## Analog Communications Questions and Answers Part-16

1.If a receiver has poor selectivity then it also have poor ________
a) sensitivity
b) blocking of unwanted signals
c) double spotting
d) reception

Explanation: Selectivity is the ability of a radio receiver to reject unwanted signals and allow only the desired signal to pass. Thus, it’s the blocking of unwanted signals. Sensitivity is the ability of a receiver to amplify weak signals to the desired level, whereas reception means to receive a signal.

2. For a large amount of power, a push pull amplifier is designed for operation as ________
a) class C
b) class B
c) class AB
d) class A

Explanation: A push pull amplifier is an amplifier whose output is a cumulative sum of the outputs of the currents produced by the two different stages of the load. It is generally used when a large amount of power is to be consumed. Of all the power amplifiers, only Class B amplifier is capable to produce such large amount of power.

3. In a Vidicon camera tube, the side of target plate facing the light has a coating of antimony trisulphide and the side facing electron gun has a coating of tin oxide.
a) True
b) False

Explanation: A vidicon camera tube is a type of camera having a photoconductive surface within. In a Vidicon camera tube, the side of target plate facing the light has a coating of Sno2 and the side facing electron gun has a coating of antimony trisulphide.

4. Autocorrelation function of which noise is a constant?
a) white noise
b) shot noise
c) transit time noise
d) extraterrestrial noise

Explanation: White noise is a random signal having equal intensities at different frequencies. The autocorrelation of white noise signal has a straight vertical line at origin and is zero for rest points. Thus the Autocorrelation function of white noise is constant.

5. Thermal noise shot noise and transit time noise is part of internal noise in communication.
a) True
b) False

Explanation: Internal noise in communication is the noise that gets generated within the receiver or communication system, like thermal noise, shot noise, transit time noise, partition noise and such like. External noise is that noise which is generated from an external source, that is not a part of the communication system.

6. In amplitude modulation, magnitude of side bands is 2ma times the carrier amplitude.
a) True
b) False

Explanation: In amplitude modulation, magnitude of side bands is m/2 times the carrier amplitude (‘a’), where ‘m’ is the modulation index.

7. In frequency modulation, there is a large increase in noise and hence decrease in the signal to noise ratio.
a) True
b) False

Explanation: In communication system, Signal-To-Noise ratio (SNR) is a measure of the amount of signal present compared to the surrounding noise. It is the ratio of signal power to noise power. Thus in frequency modulation, there is a large decrease in noise and hence increase in the signal to noise ratio.

8. The output of a battery eliminator is closed to ________
a) 70V DC
b) 70V AC
c) 6V AC
d) 6V DC

Explanation: A battery eliminator is an electronic device which is powered by an external source (except battery) such that it converts the source to a particular DC voltage which can be used as power supply for other devices. Its output is approximately 6V.

9. In India, the subcarrier frequency for transmission of color difference signals in television is approximately ________
a) 10.47 MHZ
b) 5.4 MHZ
c) 7.67 MHZ
d) 1.3 MHZ