# Analog Communications Questions and Answers – Carrier Acquistion Techniques in DSB-SC

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This set of Analog Communications Question Bank focuses on “Carrier Acquisition Techniques in DSB-SC”.

1. Flyback switched mode is used in television because it can supply many voltage outputs.
a) True
b) False

Explanation: In frequency modulation, power is determined from carrier amplitude and as carrier amplitude does not change, thus power remains constant. So if we increase the modulation index, then also it does not affect power and it will remain constant.

2. What will be the effect on power if the modulation index of a frequency modulated signal is increased?
a) increases
b) decreases
c) remains constant
d) first increases and then decreases

Explanation: In frequency modulation, power remains constant. So if we increase the modulation index then also it does not affect power, it will remain constant.

3. The equation 20sin⁡(108 t + 3 sin⁡ 109 t) represents which modulation?
a) AM
b) DSB-SC
c) PM
d) FM

Explanation: On comparing with the general equation of each modulation, we found that it represents Frequency modulation.
FM (t) = Ac sin (2πfct + βsin(2πfmt)), where symbols have their usual meaning.

4. Monopole antenna is a type of travelling wave antenna.
a) True
b) False

Explanation: Monopole antenna is a type of wire antenna like Marconi antenna, also known as quarter-wave antenna. Travelling wave antenna makes use of a travelling wave as a radiating guide and they consist of helical antenna, spiral antenna.

5. In which modulation, frequency of carrier signal is varied with the amplitude of modulating signal?
a) FM
b) AM
c) PM
d) VSB

Explanation: In Frequency Modulation, the frequency of carrier wave is varied with the instantaneous amplitude of modulating signal. The general FM modulated expression is :
FM (t) = Ac sin (2πfct + βsin(2πfmt)), where symbols have their usual meaning.

6. In Phase Modulation the phase of carrier signal is varied.
a) True
b) False

Explanation: In phase modulation, the phase shift of carrier wave is varied with respect to the instantaneous amplitude of modulating signal. The general PM modulated expression is:
PM (t) = Ac sin (2πfct + βsin(2πfmt)), where symbols have their usual meaning.

7. Power spectral density of thermal noise remains uniform upto frequency ___________
a) 1015 HZ
b) 105 HZ
c) 102 HZ
d) 1013 HZ

Explanation: Power Spectral Density is the quantity of power present in a signal in the frequency domain. Power spectral density of thermal or white noise gets affected when the frequency becomes greater than 1013 Hz and thus, it remains uniform upto 1013 Hz.

8. Carrier frequency remains constant in cell communication.
a) True
b) False

Explanation: In cell communication, the carrier frequency does not remain constant. Instead, the carrier frequency get changes continuously with respect to the instantaneous amplitude of the message signal.

9. Waves of which frequency can penetrate the atmosphere?
a) higher than 10 MHz
b) higher than 20 MHz
c) higher than 30 MHz
d) higher than 100 MHz

Explanation: Most Radio Waves penetrate the atmosphere. Thus, any wave having frequency greater than 30 MHz can penetrate the atmosphere. The waves having frequency less than it cannot penetrate the atmosphere and get absorbed into it.

10. Armstrong modulator can only generate FM signal.
a) True
b) False

Explanation: Armstrong Modulator can also generate PM along with FM. So the statement is incorrect. Thus, collectively Armstrong modulator is an Angle Modulator, where the angle of the carrier wave is varied with respect to the instantaneous amplitude of modulating signal.

Sanfoundry Global Education & Learning Series – Analog Communications.

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