ON he upper source supplies consan curren o he inegraor whose oupu is given by ou = i.d C 6. (a) Explain he Wheasone bridge and derive he balance equaion for Wheasone bridge. Menion he limiaions. (08 Marks) The circui diagram of WheasoneÊs bridge is shown in Fig 6(a). I consiss of four arms made up of resisive elemens R, R, R 3 and R 4 respecively as shown in figure. A dc volage source, conneced across he nodes A and C of he bridge, energizes he circui. A galvanomeer G is conneced across B and D of he circui indicaes he balancing condiion of he bridge. The arm consising of resisors R and R are called he raio arms of he bridge while he arm consising of he R 3 is called he sandard arm since i uses a sandard resisance for comparison and measuremen. The resisor R 4 conneced o he fourh arm of he bridge is usually having he unknown value. i is his resisance o be measured using he bridge. I can be seen ha, if he hree resisors have known values, hen he value of he fourh resisor can be accuraely found. December 0.indd 5 8//03 :55:58

13 Dec - December 0 Elecronic Insrumenaion A R R B I I 3 G D R 3 I G C R 4 Fig: 6(a) WheasoneÊs bride To analyse he balancing condiion of he bridge refer fig.6(a). Le curren I flow hrough resisors R and R and le curren I flow hrough resisors R 3 and R 4, as shown in he figure. These condiions assume ha here will be no curren hrough he galvanomeer (i.e., I G = 0). The condiion ha I G = 0 is known as he balancing condiion of he bridge. From he figure, we find ha curren I = R + R and curren I = R + R 3 4 The volage drop across R can now be obained as R BC = IR = R + R and ha across R 4 can be obained as DC 4 R 4 = I R = R + R 3 4 The ne volage across he nodes B and D is given by R R 4 BD = BC DC = R + R R + R 3 4 The bridge is said be balanced when volage BD = 0, so ha he curren flowing hrough he galvanomeer is zero. Tha is R R 4 BD = BC DC = = 0 R + R R + R Simplifying he above equaion, yields 3 4 R R4 = R + R R + R 3 4 Rearranging, simplifying, and canceling similar erms on eiher side of he above equaion yields he necessary condiion for balancing he bridge as Which may also be wrien as R R 3 = R R 4 R R R = R 3 4 December 0.indd 6 8//03 :55:58

14 Elecronic Insrumenaion December 0 Dec - This is he required balance equaion for he WheasoneÊs bridge The limiaions of Wheasone s bridge are :. The resisance of he leads and conacs becomes signi can in he low resisance measuremen ha inroduces error.. In he measuremen of high resisive values, he resisance presened by he Wheasoone s bridge becomes so large ha he galvanomeer will be insensiive due o imbalance. 3. The heaing effec of he curren ha rise he emperaure which in urn causes a change in he value of resisance in bridge arms. Excessive curren cause a permanen change in he value of resisance ha affecs he measuremen. (b) Find he equivalen parallel resisance and capaciance ha causes a wein bridge o null wih he following componen values : (06 Marks) R = 3. k, C = 5. F, R = 5k, f =.5 khz and R 4 = 00 k. Wein bridge R = 3 k, C = 5. f, R = 5k f =.5 khz, R4 = 00k C = ω RCR 3 3 R R C = + R R C R R = + R R C R R 4 3 ω 3 5kΩ 3.k = + π ( ) ( ) ( ) 00k R R 3 5k Ω 3.k = C = 00k R 3 ω RCR R k 00k 5k C 3 = ( ) ( ) R 3 =.4kΩ C3 = π = 0.8pF (c) Wrie a noe on Wagner s earh connecion. (06 Marks) December 0.indd 7 8//03 :55:58

15 B R R R 3 A S C s Headphone deecor C s C R R 4 Sray capaciances C C C 4 D Fig. 6. a Wanger earh connecion Figure 6 (a) shows he Wagner earh connecion. This circui is employed o eliminae he effecs of sray capaciances exising beween he deecor erminals and he ground C s and C S in Fig. of a bridge circui. An AC supply volage is applied o excie he nework. The bridge is made up of Resisors R, R, R 3 and R 4 as well as capaciors C, and C 4. An addiional R - C nework is conneced across, as shown. The deecor is conneced o he bridge hrough a band swich when i is in posiion. This swich, in is posiion, is conneced o he ground and his connecion is known as he Wagner ground connecion. Working Principles: Iniially, he headphone deecor is conneced o posiion of S and resisor R is adjused o for a null or minimum sound in i. Then S is moved o posiion (Wagner ground) and resisor R is adjused o for a null or minimum sound in he headphone. Now, we move S o posiion again. This acion may possibly inroduce some unbalance in he circui. R and R 3 are now adjused for a minimum sound from he deecor. S is hen moved o posiion and R is adjused furher for minimum sound. This procedure is repeaed several imes unil he headphone produces minimum sound in boh he posiions of S. This means ha swich posiions and are a he same ground poenial, which indicaes ha sray capaciances C S and C S are effecively shored ou, and hence heir effec on he balancing of he bridge is eliminaed. In a similar way, ha sray capaciances from poins B and D are can also be seen o be eliminaed hrough his mehod. 7. (a) Wha are he facors o be considered for he selecion of beer ransducer? Explain (08 Marks) The following poins are o be observed for selecing a ransducer suiable for a given applicaion:. Sensiiviy: The ransducer seleced mus produce a suf cienly large oupu even for a very small inpu.. Range: I mus cover faihfully and accuraely he enire range of operaion of he quaniy o be convered by i. 3. Errors: Transducers mus be free from errors as far as possible. This means ha he conversion mus be as faihful as possible and should be proeced from error-generaing surroundings. 4. Thermal sabiliy: The ransducer seleced for a given applicaion mus have good hermal sabiliy. 5. Ruggedness: I mus wihsand shock and vibraion. 6. Frequency response: Should be capable of producing uniform oupu hroughou he range of frequencies in which i has o operae. 7. Elecrical characerisics: The ransducer (used in elecrical conversion) mus be seleced based on is elecrical characerisics such as he value of is oupu impedance, response ime, rise ime, elecrical power-supply December 0.indd 8 8//03 :55:58

16 requiremens, ampli caion/aenuaion produced, ransfer characerisics, phase-shif produced, and so on. 8. Physical size: In he modern world of miniaurizaion, a ransducer seleced mus be as small as possible; a he same ime, i should no affec he value of he oupu produced by i. 9. Simple consrucion: The ransducer consrucional feaures mus be as simple as possible. 0. Weigh: I mus be noed ha he weigh of a good ransducer mus be as small as possible for easy handling.. Simple heory of operaion: The principle of operaion mus also be as simple as possible.. Cos: Any designer will always be cos-conscious. Thus any ransducer seleced for a given applicaion should be as inexpensive as possible. 3. Easy availabiliy: Any ransducer chosen mus be easily and speedily available from he open marke. (b) Explain he consrucion, principle and operaion of LDT. ( Marks) LDT: Figure shows he consrucion of he linear variable differenial ransformer (LDT). The differenial ransformer consiss of single primary winding and wo secondary windings wound on a hallow cylindrical former. The wo secondary windings have equal number of urns and are placed on boh sides of he primary winding. The primary winding is excied by an ac source. A movable sof iron core slides in and ou he hallow former effecing he magneic coupling beween primary and wo secondary windings. When he core is a he normal posiion (exacly a he middle of he former) he secondary volages induced are equal and hence he oupu volage is he difference of hese wo volages, o = E E = 0. 0 = 0. When he core is moved o he boom more ux links S han S he oupu volage is E, he oupu volage 0 = E E. when he moved o boom mos, he oupu volage is very negligible (almos zero). E = 0 so0 E Similarly, when he core is moved in he opposie posiion o = E =E. A he exreme end, E = 0 0 = E Thus, we find ha, as he posiion of he core changes wihin he former, he volages induced in he individual December 0.indd 9 8//03 :55:58

17 secondary coils differ; his produces an oupu volage ha is linearly proporional o he posiion of he core; hence he name linear variable differenial ransformer. The ransfer curve of he LDT is shown in Fig. 7 (c). The ransfer characerisic shows a fairly linear operaion of he LDT. 0 = 0 0 Posiion of he core ( X) X max +X max Posiion of he core (+X) 0 = 0 0 Fig. 7. c Transfer characerisic of LDT 8. (a) Explain piezo elecric ransducer, wih circui diagram. (8 Marks) Ans: Piezo means press. Then piezo elecriciy means press and generae elecriciy. I is found ha when cerain crysaline maerials such as quarz, Rochelle sal and Barium ianae placed under sress, hey generaed elecriciy. This propery is used in he consrucion of piezoelecric ransducer, where a crysal is placed beween a solid base and he force summing member as shown in Fig 8 (a). Force - summing C R oupu L C p Base Crysal (Barium Tianae Rochelle Sal Quarz) Equivalen Circui of Crysal Fig. 8.a Peizo Elecrical Transducer An exernally applied force enering he ransducer hrough is pressure par applies pressure o he op of he crysal, which produces an emf across he crysal proporional o he magniude of he exernal pressure. Equivalen elecrical circui of he crysal is also shown in Fig. 8 (a). The basic expression for he generaed emf is given by E = Q C P where E = generaed elecriciy Q = generaed charge C P = Shun capaciance For a pizeo elecric elemen under pressure, par of energy is convered o an elecrical poenial ha appears on December 0.indd 30 8//03 :55:59

18 opposie faces of he elemen, analogous o a charge on he plaes of a capacior. The res of he applied energy is convered o mechanical energy, analogues o compressions of a spring. The piezo elecric elemen reurns o is original shape and losses is charge, when he exernal pressure is removed. From hese relaionship, i is derived ha Mechanical energy convered ino elecric energy K = Applied mechanical energy or Elecrical energy convered ino Mechanical energy K = Applied elecric energy The piezo elecric effec is reversible. Tha means, hese crysals exhibi he propery of elecrosricion when an elecric field is applied across he crysal, mechanical sress is produced. An alernaing volage is applied o he crysal causes i o vibrae a is naural frequency. This propery is used in high frequency acceleromeers. (b) Compare LED and LCD ypes of displays. (06 Marks) Comparison LED LCD. Small and compac. Medium and compac. Small size. Small size 3. Brigh displayed 3. Good conras 4. Low power 4. Low- power 5. high speed 5. Medium speed (c) Wrie a shor noe on signal condiioning sysem. (6 Marks) Signal - condiioner: Signal condiioning equipmen is required o perform basic linear processes such as aenuaion, ampli caion and mahemaical operaions like inegraion and differeniaion. They can also be used o perform non-linear process such as lering, muliplicaion, sampling and modulaion/demodulaion. These funcions require proper selecion of componens and faihful reproducion of he required nal oupu. The signal condiioning / daa acquisiion sysem is an exciaion sysem/ampli caion sysem for passive ransducers, or i can be an ampli caion sysem for acive ransducer. Ampli er Fig. DC signal condiioning sysem For a passive ransducer exciaion is essenially required. The passive ransducer like srain gauge hermomeers, capaciive ransducers and poeniomeers o be excied from an exernal dc source. The acive ransducers like hermocouples, phoo diode piezoelecric ransducers does no require any exernal dc source for exciaion. Bu hese ransducers are conneced o ampli ers o amplify he low level inpu signal. A simple dc signal condiioning sysem is as shown in gure. The dc bridge is excied by a dc source and sain gauge is conneced o one arm of he wheasone s - bridge. The bridge can be basically balanced using a poeniomeer and oupu of calibraion is applied o dc ampli er he essenial requiremens of dc-ampli er are as follows. December 0.indd 3 8//03 :55:59