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Communications MCQ Set 1

1. Why a helical antenna is used for satellite tracking?
a) because of its circular polarization
b) because of its broad bandwidth
c) because of its low bandwidth
d) because of its good front to back ratio

View Answer

Answer: a [Reason:] A helical antenna generally consists of a conducting wire which is wound in the form of a helix. It generally exhibits circular polarization due to which it is mostly used for satellite tracking.

2. Which one of the following noise becomes of great importance at high frequencies?
a) random noise
b) shot noise
c) impulse noise
d) transit-time noise

View Answer

Answer: d [Reason:] If the time taken by electrons to travel from emitter to collector becomes comparable to the period of the signal which is amplified which is only possible at high frequencies called transit time. At transit time noise input admittance of the transmitter increases.

3. TDM stands for ________
a) Time Division Multiplexing
b) Time Difference Multiplexing
c) Time Division Method
d) Time Data Method

View Answer

Answer: a [Reason:] TDM stands for Time Division Multiplexing. It is used to transmit and receive independent signals over a common signal path with the help of synchronized switches.

4. Which one of the following statement is false?
a) HF mixers are generally noisier
b) Voltage of impulse noise is independent of bandwidth
c) Thermal noise is not dependent on frequency
d) Industrial noise is impulse type noise

View Answer

Answer: b [Reason:] Impulse noise includes unwanted noise which is almost instantaneous and has a sharp sound. High level of such noise may damage internal organs. It depends on bandwidth of the signal.

5. Which of broad classifications of noise are most difficult to treat?
a) noise generated in the receiver
b) noise generated in the transmitter
c) externally generated noise
d) internally generated noise

View Answer

Answer: a [Reason:] Noise which is generated inside the receiver is very much difficult to handle.

6. FDM is an analog multiplexing technique used to combines ________
a) analog signals
b) digital signals
c) both analog and digital signals
d) alternatively passes analog and digital signals

View Answer

Answer: a [Reason:] FDM stands for Frequency Division Multiplexing. It is used to combine multiple analog signals. It is usually applied when the bandwidth is greater than the combined bandwidth of signals.

7. What points must be important to remember, when we deal with random noise calculations?
a) all calculations are based on peak to peak values
b) calculations are based on peak values
c) calculations are based on average values
d) calculations are based on RMS values

View Answer

Answer: d [Reason:] All calculations of any random signals are done by using their RMS (Root Mean Square) values.

8. Which of the following statement is true?
a) Random noise power is inversely proportional to bandwidth
b) Flicker noise is demodulation to noise
c) Noise mixers is caused by inadequate image frequency rejection
d) A random voltage across a resistance cannot be calculated

View Answer

Answer: c [Reason:] Image frequency rejection is the ratio of intermediate frequency (IF) produced by desired input frequency to that which is produced by the image frequency.

9. Which of the following statement is false?
a) Modulation is used to reduce the bandwidth
b) Modulation is used to separate different transmissions
c) Modulation is used to allow the use of practical antennas
d) Modulation is used to ensure that wave is transmitted over long distances

View Answer

Answer: a [Reason:] Modulation is the process of varying one or more properties of a periodic waveform that contains the information to be transmitted. It is used to separate different transmission signal so they do not get mixed, it ensure that the wave travels a long distance. But it is not used to reduce bandwidth.

10. Which of the steps is not included in the process of reception?
a) encoding
b) decoding
c) interpretation
d) storage

View Answer

Answer: a [Reason:] Communication process is the set of some sequential steps which is involved in transferring messages as well as feedback. The steps included in process of reception are decode – to decode the message signal, interpretation – to interpret the message carefully, and storing – to store the message carefully so that it can use in future if needed.

Communications MCQ Set 2

1. The image frequency of a superheterodyne receiver ___________
a) is created within the receiver itself
b) is due to insufficient adjacent channel rejection
c) is not rejected to be the IF tuned circuits
d) is independent of the frequency to which the receiver is tuned

View Answer

Answer: c [Reason:] To eliminate image frequency of a superheterodyne receiver a sufficient attenuation on the incoming signal by the RF amplifier filter is needed. It is not rejected to be the IF tuned circuits.

2. Which one of the following circuits could not demodulate SSB?
a) Balance Modulator
b) Product Modulator
c) BFO
d) Phase discriminator

View Answer

Answer: d [Reason:] Phase discriminator cannot be used to demodulate SSB. It is used to discriminate between different phases.

3. Which of the following statement is true about a radio detector?
a) the linearity is worse than in phase discriminator
b) there is stabilization against signal strength variations
c) the final output is twice of that obtainable from a similar phase discriminator
d) the circuit is same as in a discriminator, except that the diodes are reversed.

View Answer

Answer: a [Reason:] Detector in a radio receiver extracts information that is contained in a modulated radio wave. The linearity of radio detector is worse than in phase discriminator.

4. Envelope of AM wave has the same shape as the message of baseband signal.
a) True
b) False

View Answer

Answer: a [Reason:] Modulation Envelope is observed when carrier, upper sideband and lower sideband are combined in a single impedance and observed on time versus amplitude curve. For AM, it has same shape as message of baseband signal.

5. To prevent overloading of the IF amplifier in a receiver, one should use _________
a) variable sensitivity
b) variable selectivity
c) double conversion
d) squelch

View Answer

Answer: a [Reason:] Squelch is a circuit that is preferably used to prevent overloading of the IF amplifier in a receiver.

6. In a broadcast superheterodyne receiver, the ____________
a) Local oscillator always operates below the frequency of transmitted signal
b) Input of mixer must be tuned to the signal frequency
c) Local oscillator frequency should be double than that of IF
d) RF amplifier normally works at 455 KHz above the carrier frequency

View Answer

Answer: b [Reason:] For a superheterodyne receiver, the input of mixer must be tuned to the signal frequency. Tuning means that the circuit works for that one signal frequency only.

7. Which of the following statement is false regarding Armstrong modulation system?
a) The system is basically phase, not frequency modulated
b) AFC is not needed as crystal oscillator is used
c) Frequency multiplication must be used
d) Equalization is necessary

View Answer

Answer: d [Reason:] The Armstrong method generates a double sideband carrier signal, phase shifts this signal and then reinserts the carrier to produce a frequency modulated signal. In Armstrong modulation system equalization is not necessary.

8. If the target cross section is changing, the best system for accurate tracking is __________
a) sequential switching
b) lobe switching
c) conical scanning
d) monopulse

View Answer

Answer: d [Reason:] Monopulse radar compares the received signal from a single radar pulse against itself in order to compare the signal in various aspects. If the target cross section is changing then it is the best system used for accurate tracking.

9. What do you understand by low level AM?
a) Output power is low
b) Modulation is done at high power of carrier and modulating signal
c) Collector Modulation Method is low level AM
d) Output power is high

View Answer

Answer: a [Reason:] In low level AM, modulation is done at low power of carrier and modulating signal therefore output power is low. Collector Modulation Method is high level AM.

10. After a target has been acquired, the best scanning system for tracking is _________
a) nodding
b) spiral
c) conical
d) helical

View Answer

Answer: c [Reason:] Conical is the best scanning system for tracking. But it is only preferred for tracking after a target has been acquired.

Communications MCQ Set 3

1. The discone antenna is ________
a) useful direction-finding antenna
b) used as a radar receiving antenna
c) circularly polarized
d) useful as UHF receiving antenna

View Answer

Answer: d [Reason:] A discone antenna is almost same as a biconical antenna in which one of the cones is replaced by a disc. It is mounted vertically, with the disc at top and cone beneath. It is useful as UHF receiving antenna.

2. Which one of the following is not an omnidirectional antenna?
a) Marconi
b) Discone
c) Log-periodic
d) Half-wave dipole

View Answer

Answer: c [Reason:] Omnidirectional antenna radiates radio wave power uniformly in all directions in one plane. From above given antennas, discone antenna is not an omnidirectional antenna.

3. If the value of a resistor creating thermal noise is doubled. The noise power generator is therefore ________
a) unchanged
b) doubled
c) quadrupled
d) halved

View Answer

Answer: a [Reason:] If the value of resistor creating thermal noise is doubled then also the noise power generator remains unaffected.

4. Which one of the following is not a useful quantity for comparing the noise performance of receivers?
a) Input noise voltage
b) Equivalent noise resistance
c) Noise Figure
d) Figure of merit

View Answer

Answer: a [Reason:] Noise performance of different receivers cannot be compared on the basis of input noise voltage. It can be calculated on the basis of figure of merit, or noise figure, or on the basis of equivalent noise resistance.

5. What is Synchronous TDM?
a) gives same amount of time to each device
b) gives same amount of frequency to each device
c) gives variable time to each device
d) gives variable frequency to each device

View Answer

Answer: a [Reason:] Synchronous TDM gives exactly the same amount of time to each connected device. It allocates time to every device even if a device has nothing to transmit.

6. Which of the following is the most generally used term for comparing amplifier noise characteristics?
a) signal to noise ratio
b) shot noise
c) noise factor
d) figure of merit

View Answer

Answer: c [Reason:] Noise factor measures degradation of signal to noise ratio which is caused by components in radio frequency (RF) signal chain. It is a number by which the performance of an amplifier can be specified, with lower values indicating better performance.

7. The number of lines per field in the United States TV system is _________
a) 26212
b) 525
c) 300
d) 600

View Answer

Answer: a [Reason:] The number of lines per field in the United States TV system is 26212.

8. The number of frames per second in the United States TV system is _________
a) 26212
b) 525
c) 45
d) 30

View Answer

Answer: d [Reason:] The number of frames per second in the United States TV system is 30.

9. Why interlacing is used in television?
a) to produce the illusion of motion
b) to ensure that all the lines on screen are scanned
c) to reduce the disturbances
d) to avoid flicker

View Answer

Answer: d [Reason:] Interlacing is used for doubling the perceived frame rate of a video display without consuming the extra bandwidth. It is mostly used in television to avoid flicker.

10. The signals sent by the TV transmitters to ensure correct scanning in the receiver are called luminance.
a) True
b) False

View Answer

Answer: b [Reason:] The signals sent by the TV transmitters to ensure correct scanning in the receiver are called sync. Luminance refers to the component of television which carries information on the brightness of the image.

11. As distance increases ________
a) Packet size also increases
b) Packet size decreases
c) Packet size becomes twice of its previous value
d) Packet size becomes half

View Answer

Answer: a [Reason:] Distance is directly proportional to packet size. So as distance increases packet size also increase.

12. If 32 equally probable events exists and we have to select one of them, number of required bits is?
a) 2
b) 4
c) 8
d) 5

View Answer

Answer: d [Reason:] 25 = 32, so number of bits required is 5.

Communications MCQ Set 4

1. What is the probability density function of thermal noise?
a) Poisson
b) Gaussian
c) Binomial
d) Armstrong

View Answer

Answer: b [Reason:] Thermal noise is approximately white, it means that its power spectral density is nearly equal throughout the frequency spectrum. The amplitude of the signal has a Gaussian probability density function.

2. Neutralization cancels unwanted feedback by bypassing the feedback to the neutral or ground plane.
a) True
b) False

View Answer

Answer: b [Reason:] Neutralization cancels unwanted feedback by adding feedback out of phase with the unwanted feedback.

3. What is the spectral density of white noise?
a) varies with bandwidth
b) varies with frequency
c) constant
d) infinite

View Answer

Answer: c [Reason:] Random signals are considered as white noise if they are observed to have a flat spectrum. Its spectral density remains constant.

4. Which is the true statement about noise performance of wideband FM system?
a) it is independent of modulation index
b) it is generally poorer than that of an AM system
c) it is independent of signal to noise ratio
d) it exhibits a threshold

View Answer

Answer: d [Reason:] Noise performance of wideband FM system exhibits a threshold.

5. What is the power ratio of one decibel?
a) 10:1
b) 1.26:1
c) 3:1
d) 2.5:1

View Answer

Answer: b [Reason:] Power ratio is a method used by media companies to measure revenue performance compared to the audience share it controls. Power ratio of one decibel is close to 1.26:1.

6. Bit is a unit of ________
a) Channel Capacity
b) Entropy
c) Information
d) Rate of information

View Answer

Answer: c [Reason:] Bit is a unit of information. It is used in computing and digital communications.

7. What is the maximum possible value of entropy?
a) 1
b) ∞
c) 1000
d) 18

View Answer

Answer: a [Reason:] Entropy is the measure of disorder. Higher the disorder, higher is the entropy of the system. The maximum possible value of entropy is 1.

8. If we double the value of resistor then how it affects the thermal noise power?
a) gets doubled
b) gets halved
c) gets quadrupled
d) remains unchanged

View Answer

Answer: d [Reason:] The thermal noise power remains unchanged, if we double the values of given resistors.

9. What is the auto correction function of white noise?
a) Impulse Function
b) Step Function
c) Constant
d) Linear Function

View Answer

Answer: a [Reason:] White noise is a random signal having equal intensity at different frequencies, giving it a constant power spectral density. Its auto correlation function is an impulse function.

10. Pick odd one about:
a) Cosmic noise
b) Galactic noise
c) Atmospheric noise
d) Polar noise

View Answer

Answer: b [Reason:] Galactic noise is a random noise that originates from outside the earth’s atmosphere.

Communications MCQ Set 5

1. In AM receiver, the oscillator frequency is always ________
a) lower than signal frequency
b) higher than signal frequency
c) equal to signal frequency
d) equal to 1000KHz

View Answer

Answer: b [Reason:] The frequency of oscillator in amplitude modulated system is always higher than the frequency of signal.

2. What you understand by the term discone antenna?
a) combination of disc and cone
b) combination of disc and cone with a spacing of ʎ4
c) combination of disc and cone with a spacing of ʎ2
d) same as a simple antenna

View Answer

Answer: a [Reason:] Discone antenna is a combination of disc and cone. It is usually vertically mounted, with disc at top and the cone beneath.

3. Which of the following receiver does not have amplitude limiter stage?
a) AM
b) PM
c) FM
d) Both AM and FM

View Answer

Answer: a [Reason:] Amplitude limiter removes the amplitude of output signal to a desired amplitude irrespective of variations in the input signal. Amplitude modulated system does not need amplitude limiter stage.

4. Most of the amplification of received signal is obtained in a superheterodyne receiver from ________ stage.
a) IF
b) RF
c) Power Amplifier
d) can be from IF or RF

View Answer

Answer: a [Reason:] In superheterodyne receiver, most of the received signals gets amplified from intermediate frequency (IF).

5. Padder capacitor can be used to achieve three point tracking.
a) True
b) False

View Answer

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

6. Any signal and its Hilbert transform are mutually orthogonal.
a) True
b) False

View Answer

Answer: a [Reason:] Consider f(t) be any signal and fh(t) is its Hilbert transform. If we integrate both of them under proper boundary conditions, we get 0 which shows that both are mutually perpendicular i.e. analog-communications-questions-answers-noise-phase-modulation-q6

7. If the output power of radio receiver is doubled then the volume is increased by _________
a) 3 dB
b) 2 dB
c) -3 dB
d) -2 dB

View Answer

Answer: a [Reason:] Volume of a radio receiver is increased by 3 dB if we double the output power of a radio receiver.

8. If selectivity is poor then the receiver has poor _________
a) sensitivity
b) diverse reception
c) blocking
d) transmission and reception

View Answer

Answer: c [Reason:] By selectivity, receiver responds only to the radio signals, it is tuned to and reject the other signals. If a receiver has poor selectivity then obviously its blocking of unwanted signals is also poor.

9. In FM the carrier deviation is determined by ________
a) Frequency
b) Modulating
c) Amplitude
d) Phase

View Answer

Answer: b [Reason:] In frequency modulated system, it is the modulation who is responsible for the deviation of carrier wave.

10. In systems, pre-emphasis is used to amplify ________
a) high frequency
b) low frequency
c) both high and low frequency
d) amplitude

View Answer

Answer: a [Reason:] Pre-emphasis is used in frequency modulated or phase modulated transmitters to equalize the drive power of transmitting signal in terms of deviation ratio. It is done at the transmitter. The modulating signal is passed through a simple network at the transmitter which amplifies the high frequency components.