AMIETE – ET (NEW SCHEME) – Code: AE65

Subject: ANALOG COMMUNICATIONS

Time: 3 Hours Max. Marks: 100

NOTE: There are 9 Questions in all.

· Question 1 is compulsory and carries 20 marks. Answer to Q.1 must be written in the space provided for it in the answer book supplied and nowhere else.

· Out of the remaining EIGHT Questions, answer any FIVE Questions. Each question carries 16 marks.

· Any required data not explicitly given, may be suitably assumed and stated.

Q.1 Choose the correct or the best alternative in the following: (210)

a. One of the following types of noise becomes of great importance at high frequencies. It is the ________.

(A) Shot noise (B) Transit-time noise

(C) Impulse noise (D) Random noise

b. If the carrier and one of the sideband of a 100 percent modulated AM wave is suppressed, the percentage power saving will be __________.

(A) 50 (B) 150

(C) 83.33 (D) 66.66

c. A carrier is simultaneously modulated by two sine waves with modulation indices of 0.5 and 0.4; the total modulation index is __________.

(A) 0.64 (B) 0.5

(C) 0.7 (D) 1

d. An FM signal with a modulation index m_f is passed through a frequency tripler. The wave in the output of the tripler will have a modulation index of ______.

(A) m_f/3 (B) m_f

(C) 9m_f (D) 3m_f

e. A superheterodyne receiver with an IF of 455 kHz is tuned to a signal at 1200 kHz. The image frequency is ___________.

(A) 290 kHz (B) 910 kHz

(C) 1650 kHz (D) 2100 kHz

f. A (75–j25)Ω load is connected to a coaxial transmission line of Z₀= 75 Ω, at 10 GHz. The best method of matching consists in connecting ___________.

(A) a short-circuited stub at the load

(B) a short-circuited stub at some specific distance from the load

(C) an inductance at the load

(D) a capacitance at some specific distance from the load

g. Impedance inversion may be obtained with ____________.

(A) a quarter-wave line (B) a short-circuited stub

(C) an open-circuited stub (D) a half-wave line

h. Quantization error occurs in _____________.

(A) PCM (B) TDM

(C) FDM (D) PAM

i. Microwave link repeaters are 50 typically 50 kms apart because of _________.

(A) atmospheric attenuation (B) earth’s curvature

(C) output tube power limitation (D) excess DC voltage

j. To couple a coaxial line to a parallel-wire line, it is best to use a __________.

(A) slotted line (B) balun

(C) directional coupler (D) quarter-wave transformer

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

Q.2 a. What is modulation? Explain the need for modulation. (5)

b. Explain different types of noises in communication system. (6)

c. The first stage of a two-stage amplifier has a voltage gain of 10, a 600 ohm input resistor, a 1600 ohm equivalent noise resistance and a 27 kilo-ohm output resistors. For the second stage these values are 25, 81kilo-ohm and 1 mega ohm respectively. Calculate the equivalent input-noise resistance of the two stage amplifier. (5)

Q.3 a. With neat block diagram explain the working of high level AM transmitter. (6)

b. With neat block diagram explain the working of ISB transmitter. (6)

c. The antenna current of an AM broadcast transmitter, modulated to a depth of 40% by an audio sine wave is 11A. It increases to 12A as result of simultaneous modulation by another audio sine wave. What is the modulation index due to second wave? (4)

Q.4 a. With neat waveform explain frequency modulation and derive an expression for FM wave. (6)

b. Explain pre-emphasis and de-emphasis operation in FM system. (4)

. c. In an FM system, when the audio frequency is 500 Hz and the audio frequency voltage is 2.4 V, the deviation is 4.8 kHz. If the audio frequency voltage is now increased to 7.2 V, what is the new deviation? If the audio frequency voltage is raised to 10V while audio frequency is dropped to 200 Hz, what is the deviation. Find the modulation index in each case. (6)

Q.5 a. With neat circuit diagram explain the working of Phase discriminator type FM demodulation. (6)

b. What are the factors to be considered while selecting Intermediate frequency in radio receiver? (4)

c. In a broadcast superheterodyne receiver having no RF amplifier, the loaded Q of the antenna coupling circuit is 100. If the intermediate frequency is 455 kHz, calculate (a) image frequency and its rejection ratio at 100 kHz and (b) image frequency and its rejection ratio at 25 MHz. (6)

Q.6 a. With neat circuit diagram explain the working of stereo FM receiver. (6)

b. Define the following terms with refer to transmission lines:

(i) Characteristic impedance.

(ii) Propagation constant.

(iii) Standing wave ratio.

(iv) Reflection coefficient.

(v) Stubs.

(vi) Baluns. (6)

c. A (200+j75) ohm load is to be matched to a 300 ohm line to give SWR=1. Calculate the reactance of the stub and the characteristic impedance of the quarter-wave transformer, both connected directly to the load. (4)

Q.7 a. For a wave propagating in a parallel-plane wave guide derive the expressions for cutoff wavelength, group and phase velocity. (8)

b. It is necessary to propagate a 10 GHz signal in a waveguide whose wall separation is 6 cm. What is the greatest number of half waves of electric intensity which it will be possible to establish between the two walls? Calculate the guide wavelength for this mode of operation. (8)

Q.8 a. With neat diagram explain the working of PCM system. (6)

b. State and explain Shannon-Hartley theorem. (4)

c. A system has a bandwidth of 4 kHz and a signal-to-noise ratio of 28 dB at the input to the receiver, calculate:

(i) Its information carrying capacity.

(ii) The capacity of the channel if its bandwidth is doubled, while the transmitted signal power remains same. (6)

Q.9 a. Explain the working of TDM and FDM techniques. (6)

b. What are the different types of losses in optical fibers? Explain them. (5)

c. Explain the elements of long-distance Telephony. (5)