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EC2252 Communication Theory ANNA UNIVERSITY PREVIOUS YEAR QUESTION PAPER, IMPORTANT QUESTIONS, 2 MARKS AND 16 MARKS QUESTIONS FOR ECE DEPARTMENT

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B.E./B.Tech.Degree Examinations,Apr/May 2010
Regulations 2008
Fourth Semester
Electronics and Communication Engineering
EC2252 Communication Theory
Time: Three Hours Maximum: 100 Marks
Answer ALL Questions
Part A - (10 x 2 = 20 Marks)
1. How many AM broadcast stations can be accommodated in a 100 kHz bandwidth
if the highest frequency modulating a carrier i s 5 kHz?
2. What are the causes of linear distortion?
3. Draw the block diagram of a method for generating a narrowband FM signal.
4. A carrier wave of frequency 100 MHz i s frequency modulated by a signal
20 sin(200¼ £ 103t):
What is bandwidth of FM signal if the frequency sensitivity of the modulation i s
25kHz=V .
5. When i s a random process called deterministic?
6. A receiver connected to an antenna of resistance of 50­ has an equivalent noise
resistance of 30­. Find the receiver noise ¯gure.
7. What are the characteristics of superheterodyne receivers?
8. What are the methods to improve FM threshold reduction?
9. De¯ne entropy function.
10. De¯ne Rate Bandwidth and Bandwidth e±ciency.

Part B - (5 x 16 = 80 Marks)
11. (a) (i) Draw an envelope detector circuit used for demodulation of AM and ex-
plain its operation. (10)
(ii) How SSB can be generated using Weaver's method? Illustrate with a neat
block diagram. (6)
OR
11. (b) (i) Discuss in detail about frequency translation and frequency division mul-
tiplexing technique with diagrams. (10)
(ii) Compare Amplitude Modulation and Frequency Modulation.
(6)
12. (a) (i) Using suitable Mathematical analysis show that FM modulation produces
in¯nite sideband. Also deduce an expression for the frequency modulated
output and its frequency spectrum. (10)
(ii) How can you generate an FM from PM and PM from FM?
(6)
OR
12. (b) (i) A 20 MHz i s frequency modulated by a sinusoidal signal such that the
maximum frequency deviation is 100 kHz. Determine the modulation index
and approximate bandwidth of the FM signal for the following modulating
signal frequencies,
(1) 1 kHz (2) 100 kHz and (3) 500 kHz. (8)
(ii) Derive the time domain expressions of FM and PM signals.
(8)
13. (a) (i) Give a random process, X(t) = Acos(!t+µ), where A and ! are constants
and µ is a uniform random variable. Show that X(t) is ergodic in both
mean and autocorrelation. (8)
(ii) Write a short note on shot noise and also explain about power spectral
density of shot noise. (8)
OR
13. (b) Write the details about narrow band noise and the properties of quadrature
components of narrowband noise. (16)
14. (a) Derive an expression for SNR at input (SNRc) and output of (SNRo) of a
coherent detector. (16)
OR
14. (b) (i) Explain pre-emphasis and De-emphasis in detail. (10)
(ii) Compare the performances of AM and FM systems. (6)
15. (a) (i) Find the code words for ¯ve symbols of the alphabet of a discrete memory-
less source with probability f0.4, 0.2, 0.2, 0.1, 0.1g, using Hu®man coding
and determine the source entropy and average code word length. (10)
(ii) Discuss the source coding theorem. (6)
OR
15. (b) (i) Derive the channel capacity of a continuous band limited white Gaussian
noise channel. (10)
(ii) Discuss about rate distortion theory. (6)

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EC2252 Communication Theory nov/dec 2010 ANNA UNIVERSITY PREVIOUS YEAR QUESTION PAPER, IMPORTANT QUESTIONS, 2 MARKS AND 16 MARKS QUESTIONS FOR ECE DEPARTMENT

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B.E./B.Tech.Degree Examinations, November/December 2010
Regulations 2008
Fourth Semester
Electronics and Communication Engineering (ECE)
EC2252 Communication Theory
( Common to PTEC 2252 Communication Theory for B.E.(Part -Time)
Third Semester ECE - Regulations 2009)
Time: Three Hours Maximum: 100 Marks
Answer ALL Questions
Part A - (10 x 2 = 20 Marks)
1. How many AM broadcast stations can be accommodated in a 100 kHz bandwidth
if the highest frequency modulating a carrier i s 5 kHz?
2. State the applications of FDM.
3. Illustrate the relationship between FM and PM, with block diagrams.
4. Compare the transmission bandwidth required for Narrowband FM and Wide band
FM.
5. De¯ne a random variable. Specify the sample space and the random variable for a
coin tossing experiment.
6. What i s white noise? Give its characteristics.
7. De¯ne threshold e®ect in AM receiver.
8. De¯ne pre-emphasis and de-emphasis.
9. A source generates 3 messages with probability 0.5, 0.25, 0.25. Calculate source
entropy.
10. De¯ne Rate Bandwidth and Bandwidth effciency.

Part B - (5 x 16 = 80 Marks)
11. (a) (i) With the help of a neat diagram, explain the generation of DSB-SC using
Balanced modulator. (8)
(ii) Write about the coherent detection method in detail for DSB-SC and SSB-
SC. What happens when there i s phase mismatch? (8)
OR
11. (b) (i) Explain the concept of Frequency Translation. (4)
(ii) With aid of block diagram explain the principle of FDM. (8)
(iii) Illustrate the formation of Basic group and super group. (4)
12. (a) (i) De¯ne frequency modulation. Draw the FM waveform. Derive an expres-
sion for single tone frequency modulation.
(2 + 2 + 6)
(ii) Compare Narrowband and Wideband FM. (6)
OR
12. (b) (i) A 20 MHz i s frequency modulated by a sinusoidal signal such that the max-
imum frequency deviation i s 100 kHz. Determine the modulation index
and approximate bandwidth of the FM signal for the following modulating
signal frequencies,
(1) 1 kHz (2) 100 kHz and (3) 500 kHz. (8)
(ii) Derive the time domain expressions of FM and PM signals.
(8)
13. (a) (i) List the di®erent types of random process and give the de¯nitions. (10)
(ii) Write short notes on shot noise. (6)
OR
13. (b) (i) A mixer stage has a noise ¯gure of 20 dB and this i s preceded by an
ampli¯er that has a noise ¯gure of 9 dB and an available power gain of 15
dB. Calculate the overall noise ¯gure referred to the input. (8)
(ii) A receiver has a noise ¯gure of 12 dB and it is fed by a low noise ampli¯er
that has a gain of 50 dB and a noise temperature of 90 K. Calculate the
noise temperature of the receiver and the overall noise temperature of the
receiving system. Take room temperature as 290 K. (8)
14. (a) Derive the expression for ¯gure of merit of a AM receiver using envelope de-
tection. What do you infer from the expression? (16)
OR
14. (b) De¯ne and explain FM Threshold e®ect. With suitable diagram, explain
threshold reduction by FMFB demodulator. (16)
15. (a) (i) An Analog signal i s band limited to `B' Hz and sampled at Nyquist rate.
The sampled signals are quantized into 4 levels. Each level represents
one message. The probability of occurrence of the four messages are
p1=p3=1/8; p2=p4=3/8. Find out information rate of the source. (6)
(ii) Five source messages are probable to appear as m1 = 0:4, m2 = 0:15,
m3 = 0:15, m4 = 0:15, m5 = 0:15. Find coding e±ciency for (1) Shanon-
Fano coding, (2) Hu®man coding.
(10)
OR
15. (b) (i) Derive the channel capacity for Binary Symmetric channel.
(6)
(ii) Derive the channel capacity for band limited, power limited Gaussian
Channel. (10)

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