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## Communications MCQ Set 1

1. Calculate power in each sideband, if power of carrier wave is 176W and there is 60% modulation in amplitude modulated signal?
a) 13.36W
b) 52W
c) 67W
d) 15.84W

Answer: d [Reason:] Modulation index = 0.6 and Pc = 176W. Power in sidebands may be calculated as

2. For 100% modulation, power in each sideband is ________ of that of carrier?
a) 50%
b) 70%
c) 60%
d) 25%

Answer: d [Reason:] Modulation index = 1. Power in sidebands may be calculated as

3. Overmodulation results in?
a) Distortion
b) Weakens signal
c) Strengthens the signal
d) provides immunity to noise

Answer: a [Reason:] When instantaneous level of modulating signal exceeds the value necessary to provide 100% modulation. Overmodulation results in distortion of the modulating signal.

4. Maximum power efficiency of an AM modulator is?
a) 25%
b) 50%
c) 75%
d) 100%

Answer: b [Reason:] Efficiency, ή = μ22. For maximum efficiency µ = 1 so ή = 122 = 12 and ή% =12 x100 =50%.

5. Noise performance of a square law demodulator of AM signal is?
a) Better than that of synchronous detector
b) Weaker than that of synchronous detector
c) Better than that of envelope detector
d) Weaker than that of envelope detector

Answer: a [Reason:] Process of recovering message signal from received modulated signal is called modulation. It is exactly opposite to modulation. There are two types of AM demodulators, Square Law Demodulator and Envelope Demodulator. Noise performance of it is far better than synchronous detector.

6. For getting 100% modulation, carrier amplitude should ________
a) exceed signal amplitude
b) be equal to signal amplitude
c) be lesser than signal amplitude
d) It is not possible to achieve 100% modulation

Answer: b [Reason:] Modulation, m = Vm/A where Vm is maximum baseband amplitude and A is maximum carrier amplitude. So for getting m = 1, Vm should be equal to A.

7. For 100% modulation, total power is?
a) same as the power of unmodulated signal
b) twice as the power of unmodulated signal
c) four times as the power of unmodulated signal
d) one and half times as the power of unmodulated signal

Answer: d [Reason:] Total power, Pt = Pc (1 +m22) where m is equal to 1 so Pt = Pc (1 +122). On solving it we get Pt = 1.5 Pc.

(Q.8 – Q.11) An AM signal is represented by x(t) = (30 + 2Sin(700πt)) Cos(2πt x 10²t)V.
8. The modulation index is
a) 0.7
b) 0.066
c) 0.341
d) 0.916

Answer: b [Reason:] Given equation can be written as 30(1 + 0.066 Sin(700πt)). So modulation index is 0.066.

9. Carrier power of the wave is?
a) 555W
b) 675W
c) 450W
d) 310W

Answer: c [Reason:] Hence Pc = 450W.

10. Find the total power of amplitude modulated wave?
a) 453W
b) 675W
c) 789W
d) 451W

11. What is its sideband power?
a) 4W
b) 1W
c) 3W
d) 2W

Answer: b [Reason:] Sideband power is (Pt – Pc) i.e. 451 – 450 = 1W.

## Communications MCQ Set 2

1. Notch is a ________
a) High pass filter
b) Low pass filter
c) Band stop filter
d) Band pass filter

Answer: c [Reason:] Notch filter is a band stop filter since it allows most frequencies to pass through it, except a specific range. It is just opposite of a band-pass filter.

2. Sin wave is ________
a) Aperiodic Signal
b) Periodic Signal
c) Random Signal
d) Deterministic Signal

Answer: b [Reason:] Periodic signal is that which repeat it’s value after some interval. Sin wave is a periodic function since it’s value may be determined at any point of time since it repeats itself at regular intervals of time.

3. What is the role of channel in communication system?
a) acts as a medium to send message signals to its medium
b) converts one form of signal to other
c) allows mixing of signals
d) helps to extract original signal from incoming signal

Answer: a [Reason:] Channel acts as a medium to transmit message signal to its medium. Noise gets involved in signal in channel only. Transducer converts one form of energy to other form.

4. Sum of a periodic and aperiodic signal always be a aperiodic signal.
a) True
b) False

Answer: b [Reason:] No, the sum of a periodic and aperiodic signal can be periodic. For example Let f(x) = sin(x), which is a periodic function with period 2π and g(x) = −sin(x) + sin(√2x), which is an aperiodic function. Now the sum of both i.e. f(x) + g(x) = sin(√2x) which is a periodic function.

5. Noise is added to a signal ________
a) In the channel
b) At receiving antenna
c) At transmitting antenna
d) During regeneration of information

Answer: a [Reason:] Noise is an unwanted signal that gets mixed with the transmitted signal while passing through the channel. The noise interferes with the signal and provides distortion in received signal.

6. Agreement between communication devices are called ________
a) Transmission medium
b) Transistor
c) Protocol
d) Data

Answer: c [Reason:] Protocol allows two or more entities of communication system to transmit information. Transmission medium is the path through which information travels.

7. What is the advantage of superheterodyning?
a) High selectivity and sensitivity
b) Low Bandwidth
d) Low fidelity

Answer: a [Reason:] The main advantage of superheterodyning is that it has high selectivity and sensitivity. Its bandwidth remains same. Moreover, it has high adjacent channel rejection.

8. Low frequency noise is ________
a) Flicker noise
b) Shot noise
c) Thermal noise
d) Burst noise

Answer: a [Reason:] Power spectral density of flicker noise increases with decrease in frequency. It is generated due to the fluctuations in the density of carrier.

9. Relationship between amplitude and frequency is represented by ________
a) Time-domain plot
b) Phase-domain plot
c) Frequency-domain plot
d) Amplitude-domain plot

Answer: c [Reason:] Relationship between amplitude and frequency is represented by frequency-domain plot. While a time-domain graph shows how a signal changes over time.

10. A function f(x) is even, when?
a) f(x) = -f(x)
b) f(x) = f(-x)
c) f(x) = -f(x)f(-x)
d) f(x) = f(x)f(-x)

Answer: b [Reason:] Geometrically a function f(x) is even if graph of function is symmetric. Algebraically, for any function f(x) to be even, f(x) = f(-x).

## Communications MCQ Set 3

1. Pre-emphasis is used to amplify _________ frequencies.
a) low
b) high
c) both low and high
d) neither low nor high

Answer: b [Reason:] 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 modulation
d) after modulation

Answer: b [Reason:] 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 converse frequencies
d) to reduce distortion

Answer: b [Reason:] 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.

4. For a phase modulated signal, the frequency deviation is proportional to ________
a) frequency only
b) amplitude only
c) both frequency and amplitude
d) phase only

Answer: b [Reason:] 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

Answer: b [Reason:] For a phase modulated signal, the frequency deviation is proportional to amplitude.

6. Which is the true statement about frequency deviation in frequency modulation?
a) frequency deviation is constant
b) frequency deviation is zero
c) frequency deviation is proportional to modulating frequency
d) frequency deviation is proportional to amplitude of modulating signal

Answer: d [Reason:] 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) lower bandwidth is required
d) carrier is of any shape

Answer: c [Reason:] In frequency modulation, frequency of carrier gets varied with respect to the wave being propagated. FM has many advantages over AM but requirement of lower 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

Answer: d [Reason:] Total possible outputs will be 27 which is equal to 128.

9. Push pull amplifier is a class B amplifier.
a) True
b) False

Answer: a [Reason:] Push pull amplifier uses a pair of active devices that alternately supply current to a connected load and also absorb current from a connected load. It is a class B amplifier.

10. 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

Answer: d [Reason:] 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.

## Communications MCQ Set 4

1. Costa’s receiver is used for ________
a) coherent detection of AM-SC signal
b) asynchronous detection of AM-SC signal
c) amplitude modulation of signal
d) frequency modulation of signal

Answer: a [Reason:] Costa’s receiver is based on phase-locked loop (PLL). It is generally used for coherent detection of AM-SC signal.

2. Helical antenna is mainly used for ________
a) bandwidth calculation
b) television production
d) satellite tracking

Answer: d [Reason:] Helical antenna consists conducting wire. These conducting wires is wound around it in the form of helix. It is mainly used for satellite tracking.

3. Sampling frequency of a compact disk is about ________
a) 30 KHz
b) 50 KHz
c) 10 KHz
d) 70 KHz

Answer: c [Reason:] Sampling frequency is the number of samples per second. For a CD it is about 10 KHz.

4. In frequency modulation, there is a large increase in noise and hence increase in the signal to noise ratio.
a) True
b) False

Answer: b [Reason:] In frequency modulation, there is a large decrease in noise and hence increase in the signal to noise ratio.

5. Image frequency is given by ________
a) fs + fi
b) fs ± fi
c) fs + 2fi
d) fs ± 2fi

Answer: c [Reason:] Image frequency is generally observed in heterodyne receivers. It is an undesired input frequency which is further equal to the sum of station frequency and twice of intermediate frequency. It is usually responsible for interference.

6. Atmospheric noise, extraterrestrial noise and man-made noise is part of internal noise in communication.
a) True
b) False

Answer: b [Reason:] Atmospheric noise, extraterrestrial noise and man-made noise is part of external noise in communication. External noise are those noises whose sources are external.

7. Thermal noise is a synonym of ________
a) Atmospheric noise
b) shot noise
c) white noise
d) transit time noise

Answer: c [Reason:] Thermal noise is a type of internal noise in communication. It can also be called white noise or johnson noise.

8. Distance at which a sky wave is received back on earth ________ with angle of incidence.
a) increases
b) decreases
c) can be increase or decrease
d) can neither be increase or decrease

Answer: b [Reason:] Distance at which a sky wave is received back on earth decreases with angle of incidence for all angle of incidence.

9. In a Vidicon camera tube, both sides of the target plate have the coating of Sno2.
a) True
b) False

Answer: b [Reason:] In a Vidicon camera tube, the side of target plate facing the light has a coating of Sno2 and the side facing electron gun has a coating of antimony trisulphide.

10. Ground wave can be used for communication upto ________
a) 5 KHz
b) 12 KHz
c) 16 KHz
d) 25 KHz

Answer: c [Reason:] Ground wave is that wave which reaches a receiver from a transmitter directly without reflecting from ionosphere. It can be used for communication upto 16 KHz.

## Communications MCQ Set 5

1. Quantization error occurs in ________
a) Time Division Multiplexing
b) Frequency Division Multiplexing
c) Pulse Code Modulation
d) Pulse Width Modulation

Answer: c [Reason:] The difference between an input value and its quantized value is called quantization error or we can also see it as that when an analog number is not converted into an exact digital number. It takes place in pulse code modulation (PCM).

2. Waves of frequency 60MHz can penetrate the atmosphere.
a) True
b) False

Answer: a [Reason:] Any frequency above 30MHz can penetrate the atmosphere. So 60 MHz can penetrate the atmosphere.

3. Ground wave travels along surface of ground.
a) True
b) False

Answer: a [Reason:] Ground wave propagation uses the area between surface and ionosphere for transmission. So the statement is true.

4. Autocorrelation function of which noise is a constant?
a) white noise
b) shot noise
c) transit noise
d) extraterrestrial noise

Answer: a [Reason:] White noise or thermal noise or Johnson noise is an interior type of noise in communication system. It is a random signal having equal intensities at different frequencies which gives it a constant power and autocorrelation function.

5. TM stands for _________
a) Trans positional Modulation
b) Transpose Modulation
c) Thermal Modulation
d) Tuned Modulation

Answer: a [Reason:] TM stands for transpositional modulation. In transpositional modulation, the waveform is modified resulting in a signal where each quarter cycle is transposed in modulation process.

6. In which of the modulation, the waveform is modified resulting in a signal where each quarter cycle is transposed.
a) AM
b) FM
c) PM
d) TM

Answer: d [Reason:] In TM, the waveform is modified resulting in a signal where each quarter cycle is transposed in modulation process.

7. Which of the following is a reflector antenna?
a) Marconi reflector antenna
b) Corner reflector antenna
c) Helical antenna
d) Bow tie antenna

Answer: b [Reason:] It is corner reflector antenna that comes under reflector antenna. Helical antenna and bow tie antenna comes under travelling wave antenna and log antenna respectively.

8. Carbon microphone has a bidirectional feature.
a) True
b) False

Answer: b [Reason:] Carbon microphone does not have a bidirectional feature. It is ribbon microphone that has bidirectional feature.

9. In India, the subcarrier frequency for transmission of color difference signals in television is approximately ________
a) 10.47 MHZ
b) 5.4 MHZ
c) 7.67 MHZ
d) 1.3 MHZ