B4E571 Pages: 2

PART 8
Answer any 2 questions. Question No.4 is compulsory.

. (a) Prove that balanced modulator suppresses the carrier. (7)
(b) An AM radio uses a superhetrodyne receiver. The mixer translates the carrier
frequency ‏عر‎ to a fixed IF of 455 kHz by using a local oscillator of frequency 10. The
broadcast-band frequencies range from 555 kHz to 1605 kHz. (a) Determine the range
of frequency tuning and capacitor tuning that must be provided in the local oscillator
(i) when 1015 higher than ‏عكر‎ (and (ii) when {© is lower than £. Based on the results
obtained in (a), explain why the usual AM radio receiver uses fro higher than ‏.عكر‎ (8)
(a) With a neat diagram explain the Weaver’s method for SSB generation. State the
advantages and disadvantages of this method (10)
(b) What is ISB? Compare SSB and ISB. (5)
. (a) With neat block diagrams, compare single versus double conversion super
heterodyne receivers. (8)
(b) If the signal v(t)= 20 sin(2x x 10% + 10 sin2x x 10°t) represents a FM signal,
determine i )The carrier frequency ii) The modulating frequency iii) The modulation
index, iv) band width required, v) average power if the load resistance is 50Q. Also
write the expression for FM signals if the modulating frequency is doubled. (7)
PART C

Answer any 2 questions. Question No.7 is compulsory.

. Describe the working of a Varactor Diode FM Modulator and balanced slope detector.

(20)
. (a) With a neat block diagram explain FM transmitter using indirect method. (10)

(b) Given FM and PM modulators with the following parameters: Deviation
sensitivity as 1.2 kHz/v & 1.2rad/volt respectively. Carrier: 20 cos(2a x 500x10°t) ;
Modulating signal: 5605 (27 x 101) (i) Determine the modulation indices, bandwidth
and sketch the output spectrum for both modulators. (ii) Half the modulating
frequency and Determine the modulation indices and sketch the output spectrum for
both modulators. Assume random value for the Bessel coefficients (10)

. (a) Why pre-emphasis and de-emphasis are used in FM? Draw the circuit diagrams
and the characteristics of pre-emphasis and de-emphasis circuits. (10)

(b) Describe functional block diagram of a standard telephone set. (10)

KKK

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