COMMUNICATION THEORY Question Bank For UNIT 1

UNIT -1

1.      Represent an amplitude modulated wave as a function of time with amplitude sensitivity of the modulator as the constant. (AU: Dec 13)

The amplitude sensitivity of the modulator is basically variation in amplitude of the carrier with respect to modulating signal. Refer AM envelope diagram.

2.      For an AM system, the instantaneous values of carrier and modulating signal are 60 sinw_ct and 40 sinw_mt respectively. Determine the modulation index. (AU: May 14)

E_c=60V E_m=40

Modulation index = E_m/E_c = 40/60 =0.6667

3.      State the difference between SSB and VSB. (AU: May 14)

Single side band	Vestigial side band
Only one side is transmitted	One side and part of other side is transmitted
Frequencies near edges are attenuated	Frequencies near edges are not attenuated
Bandwidth requirement is low	Bandwidth requirement is high

4.      What is meant by frequency translation? (AU: Dec 11)

In the single sideband modulation, the message spectrum is shifted by an amount equal to carrier frequency. This is called frequency translation.

5.      A transmitter radiates 10KW without modulation and 11.125 KW after modulation. Determine modulation index m. (AU: May 11)

P _total = P_c ((1+m²⁾/2) using this formula, m=0.474

6.      State the theorem by which average power of the signal is calculated. (AU: Dec 11)

Parseval’s theorem states that if x(t) is the periodic power signal with fourier co-efficient x(k), then average power in the signal is given by

P = ∑|X(k)|²

7.      Mention the important limitations of amplitude modulation system. (AU: May 11)

AM signal has varying amplitude. Hence the transmitter power is not constant. Also, it is very difficult to identify noise, since amplitude level is not constant.

8.      What is meant by amplitude modulation? (AU: Dec 05)

The process of changing the amplitude of the carrier signal with respect to message signal is called amplitude modulation.

9.      Why do we need modulation? (AU: May 11)

·         If the message signal is directly transmitted, it gets attenuated highly

·         The size of antennas needed is huge without modulation

·         The signal cannot be transmitted over longer distance

·         Message signals get interfered and distorted in channel without modulation

·         Multiplexing is not possible without modulation

10.  Determine the range of tuning oscillator of a super heterodyne receiver when f_LO >f_c. The  broadcast frequency is 540 Hz to 1600 Hz. Assume f_IF=455 KHz.

F0 =f_c + f_IF = (455.54 to 456.6) KHz

16 marks

1. Derive the equation of AM wave. Also draw  the modulated wave for various modulation index (AU: May 13) (8 Marks)

A 1000 KHz carrier is simultaneously AM modulated with 300, 800 and 1.5 KHz audio sine waves. What will be the frequencies present in the output? (AU:May 13) (4 marks)

Explain how Am signal is generated using non-linear modulator circuit? (AU: Dec 12) (4 marks)

2. Derive the equation for suppressed carrier system and draw its frequency spectrum. (AU: May 14) (8 marks)

Explain the generation of DSB-SC signal using balanced and ring modulators (AU: Dec 10) (8 marks)

3. Derive the equation for SSB modulation and demodulation (6 marks)

Discuss the generation of SSB with filter method, phase shift method and weaver’s methods. (AU: May 10) (12 marks)

4. Explain the working of super heterodyne receiver with suitable block diagram. (AU: Dec 13) (16 marks)
5. Explain in detail about modulation and demodulation of VSB using various methods. (AU: May 14) (16 marks)