# Analog Communications Questions and Answers – Comparison Between FM and PM

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This set of Analog Communications Multiple Choice Questions & Answers (MCQs) focuses on “Comparison Between FM and PM”.

1. Pre-emphasis is used to amplify _________ frequencies.
a) low
b) high
c) both low and high
d) local oscillator

Explanation: Pre-emphasis is used in frequency modulated transmitters to equalize the drive power of transmitting signal in terms of deviation ratio. It is done at the transmitter. It is used to amplify high frequencies.

2. De-emphasis circuit is used ________
a) before detection
b) after detection
c) before encoding
d) after encoding

Explanation: De-emphasis means attenuation of those frequencies by the amount by which they are boosted. It is done at the receiver end i.e. it is used after detection.

3. Why frequency fogging is used in a carrier system?
a) to reduce noise
b) to reduce cross talk
c) to converge frequencies
d) to reduce distortion

Explanation: The interchanging of the frequencies of carrier channels to accomplish specific purposes. It is used to prevent feedback and oscillation. It is also used to reduce cross-talk and also to correct for a high frequency response slope in the transmission line.
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4. For a phase modulated signal, the frequency deviation is proportional to ________
a) frequency only
b) amplitude only
c) only width
d) phase only

Explanation: For a phase modulated system, it is amplitude which is directly proportional to the deviation.

5. The frequency deviation is proportional to frequency in phase modulated signal.
a) True
b) False

Explanation: In phase modulation, the phase of the carrier signal is varied with respect to the amplitude of the message signal. For a phase modulated signal, the frequency deviation is proportional to amplitude.
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6. Which is the true statement about frequency deviation in frequency modulation?
a) frequency deviation is proportional to carrier signal frequency
b) frequency deviation is proportional to amplitude of carrier signal
c) frequency deviation is proportional to modulating frequency
d) frequency deviation is proportional to amplitude of modulating signal

Explanation: In frequency modulated system, the frequency deviation is proportional to the amplitude of the modulating signal.

7. Which of the following is not necessarily an advantage of FM over AM?
a) less modulating power is required
b) better noise immunity is provided
c) higher bandwidth is required
d) carrier is of any shape

Explanation: In frequency modulation, frequency of carrier gets varied with respect to the wave being propagated. FM has many advantages over AM but requirement of higher bandwidth is not an efficient condition.

8. What is number of possible outputs if there is 7 line digital input?
a) 64
b) 32
c) 16
d) 128

Explanation: Total possible outputs will be 27 which is equal to 128.

9. What is the frequency of the stereo sub carrier signal in FM broadcasting?
a) 19 KHz
b) 45 KHz
c) 55 KHz
d) 38 KHz

Explanation: Stereo broadcasting is made possible by using a subcarrier on FM radio stations, which takes the left channel and “subtracts” the right channel from it. A subcarrier is basically a sideband of a radio frequency carrier wave, which is modulated to send additional information. The frequency set for stereo sub carrier signal in FM broadcasting is 38 KHz.

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