Communication Theory - Unit 1 (Amplitude Modulation Sys) Question Bank with Answers

Communication Theory



1. State any two important spectral properties of periodic power signals.
  • Linearity
  • Time shift
  • Frequency Shift.
2. Define amplitude modulation. Give the expression for AM wave.

It is the process by which the amplitude of the carrier wave is changed in Accordance with the instantaneous value of the message signal,

3. A transmitter radiates 9 kW without modulation and 10.125 kW after modulation. Determine depth of modulation.

4. A 5O0 W carrier is modulated to a depth of 60 percent Calculate the total power in modulated wave.

5. Define VSB transmission.Mention its application

One of he sideband is partially Suppressed and portion of the other sideband is transmied. This portion compensates the suppression of the sideband. It is called vestigial sideband transmission.

VSB is mainly used in TV transmission

6. Compare low level modulation and high level modulation.

7 An AM transmitter radiates 10 KW of power without modulation With 0 65 depth of modulation
what is the radiated power?

8. A carrier is amplitude modulated to a depth of 75 percent. Calculate the total Power in the modulated wave, if the carrier power is 40 watt.

9. SSB is suitable for speech signals and not for video signals. Why?
To generate SSB,the message spectrum must have an energy gap centered at the origin. . This criteria is satisfied by speech signais, for which the energy gap is from -300 Hz to +300 Hz(600 Hz wide).
Hence any one sideband can be easily isolated with the help of practical band pass fliters. But in case of video signals, there is no such energy gap near origin. In other words, the video signal extends from DC origin. Hence lower and upper side bands are practically joined at origin. Hence with the help of practical band pass filters, it is not possible to isolate one sideband from other. Hence for video signals vestigial sideband transmission is suitable, rather than SSB.

10. As related to AM what is over modulation, under modulation and 100% modulation?
When does a carrier is said to be over, under moaulated in Amplitude modulation?

In the case of Under modulation, modulation index ma<1(Le.)Vmc. Here. the envelope of Amplitude modulated signal does not reach the Zero amplitude axis. Hence the Message signal is fully preserved in the envelope of the AM Wave. In the case of Over modulation, modulation index m>1(i.e.)Vm>Vc. Here the envelope of Amplitude modulated signal crosses the zero axis.

In-the case critical modulation modulation index ma=1(i e )Vm=Vc. Here the envelope of the modulate signal just reaches the zero amplitude axis.. The message signal remains preserved.

11.Define Modulation index of an signal.

It is defined as the ratio of the maximum modulating voltage to the maximum carrier voltage. It is also called as ‘Depth of modulation'.

12. Draw the Spectrum of DSBFC. 

13. Distinguish between low level and high level modulator.

14. Define FDM & frequency translation.
Frequency translation:
The single sideband modulation basically performs frequency translation. it is also called frequency changing, mixing or heterodyning. the single sideband modulation the message spectrum is shifted by an amount equal to the carrier frequency fc
For voice transmission, the frequency range of 300 to 3400 Hz is used. when more number of such voice channels are to be transmitted then each channel is given a fixed frequency slot and then transmitted. This is called Frequency Division Multiplexing.

15. What do you mean by image frequency ?
The local osccillator (fo), input signal frequency (f) and If some other frequency, f = fsi + 2fi appears at the input of mixer, then it produces fi at the output of the mixer fsi image frequency.

16. Why we need modulation?
Needs for modulation:
1 .Ease of transmission
2. Multiplexing
3 .Reduced noise
4.Narrow bandwidth
5.Frequency assignment
6.Reduce the equipments limitations.
17. What is the power saving in DSB-SC-AM and SSB-SC AM?
Power saving in DSB-Sc-AM is 66.7%.
Power saving in SSB-Sc-AM is 83.3%.

18. What are the advantages of VSB-AM?
1. It has bandwidth greater than SSB but less than DSB system.
2. Power transmission greater thaƱ DSB but less than SSB system.
3. No low frequency component lost. Hence it avoids phase distortion.
19. How will you generating DSB-SC-AM?
Give the two methods of generating D SB-SC-AM.
There are two ways of generating DSB Sc-AM such as
  • balanced modulator
  • ring modulators
20.Compare linear and non-linear modulators.

21. What are advantages of ring modulator?

1. Its output is stable.
2. It requires no external power source to activate the diodes.
3. virtually no maintenance.
4.Long life

22. DSB-SC and SSB-SC:

I. Suppression of carrier results in economy of power.
2. It is commonly used in carrier current telephony system, in which one sideband is filtered out to reduce the width of the channel required for transmission.
3. It offers secrecy.
4. It increases the efficiency because the carrier is suppressed.
. 1. Bandwidth of SSB is half that of DSB-SC AM. Thus twice the number of channels can be accommodated at a given frequency spectrum.
2. No carrier is transmitted, hence possibility of interference with other channels are avoided.
3. It eliminates the possibility of fading. Fading occurs due to multipath propagation of electro magnetic waves.

23. Give the methods of generating SSB-SC-AM. And mention Some applications of SSB-SC
The two methods of generating the SSB-SC waves are
  1. Frequency discrimination or Filter method.
  2. Phase discrimination method.

1. Police Wireless cornmuniĆ³ation
2. SSB telegraph system ,
3. Point to point radio telephone communication
4. VHF and UHF communication systems.

24. What is single tone and multi tone modulation?

If modulation is performed for a message signal with more than one frequency component then the
modulation is called multi tone modulation.
If modulation is performed for a message signal with one frequency component then the modulation is
called single tone modulation.

25. Compare AM with DSB-SC and SSB-SC.

26. A transmitter supplies 8 Kw to the antenna when modulated. Determine the total power radiated when modulated to 30%.

27. The antenna current of an AM transitter is 8A when only carrier is sent. It increases to 8.93A when the carrier is modulated by single sine wave. Find the percentage modulation.

Solution :

Ic = 8A
It = 8.93 A
m = 0.8

28. A 1MMz carrier is amplitude modulated by 400Hz modulating signal to a depth of 50%.The modulated carrier power is 1KW.Calculate the power of the unmodulated signal.

Solution : 

Part B
l.Describe the generation of SSB wave using phase shift method. (16)

2.How does a balanced modulator produce an output consisting of sidebands only without carrier? (16)

3. Explain the generation and detection of AM signals.(16).
4.Discuss the detections scheme for AM(with carrier).(16)
5. Explain the operation of a ring modulator. State its advantages. (16) .
6.(i)Explain frequency translation (6)
(ii) Explain frequency division multiplexing.(6)
ii) Compare AM ard FM (4)
7. Draw the block diagram for the generation and demodulation of a VSB signai and explain the principle of operation. (16)

8. . Explain the double side suppressed carrier modulation technique. (16)
9.Explain about coherent detector to detect SSB-SC signal. (16)
10.Discus the coherent detection of DSB-SC modulated wave with a block diagram,. of detector and  explain. (16)

11.Expain the generation of AM signals using square law modul . (16)
12 .i) Explain the detection of AM signals using envelope detector. , (8)

ii) How SSB can be generated using Weaver’s method? Illustrate with a neat block diagram.. (8)