# Analog Communications Questions and Answers – Switching Circuit

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This set of Analog Communications Multiple Choice Questions & Answers (MCQs) focuses on “Switching Circuit”.

1. If we compared to the USB, the power in the LSB is ________
a) one-fourth of USB
b) twice of USB
c) same as USB
d) half of USB

Explanation: Generally, USB (upper sideband) and LSB (lower sideband) contain same information and are identical to one another. Thus, both entail same power.

2. What will be the effect on power in amplitude modulated system if we increase the modulation index?
a) increases
b) becomes twice
c) remains same
d) decreases

Explanation: If we increase or decrease the modulation index, accordingly the power of amplitude modulated system will also increase or decrease. This is because the power is directly proportional to the modulation index.

3. What is the modulation index for getting maximum power in amplitude modulated system?
a) 1
b) 12
c) 0.3
d) 0.8

Explanation: For getting maximum power, it is required to set the maximum and minimum voltage of the modulated wave such that we get modulation index equal to unity. This implies 100% modulation.
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4. How can we derive FM from PM?
a) by integrating the modulating signal before applying to the PM oscillator
b) by integrating signal out of the PM oscillator
c) by differentiating the modulating signal before applying to the PM oscillator
d) by differentiating signal out of the PM oscillator

Explanation: Frequency modulation can be derived from phase modulated system, if we integrate our modulating signal before applying our wave to PM oscillator. Likewise, Phase modulation can be obtained from frequency modulated system, if we differentiate our modulating signal before applying our wave to FM oscillator.

5. FM bandwidth can be approximated by ________
a) Carson’s rule
b) Armstrong’s rule
c) Bessel’s rule
d) Pythagoras rule

Explanation: According to Carson’s rule, the required bandwidth is twice the sum of the maximum frequency deviation and the maximum modulating signal frequency.
B = 2(fd+fm)Hz.

6. Which effect is observed when FM reception deteriorates abruptly due to noise?
a) Threshold effect
b) Capture effect
c) Noise effect
d) Limit effect

Explanation: Signal to Noise ratio is the measure of signal present with respect to the surrounding noise. Threshold effect has low output signal to noise ratio. It contains large noise as compared to input signal in envelope detector. In threshold effect, detection of message signal is difficult.

7. FM stereo _________
a) is implemented using an SCA signal
b) has a higher signal to noise ratio than mono FM
c) is not compatible with mono FM
d) uses DSBSC AM modulation

Explanation: A stereo FM contains three major sections :- monomode, stereo mode and section common to both mono and stereo mode. The stereo section is much complicated than others. It uses DSBSC AM modulation, in which the carrier is suppressed and only the two sidebands are transmitted, for its process.

8. Signal to noise ratio of a frequency modulated system is better than amplitude modulated system.
a) True
b) False

Explanation: Signal to noise ratio is the ratio of signal power to the noise power. It is often described in decibels. Signal to noise ratio is the ratio of signal power to the noise power. It is the measure of signal present with respect to the surrounding noise. It is directly proportional to the modulation index and thus it increases with modulation index. It is better for frequency modulated systems.

9. What will be the effect on power if the distance between the sidebands and center frequency increases?
a) power decreases
b) power increases
c) power remains same
d) power becomes twice of its previous value

Explanation: If the distance between the central frequency and sidebands increases, the effective power decreases as bandwidth increases.

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