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EC2311 Communication Engineering - Unit 1 and 2 Question Paper with Answer

Anna University
Department of Electrical and Electronics Engineering

Semester : 5
Year : 3
Department : B.E EEE
Subject Code : EC2311
Subject Name : Communication Engineering

Note :
Page no to the Answers was given with respect to the Book "Communication Engineering" Lakshmi Publication - K.Muralibabu (Third Edition - June 2012)

Part A

1. Distinguish Between Low level and High level modulation.
Ans :
In low level modulation, modulation takes place prior to the output element of the final stage of the transmitter.It requires less power to achieve a high percentage of modulation. In high level modulators, the modulation takes place in the final element of the final stage where the carrier signal is at its maximum amplitude and thus ,requires a much higher amplitude modulating signal to achieve a reasonable percent modulation.

2. A 1Mhz carrier with an amplitude of 1 volt peak is modulated by a 1 KHz signal with ma = 0.5. Sketch the voltage specturm.

3. Define Am-Vestigial sideband.
Ans : AM vestigial sideband is a form of amplitude modulation in which the carrier and one complete sideband are transmitted,but only part of the second sideband is transmitted

4. Define Deviation ratio.
Ans : Deviation ratio is the worst case modulation index and is equal to the maximum peak frequency deviation divided by the maximum modulating signal frequency. Mathematically ,the deviation ratio is DR= Df (max) / fm(max)

5.Define image frequency.
Ans : An image frequency is any frequency other than the selected radio frequency carrier that ,if allowed to enter a receiver and mix with the local oscillator ,will produce a cross product frequency that is equal to the intermediate frequency.

6. Define Modulation index and percent modulation for an AM wave.

Modulation index is a term used to describe the amount of amplitude change present in an AM waveform .It is also called as coefficient of modulation.
Mathematically modulation index is m = Em/ Ec
Where m = Modulation coefficient
Em = Peak change in the amplitude of the output waveform voltage.
Ec = Peak amplitude of the unmodulated carrier voltage.
Percent modulation gives the percentage change in the amplitude of the output wave when the carrier is acted on by a modulating signal.

7. Write the disadvantage of DSB-SC - AM.
Ans : Even though the carrier is suppressed the bandwidth of DSBSC remains same as DSBFC.

8. What do you meant by AGC?
AGC stands for Automatic Gain Control. An automatic circuit in a receiver, or in firmware in some receivers and modems, used to maintain a constant level of audio gain output even though the incoming analog signal may be varying in strength.

9.Define Indirect FM.
In indirect frequency modulation ,phase of a constant amplitude carrier directly proportional to the amplitude of the modulating signal at a rate equal to the frequency of the modulating signal.

10.Mention the relationships betwwen carrier and total power.
The relations between carrier power and AM wave power can be written as :
Ptotal / Pc = 1 + m2 / 2

Part B

Ans : Page no : 5.1 and 5.4

or

11 (b) i. Derive an expression to illustrate that when a modulating signal with only one frequency component to frequency modulates a carrier. Aslo show the resulting FM signal has got infinite sideband frequencies.
(8 marks)
Ans : Wide Band FM - Page no : 6.14

11 (b) ii. Explain the generation of FM using Reactance Tube Modulator.(8Marks)
Page no : 7.2

12. (a)Explain SSB-SC AM and it's transmission bandwidth,Frequency,Spectrum,Phasor diagram,Power Calculation and generation of SSB-SC using phase shift method. (16marks)
Page no : 4.11

or

(b) Explain two methods of generation of DSB-SC AM.
Page no : 4.5

13.(a). Discuss about FM demodulators. (16 marks)

or

(b) i. Explain Narrow band FM with suitable diagram. (8 marks)
Page no : 6.13

ii. In a FM system the frequency deviation is a 1 kHz/V. A sinusoidal modulating voltage of amplitude 15v and frequency 3 kHz is applied. Find maximum frequency deviation and modulation index.

14. (a). Explain generation of FM using Armstrong method. (16marks).
Page no : 7.5

or

Page no :5.3

15(a).(i) Explain power calculations in DSB-SC AM. (6marks)
Page no : 4.4
(ii).Explain VSB (10 marks)
Page no : 4.21

or

(b) Discuss generation of SSB-SC using Modified phase shift method and Frequency disrimination method.
Page no : 4.14