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Posted By: jothi vignesh Member Level: Gold Posted Date: 07 Jul 2008
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2005 Anna University B.E COMMUNICATION ENGINEERING Question paper
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Code: A - 15 Subject: COMMUNICATION ENGINEERING
Time: 3 Hours Max. Marks: 100
NOTE: There are 11 Questions in all.
Question 1 is compulsory and carries 16 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.
Answer any THREE Questions each from Part I and Part II. Each of these questions carries 14 marks.
Any required data not explicitly given, may be suitably assumed and stated
Q.1 Choose the correct or best alternative in the following: (2x8)
a. FM discriminator changes FM signal into
(A) AM signal. (B) FM signal of higher intensity.
(C) PM signal. (D) Single side band AM signal.
b. The pre-emphasis circuit in Frequency Modulation system is used
(A) to increase the amplitude of low frequency component of the signal.
(B) to increase the amplitude of high frequency component of the signal.
(C) after the modulation.
(D) prior to modulation.
c. Increasing the peak power by 12 dB in a radar system will increase its range by a factor of about
(A) 2. (B) 4.
(C) 12. (D) 16.
d. Demodulation of SSB-SC requires
(A) an envelop detector. (B) a synchronous detector.
(C) a discriminator. (D) an integrator.
e. The effect of noise in a communication system is most adverse with reference to
(A) channel. (B) encoder.
(C) source / transmitter. (D) receiver.
f. Occurrence of turning up of tail when a coin is tossed has the information equal to
(A) 0 bit. (B) 4 bit.
(C) 8 bit. (D) 1 bit.
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Index Next>>g. If x and y are two independent Gaussian random variables with average zero and with variance , the joint density function is defined as
(A) (B)
(C) (D)
h. Assuming R = G = B = 1 V, fully conducted red is
(A) 0.3 (B) 0.59
(C) 0.11 (D) 1.0
PART I
Answer any THREE Questions. Each question carries 14 marks.
Q.2 a. With the help of neat diagrams and waveforms, explain generation of DSBFC-AM wave by using a class - C amplifier. What is the maximum collector voltage that the BJT (the active device) should be capable of standing in such modulators? (10)
b. An unmodulated carrier wave has mean power of Po. When it is amplitude modulated by a sinusoid, the power becomes P. Show that the modulation index (4)
Q.3 a. Show that the signal (7)
represents an SSB-SC
(i) Find the expression for the missing side band.
(ii) Write expression for the DSB-SC.
b. If v(t) above is multiplied by , and then sent through a low-pass filter to recover the modulation (7)
(i) Show that the modulation is completely recovered if the cutoff
frequency of the low -pass filter is .
(ii) If the multiplying signal were , find the recovered
signal?
Q.4 a. How do we generate an FM signal? Define modulation index for single tone modulated FM signal. Under what conditions FM signal becomes narrow band FM. (7)
b. An FM broadcast station uses maximum frequency deviation of 75 KHz. The maximum signal frequency is limited to 15 KHz. Estimate the bandwidth of the FM signal. What would be the estimated bandwidth if the maximum signal frequency is limited to 4 KHz. (7)
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<>Q.5 a. What is Pulse Code Modulation (PCM)? How do we obtain a PCM signal? Explain briefly the generation of PCM by using block diagrams. (7)
b. A telephone signal has maximum frequency of 4 KHz. It is limited in voltage between -1V and +1V, and has unit variance. It is transmitted by using PCM. The required signal to quantisation noise ratio is 40 dB. What is the minimum bandwidth required for transmission? (7)
Q.6 a. With the help of neat diagrams, explain the principle of Delta Modulation (DM) system. What are its main disadvantages? How does Adaptive Delta Modulation (ADM) overcomes the problems of DM system? (7)
b. What is meant by sampling? Explain the reasons to prefer flat-top sampled signals as compared to the natural - sampled ones. (7)
PART I
Answer any THREE Questions. Each question carries 14 marks.
Q.7 a. Explain the operation of a Pulse Radar with the help of block diagrams. What are the salient parameters of a Radar and how do each of these affect the Radar performance, explain in brief. (7)
b. Derive Radar Range equation. What parameters affect the maximum radar range? (7)
Q.8 a. The auto correlation of a real-valued wide sense stationary process is defined by
(6)
Show that
(i.e. is an even function)
b. Explain the terms : Entropy, mutual information capacity, channel capacity. What are their significances in communication? (8)
Q.9 a. What is the channel capacity of a white, band limited gaussian channel. Derive expression giving channel capacity of a noisy channel when bandwidth tends to be very large. (7)
b. How does coding help for error detection and error correction in the received signal? Explain the use of Block codes and explain how coding/ decoding is done? (7)
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<>Q.10 a. With a neat block diagram explain the functioning of a colour Television system. (7)
b. Name a few TV cameras and describe, in detail the operation & limitations
of one such camera. (7)
Q.11 Write short notes on any TWO of the following:
(i) A sonar system.
(ii) Radio direction finder.
(iii) Sampling theorem. Give its proof.
(iv) Generation of single side band and its detection. (7+7)
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