Anna University Chennai Question Bank, ANNA UNIVERSITY QUESTION BANKS FOR ECE, EC2252 Communication Theory, ECE, ECE FIFTH, ECE FOURTH, QUESTION BANKS,

**COMMUNICATION THEORY SEM / YEAR: V / III**

**DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING**

**QUESTION BANK**

**SUBJECT CODE **: EC1301 **SEM
/ YEAR : **V / III

**SUBJECT NAME : **COMMUNICATION THEORY

**UNIT I – AMPLITUDE MODULATIONS**

**PART A**

**All questions – Two Marks:**

1. As related to AM, what is over
modulation, under modulation and 100% modulation?

2. Draw the frequency spectrum of
VSB, where it is used

3. Define modulation index of an AM
signal

4. Draw the circuit diagram of an
envelop detector

5. What is the mid frequency of IF
section of AM receivers and its bandwidth.

6. A transmitter radiates 9 kW
without modulation and 10.125 kW

after modulation. Determine depth of
modulation.

7. Draw the spectrum of DSB.

8. Define the transmission efficiency
of AM signal.

9. Draw the phasor diagram of AM
signal.

10. Advantages of SSB.

11. Disadvantages of DSB-FC.

12. What are the advantages of
superhetrodyne receiver?

13. Advantages of VSB.

14. Distinguish between low level and
high level modulator.

15. Define FDM & frequency
translation.

16. Give the parameters of receiver.

17. Define sensitivity and
selectivity.

18. Define fidelity.

19. What is meant by image frequency?

20. Define multitone modulation.

21. Need for modulation.

22. Application of AM.

23. What is meant by diagonal
clipping and negative peak clipping?

24. Define envelope.

25. Distinguish between linear and
non linear modulator.

26. What are the limitations of AM.

27. Draw the envelope of AM.

28. Differentiate phase modulation and frequency modulation.

29. Suggest one application for AM, SSB, DSB and VSB
modulation techniques and

justify your answer.

30. When a signal m(t) = 3 cos (2p x 103t ) modulates a
carrier c(t) = 5 cos (p x 106t),

find the modulation index and
transmission bandwidth if the modulation is AM.

**PART B**

1. Explain the generation of AM
signals using square law modulator. **(16)**

2. Explain the detection of AM
signals using envelope detector. **(16)**

3. Explain about Balanced modulator
to generate DSB-SC signal. ` **(16)**

4. Explain about coherent detector to
detect SSB-SC signal. **(16)**

5. Explain the generation of SSB
using balanced modulator. **(16)**

6. Draw the circuit diagram of Ring
modulator and explain with its operation? **(16)**

7. Discus the coherent detection of
DSB-SC modulated wave with a block diagram of

detector and Explain. **(16)**

8. Explain the working of
Superheterodyne receiver with its parameters. **(16)**

9. Draw the block diagram for the
generation and demodulation of a VSB signal and explain

the principle of operation. **(16)**

10. Write short notes on frequency
translation and FDM? **(16)**

**UNIT II – ANGLE MODULATION**

**PART A**

**All questions – Two Marks:**

1. What do you mean by narrowband and
wideband FM?

2. Give the frequency spectrum of
narrowband FM?

3. Why Armstrong method is superior
to reactance modulator.

4. Define frequency deviation in FM?

5. State Carson’s rule of FM bandwidth?

6. Differentiate between narrow band
and wideband FM.?

7. What are the advantages of FM.?

8. Define PM.

9. What is meant by indirect FM
generation?

10. Draw the phasor diagram of narrow
band FM.

11. Write the expression for the
spectrum of a single tone FM signal.

12. What are the applications of
phase locked loop?

13. Define modulation index of FM and
PM.

14. Differentiate between phase and
frequency modulation.

15. A carrier of frequency 100 MHz is
frequency modulated by a signal x(t)=20sin (200¶x103

t ). What is the bandwidth of the FM
signal if the frequency sensitivity of the modulator is

25 KHz per volt?

16. What is the bandwidth required
for an FM wave in which the modulating frequency signal

is 2 KHz and the maximum frequency
deviation is 12 KHz?

17. Determine and draw the
instantaneous frequency of a wave having a total phase angle

given by Ã¸(t)= 2000t +sin10t.

18. Draw the block diagram of PLL.

**PART B**

1. Explain the indirect method of
generation of FM wave and any one method of

demodulating an FM wave**. (16)**

2. Derive the expression for the
frequency modulated signal. Explain what is meant by

narrowband FM and wideband FM using
the expression**. (16)**

3. Explain any two techniques of
demodulation of FM. **. (16)**

4. Explain the working of the
reactance tube modulator and drive an expression to show

how the variation of the amplitude of
the input signal changes the frequency of the output

signal of the modulator**. (16)**

5. Discuss the effects of
nonlinearities in FM. **. (8)**

6. Discuss in detail FM stereo
multiplexing. **. (8)**

7. Draw the frequency spectrum of FM
and explain. Explain how Varactor diode can be used

for frequency modulation. **. (16)**

8. Discuss the indirect method of generating a wide-band
FM signal**. (8)**

9. Draw the circuit diagram of Foster-Seelay
discriminator and explain its working**. (16)**

10. Explain the principle of indirect method of generating
a wide-band FM signal with a neat

block diagram**. (8)**

**UNIT III – NOISE PERFORMANCE OF DSB, SSB RECEIVERS**

**PART A**

**All questions – Two Marks:**

1. Define noise figure.

2. What is white noise

3. What is thermal noise? Give the
expression for the thermal noise voltage across a

resistor

4. What is shot noise

5. Define noise temperature.

6. Find the thermal noise voltage
developed across a resistor of 700ohm. The bandwidth of

the measuring instrument is 7MHz and
the ambient temperature is 27’C.

**PART B**

1. Derive the effective noise
temperature of a cascade amplifier. Explain how the various

noise are generated in the method of
representing them. **. (16)**

**2. **Explain
how the various noises are generated and the method of representing them. **.(16)**

3. Write notes on noise temperature
and noise figure**. (8)**

4. Derive the noise figure for
cascade stages**. (8)**

5. What is narrowband noise discuss
the properties of the quadrature components of a

narrowband noise. **(8)**

6. What is meant by noise equivalent
bandwidth? Illustrate it with a diagram(8)

**7. **Derive
the expression for output signal to noise for a DSB-SC receiver using coherent

detection. **. (16)**

8. Write short notes on noise in SSB.
**(16)**

**9. **Discuss
the following: **. (16)**

i) noise equivalent bandwidth **(4)**

ii) narrow band noise **(4)**

iii) noise temperature **(4)**

iv) noise spectral density **(4)**

12. How sine wave plus noise is
represented? Obtain the joint PDF of such noise

component. **(16)**

**UNIT IV – NOISE PERFORMANCE OF AM & FM RECEIVERS**

**PART A**

**All questions – Two Marks:**

1. How to achieve threshold reduction
in FM receiver?

2. What is meant by FOM of a
receiver?

3. What is extended threshold
demodulator?

4. Draw the Phasor representation of
FM noise.

5. Define pre-emphasis and
de-emphasis.

6. What is capture effect in FM?

7. What is the SNR for AM with small
noise case?

8. What is threshold effect with
respect to noise?

9. Define SNR.

10. Define CSNR.

11. Discuss the factors that
influence the choice of intermediate frequency in a radio receiver.

**PART B**

1. Define Hilbert Transform with a
suitable example. Give the method of generation and

detection of SSB waver. **. (16)**

2. Discuss the noise performance of
AM system using envelope detection. **(16)**

3. Compare the noise performance of
AM and FM systems. **(16)**

4. Explain the significance of
pre-emphasis and de-emphasis in FM system? **(8)**

5. Derive the noise power spectral
density of the FM demodulation and explain its

performance with diagram**. (16)**

6. Draw the block diagram of FM
demodulator and explain the effect of noise in detail.

Explain the FM threshold effect and
capture effect in FM? **(16)**

7. Explain the FM receiver with block
diagram**. (8)**

**UNIT V – INFORMATION THEORY**

**PART A**

**All questions – Two Marks:**

1. What is entropy?

2. What is prefix code?

3. Define information rate.

4. What is channel capacity of binary
synchronous channel with error probability of 0.2?

5. State channel coding theorem.

6. Define entropy for a discrete
memory less source.

7. What is channel redundancy

8. Write down the formula for the
mutual information.

9. When is the average information
delivered by a source of alphabet size 2, maximum?

10. Name the source coding
techniques.

11. Write down the formula for mutual
information.

12. Write the expression for code
efficiency in terms of entropy.

13. Is the information of a
continuous system non negative? If so, why?

14. Explain the significance of the
entropy H(X/Y) of a communication system where X is the

transmitter and Y is the receiver.

15. An event has six possible
outcomes with probabilities ½.1/4,1/8,1/16,1/32,1/32. Find the

entropy of the system.

**PART B**

1. Discuss Source coding theorem,
give the advantage and disadvantage of channel coding

in detail, and discuss the data
compaction**. (16)**

2. Explain in detail Huffman coding
algorithm and compare this with the other types of

coding**. (8)**

3. Explain the properties of entropy
and with suitable example, explain the entropy of binary

memory less source**. (8)**

4. What is entropy? Explain the
important properties of entropy. **(8)**

5. Five symbols of the alphabet of
discrete memory less source and their probabilities are

given below. **(8)**

S=[S0,S1,S2,S3,S4]

P[S]=[.4,.2,.2,.1,.1]

Code the symbols using Huffman
coding.

6. Write short notes on Differential
entropy, derive the channel capacity theorem and

discuss the implications of the
information capacity theorem. **. (16)**

7. What do you mean by binary symmetric
channel? Derive channel capacity formula for

symmetric channel**. (8)**

8. Construct binary optical code for
the following probability symbols using Huffman

procedure and calculate entropy of
the source, average code Length, efficiency,

redundancy and variance? 0.2, 0.18,
0.12, 0.1, 0.1, 0.08, 0.06, 0.06, 0.06, 0.04 **(16)**

9. Define mutual information. Find
the relation between the mutual information and the joint

entropy of the channel input and
channel output. Explain the important properties of

mutual information. **. (16)**

10. Derive the expression for channel
capacity of a continuous channel. Find also the

expression for channel capacity of
continuous channel of a infinite bandwidth. Comment

on the
results. **(16)**

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