# Analog Communications Questions and Answers – Modulation Using Non-Linear Devices (Balanced Modulator)

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This set of Analog Communications Interview Questions and Answers focuses on “Modulation Using Non-Linear Devices (Balanced Modulator)”.

1. The approximate aspect ratio for television is ________
a) 2 : 1
b) 3 : 2
c) 4 : 3
d) 3 : 4

Explanation: Aspect ratio is an image projection attribute that describes the proportional relationship between the width of an image and its height. The most common video-graphic aspect ratio is 4:3.

2. Signal to Noise ratio for a hi-fi system is equal to ________
a) 50 dB
b) 100 dB
c) 75 dB
d) 10 dB

Explanation: Signal to Noise ratio compares signal power to noise power. It determines how much signal is present in a transmitted wave compared to presence of noise. It is generally expressed in decibels(dB). For a hi-fi system, it should be equal to 50dB.

3. What is the use of SSB?
a) It has lesser bandwidth
b) It has large bandwidth
c) It has infinite bandwidth
d) It has zero bandwidth

Explanation: In SSB-SC(Single Side Band Suppressed Carrier), the carrier is suppressed and either of the upper side-band and lower-sideband, are transmitted. This reduces it’s bandwidth to the frequency of the message signal.
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4. Approximate how many times odd and even fields of television system are scanned?
a) 25 times each
b) 50 times each
c) 50 times each but alternately
d) 25 times each but alternately

Explanation: In television system in India, each of odd and even fields are scanned 25 times but alternately.

5. Any signal and its Hilbert transform have same energy density spectrum.
a) True
b) False

Explanation: Hilbert transform of a signal results in a signal with same amplitude but a phase shift of -90 degrees. Thus, a signal and it’s Hilbert transform have same amplitude spectrum i.e. they have same energy density spectrum and also correlation spectrum.
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6. If the spectrum of a narrow band noise is symmetrical and it has a power density spectrum 4×10-6. Power density of quadrature component is ________
a) 2 x 10-6
b) 5 x 10-6
c) 3 x 10-6
d) 4 x 10-6

Explanation: Power density is the amount of power per unit volume. Quadrature component is a amplitude modulated sinusoid which is offset in phase by one quarter cycle.
Power density of quadrature component is

7. What is the capacitive reactance for DC signals?
a) zero
b) very low
c) equal to one
d) infinite

Explanation: Frequency of DC signals is zero. For zero frequency, capacitive reactance XC is infinite as XC = 1/(f*C), where f is the frequency and C is the capacitance.

8. If a FM signal having modulation index mf is passed through a frequency tripler, then the modulation index of output of frequency tripler is ________
a) mf
b) 3mf
c) 13 mf
d) 19 mf

Explanation: A frequency tripler is a frequency multiplier in which an electronic circuit generates an output signal whose output frequency is a harmonic (multiple) of its input frequency. When a FM signal is passed through a frequency tripler, it increases its modulation index 3 times. So the modulation index of output is 3mf.

9. What are the two major drawbacks of delta modulation?
a) Slope Overload and Granular noise
b) Slope Overload and Serration noise
c) Serration noise and Granular noise
d) Slope Overload and Channel Noise

Explanation: Delta modulation is an analog-to-digital and digital-to-analog signal conversion technique used for transmission of voice information where quality is not of primary importance. It is the simplest form of differential pulse-code modulation (DPCM) where the difference between successive samples are encoded into n-bit data streams. The two major drawbacks in delta modulation are slope overload and granular noise. When step size becomes too small to track the original waveform it generates an error known as slope overload and granularity occurs when step size is too large.

10. Beam width of antenna is expressed in ________
a) metres
b) degrees
d) volt

Explanation: Angular separation between the two half-power (-3dB) points is known as beam width. It is usually expressed in degrees.

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