electrical engineering_1.pdf

7/27/2019 ELECTRICAL ENGINEERING_1.PDF

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Serial No. [ (}4) EXAM20 12 FDTNMFM FA

ELECTRICAL ENGINEERINGPaperI

(Time Allowed Three Hours (Maximum Marks 300)

INSTRUCTIONSEach question is printed both in Hindi and in English.Answers must be written in the medium specified in the Admission Certificate issued toyou, which must be stated clearly on the cover of the answer-book in the space providedfor the purpose. No marks will be given for the answers written in a medium other thanthat specified in the Admission Certificate.Candidates should attempt Question Nos. I and 5 which are compulsory, and anythree of the remaining questions selecting at least one question from each Section.

The number of marks carried by each question is indicated at the end of the question.Assume suitable data if considered necessary and indicate the same clearly.Symbols/Notations carry their usual meanings, unless otherwise indicated.2 Graph sheets are attached to this question-paper for appropriate use. These are to becarefully detached and then securely attached to the answer-book.Important : Whenever a Question is being attempted, all its parts/sub-parts mustbe attempted contiguously. This means that before moving on to the next Questionto be attempted, candidates must finish attempting all parts/sub-parts of the previousQuestion attempted. This is to be strictly followed.Pages left blank in the answer-book are to be clearly struck out in ink. Any answersthat follow pages left blank may not be given credit.

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SECTIONA1. (a) Determine the transmission matrix for the circuit shown and explain why open circuit

impedance parameters can not be defined for this circuit.2i+z '4V2I

[(b) Using duality property show that the Fourier transform of 1+j2nt is equal toet u(-0 where u(t) is the unit step.2(c) A continuous time signal is given byx(t) = A cos (8000 nt) cos (2000 nt)The signal is sampled with a sample period of 3x10 seconds. Can we recover thesignal from the sampled version using an appropriate low pass filter ?2

(d) A lossless transmission line having a length of 100 metre is terminated by a resistiveload of 200 ohms. It is fed by a generator of voltage ZO volts and has impedance100 ohms. If characteristic impedance of the line is 300 ohms, calculate the transmissionloss and return loss in decibels.2(e) Assuming ideal op-amps, determine the expression for the input impedance of thegiven circuit and a simple passive equivalent of the circuit looking into terminal .12

2 (Contd.)0


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30 (Contd.)

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2. (a) Show that there can be no value of RLin the circuit given in Fig. that will make itresonant.0

Fig.(b) A system consists of two blocks connected in cascade. The first block has impulse

response h i (t) = 2u(t). The second block is shown below :L

Ob 6 b6)Input OutputOFind the impulse response of the overall system using the relationh(t) = h i (t) * h2(t).0(c) Determine the output current of the Widlar current source; given that V = 15 volts,= 10-1 4 amp., p = 200, VT = 25 mV.0+v

-v 4O (Contd.)


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2. (30 z t - q - T r k -4fib fix 4 RI mi40(10It Trr4 rtrf tI(4 ml 31144 174 91 " 11 1 I04 0(4 )444 *741 vti 4 4tea t i TO pia, - m e ' r 3 1.1414 3fIfitif gh 1 (t) = 2u(t) VITT colt na f4c4r4r Trm tL

00R0c i m" t 444 3 r r 3T r 4% i%-zr r , '14tT h( t ) = h i (t) * h2 (t) Th-t vzhYt Th-tt- rwr m91 t 1 -20(TT) %Mit ERT TkaTht NIT UM Aid 1147; RLif g

+ V = 15= 1014 am p., 13 = 200, VI= 25 m V.C o 20V-v 50 (Contd.)


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3. (a) (i) Find Y and Z parameters for the following circuit :

(ii) State Tellegen's theorem for network analysis.0(b) A particular lossless material has 12 = 4 and e r = 9. A 10 MHz uniform plane

wave is propagating in the ay direction with E x o0= 400 V/m and E = Eg o = 0 atP(0.6, 0.6, 0.6) at t = 60 ns.Determine :-(i) 13, A., v p and ri(ii) E(t)(iii) 1-1(0.0(c) Figure shows a sinusoidal oscillator. By an appropriate analysis, determine the conditionof oscillation and frequency of oscillation in terms of circuit elements assuming idealop-amps.0

6 Contd.)


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3. OD (i)4 7 -r igrq15211 Z IA Nei ' l It i T htikg :(ii) 4i4 idziur * tt-4-4 v44 m) 347 mtrAg0s1 Wilsr 6P -k i ta. W114 * fkg M r = 4 ci t i f E r = 9 g 17T 10 MHz (Pick! nor ayfI I 4 rituur -r-di t u151 E = 400 V/m UM E = E = 0(0.6, 0.6, 0.6)xooL I T= 60 ns.41d ttr47 :-(i) p, k, vp au!(ii) E(t)(iii) H(t)0v+ 41 cl tT 'eM1h'41fifi t I 3T TPT op-amps Me t to'llh

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7 Contd . )0


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4. (a) The block diagram of a discrete time LTI system is given below :

Determine the transfer function H(z).0(b) A 50 ohm lossless line connects a matched signal of 100 kHz to a load of 100 ohm.Load power is 100 mW. Estimate :(i) Voltage reflection coefficient of load.(ii) VSWR of the load.(iii) Position of first V n - u n and Vm ax(iv) Impedance at Vm m and Vm a x and values of Vm a x and Vm m .Define the terms Attenuation loss, Reflection loss, Transmission loss, Return loss

and Insertion loss.0(c) Explain the operation of the given circuit by sketching relevant waveforms assumingthat positive and negative saturation levels of output voltage of the op-amp comparatorare + Vsati and V sat2 respectively and are not equal. Determine an expression for thefrequency of the generated output voltage.0K8 Contd.)


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M 4 1 - d ((z )heftq I0(4) Wr. 50 Q M1 nii)(i44 viI00 kHzitd00 Qi4q1- -atgols 4r-47 100 mw RI 31 W-f f metD7 :(i) Mls mr c)-7 - d-4r irrra-44(II) M 16111 crEf7g753TIT(iii) 944 vm i n 31TT Vm ax Mt(iv) V.in44 vnia, mt edKtur4 our v.,11-11inr a l bra OW, ITCF44 wf4, 414(ur mf4, 4i1rif1 3Tk dui Myr ff'r41k4 r I t u merr. I0(4) RQ , 117 I T R I T Z T kr4f4U CR.1 1d TERsZ T herr 1116vrt op-amps g-ri7 mt M-44afarT - 1 1 c 4 -1.6 ffzT iIcHt+ vv S a t tlog< 941 tkif4e t sursat,c4vIch 41R1 4ti h4gI0R

9 (Contd.)0


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SECTIONB5. (a) Show how a D flip-flop can be converted to SR flip-flop.2(b) Convert decimal 65,535 to its hexadecimal and binary.2(c) A 2-pole full pitch concentrated coil of n turns is placed in the stator of an electric

machine in two slots diagonally opposite to each other. A permanent magnet 2-polerotor is rotating at N rpm producing a flux per pole 4) with its flux density sinusoidallyvarying in space in the airgap. At time t = 0 rotor field axis is in line with the statorcoil axis. Write an expression for the flux linking the stator coil as a function of time.Derive from this expression the r.m.s. value of the voltage induced in the stator coil.Draw the waveform of flux linkage and voltage generated. Write expression for frequencyof voltage in terms of speed.2(d) Draw the simplified circuit diagram for a voltage step-up DC-converter and show thatwhen duty cycle is zero, the output of DC-converter is equal to source voltage whereasDC voltage is infinity if the duty cycle is unity.2(e) Evaluate whether the following function given by expression0or x4is a probability density function (PDF).2

6. (a) Indicate how many flip-flops are required to construct each of the following counters :(i) mod-3(ii) mod 6(iii) mod-9.0(b) Two winding 10 kVA, 500/50 V transformer is converted to an auto-transformer toobtain the rating of :(i) 550/50 V,(ii) 500/550 V, and(iii) 500/450 V.Compute the kVA rating of auto transformer for the above connections, VA transferredconductivity and magnetically.0(c) (i) Consider a superhet receiver with fLe = f + ft1, fF = 455 kHz, f = 2 MHz.The RF amplifier is preceded by a first order RLC band pass filter withf = 2 MHz and B = 0.5 MHz. Assume the IFBPF is nearly ideal and mixer has0unity gain. What is the minimum spurious frequency input rejection ratio in dB ?fx(x) =

100 (Contd.)


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f t@TO-Rffq-ter Th) ;4 SR ftfficf-VN 4 w4dr .qr ktebdr t ? 121J-1(14 65,535EI* bilsviit'l 4 lft449 mar l2lm 1 =47 Hill2z-c 4, -ah m-of to41;cp g,atd-uff ticMt-d- T -;r4t itzrar Izwrz i t-ga -az7N rpm TR Tri9 t


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(ii) If the above receiver has a LO with a second harmonic whose voltage level is halfthat of the fundamental component, determine what input frequencies will beaccepted and at what power level in dB as compared to the correct input. 20

7. (a) What would be the binary address for a memory system having a capacity of 102 4 bits.For the above memory system, what is the decimal address for the binary address1011001101 ? What is the address in hexadecimal ?0(b) (i) Explain with diagram the soft starting of induction motor to start from zerospeed to rated speed. Briefly compare with a star-delta starter used for the samepurpose.(ii) A 3-phase, 4-pole 50 Hz induction motor is controlled by a variable frequencyinverter that maintains the ratio of voltage to frequency across the motor constant.

Torque-speed curve in stable region is a straight line. It has a maximum torque of2 p.u. and slip speed at maximum torque at all frequency is constant at 150 rpm.Draw a family of Torque speed characteristics (on a graph paper) at 10, 20, 30, 40& 50 Hz with usual assumptions. It is driving a load whose torque is directlyproportional to speed giving rated torque at 50 Hz. Graphically calculate speedsat all the above frequencies.0(c) (i) In an FM system, when the audio frequency (AF) is 500 Hz and AF voltage is2.4 V, the deviation is 4.8 kHz. If the AF voltage is now increased to 7.2 V, whatis the new deviation ? If the AF voltage is raised to 10 V while AF is dropped to200 Hz, what is the deviation ? Evaluate modulation index in each case.

(ii) If x(t) = cos 200 in, determine AM transmission bandwidth B andaverage transmitted power ST for the AM modulated signal assuming Ae = 10 and

(modulation index) = 0.6.What will be the values of BT and S F in the case of DSB transmission ?08. (a) A d.c. shunt motor is driving a constant torque load. If the voltage across the motor ishalved explain how the armature current and speed would be changed.0

(b) A 11 kV dc source having internal resistance of 1 ohm supplies 600 A to a 5 kV,3 phase, 6-pulse 50 Hz inverter. Compute the :(i) reactive power absorbed by the inverter,(ii) effective value of the ac line current,(iii) the required firing angle,(iv) dc voltage generated by the inverter, and(v) dc current carried by each SCR.0

12 Contd.)0


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(ii) TiR 3u f r h 31f4iirfh 4 Lo t ##I vrwr ftfflir 611=4- , fErA alp-dcif ka- ,acm 4T 31RIT g , tl fk-- 4 -1 =1 VNT 1 ART A1P4g ThIff t#1 i dsr stirdbf OlAzi?tiff UT T d B 4 1*#-c-R 'T T ?07. (30 f+711 T.Ptff-41 Wrdi 102 4 ittsiA itor crw +II ?rfiert stA tior 101100110177 1 1 - 1 c 1 4 t id y q - 4 tr g ? cits- imtctqr 4-dt glJir ?0flUJ; Itz- 4Ic1 #lt 41M lAafb411"4- -tr k--d r ,Atiicicti 4 vim Af - - 13 ter, 4 1r #(ur Art A 1,1i wit4-4 strrkEtr Trztr4 1-Cc - c r a Tup 3F-T ir -d Th) R zu 4,74 (6cIT g I f-qT4t 4 4 4-dltpf-tiIci = N I L ,,T(=11 g I. w-- p.u. t our struffKitt 4ffrEpti4 -( U,c1441 strlft trt (fir trici 150 rpm Irt Prt -rot g 1 (cticrc

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(c) (i) A commercial FM radio station alternates between music and talk show/call informats. The broadcasted CD music is band limited to 15 kHz based on convention.Assuming D (deviation ratio) = 5 is used for both music and voice, what percentageof the available transmission bandwidth is used during the talk show if we takeW = 5 kHz for voice signals ?(ii) An AM system with envelope detection has (S/N)D = 30 dB under full loadtone modulation condition with W = 8 kHz. If all bandwidths are increasedaccordingly, while other parameters are held fixed, what is the largest usuablevalue of W ?0

1 40 (Contd.)


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Serial No. FDTNMFMFA

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