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

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This set of Analog Communications MCQs focuses on “Comparison Between PM and AM”.

1. Which of the following oscillator is not used in frequency modulation?
a) Colpitts Oscillator
b) Crystal Oscillator
c) Hartley Oscillator
d) Both Colpitts and Crystal Oscillator

Explanation: Crystal Oscillator uses the mechanical resonance of a vibrating crystal which is made up of piezoelectric material to create an signal with a precise frequency. It is used to produce a specific fixed frequency. It has no role in frequency modulated systems.

2. In frequency modulation, noise gets decreased if we decrease deviation.
a) True
b) False

Explanation: In frequency modulation, noise is inversely proportional to the frequency deviation. Thus noise also starts decreasing if we increase deviation.

3. Which device is most commonly used for detection in radio receiver?
a) Triode
b) Capacitor
c) Diode
d) Transistor

Explanation: In radio receivers, diode is most preferably used for detection purposes.

4. ________ modulator is an indirect way of generating FM.
a) Varactor FET
b) Reactance FET
c) Armstrong
d) Reactance bipolar transistor

Explanation: FM can be generated in two ways. One is direct and other is indirect way. In indirect way, there is an Armstrong method which is used to generate FM system, while other methods stated in the options are direct ways.

5. Three point tracking is achieved by __________
a) Double conversion
b) Variable selectivity
c) Variable Sensitivity

Explanation: To achieve three point tracking a capacitor is connected in series with local oscillator coil. This capacitor is called padder capacitor.

a) Synchronous
b) Coherent
d) Envelope

Explanation: Household receivers generally uses envelope detector.

7. High IF in superheterodyne receiver ____________
a) improves fidelity
b) improves selectivity
c) improves sensitivity
d) increases tracking problems

Explanation: In superheterodyne receiver, most of the received signals gets amplified from intermediate frequency (IF). High intermediate frequency helps in tracking problems.

8. If the step size of a system is 0.0721, find its quantization power?
a) 3.25 x 10-3
b) 4.33 x 10-4
c) 4.33 x 10-2
d) 3.25 x 10-1

Explanation: Quantization noise is a type of quantization error generated by quantization due to analog-to-digital conversion communication systems. It is an approximate error between the analog input voltage to the ADC and the output discritized value.
Therefore, Quantisation power = ∆2/12, where ∆=0.0721
= (0.07212)/12 = 4.33 X 10-4.

9. Find the step size of a signal with peak to peak amplitude of 8V and is quantized into 40 levels?
a) 0.8
b) 1
c) 0.2
d) 0.6

Explanation: Step size is the voltage difference between two consecutive digital levels. Quantization step size is the smallest possible difference in amplitude.
Step size,