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Communications MCQ Set 1

1. Adding improvement to the signal subset choice by starting with the densest possible lattice for the space is called as
a) Optimum signal constellation boundaries
b) Higher density lattice structures
c) Trellis coded modulation
d) None of the mentioned

View Answer

Answer: b [Reason:] Higher density lattice structures are formed by adding improvement to the signal subset choice by starting with the densest possible lattice for the space.

2. Choosing a closely packed signal subset from any regular lattice or array of candidate points is called as
a) Optimum signal constellation boundaries
b) Higher density lattice structures
c) Trellis coded modulation
d) None of the mentioned

View Answer

Answer: a [Reason:] Optimum signal constellation boundaries are obtained by choosing closely packed signal subset from any regular lattice or array of candidate points.

3. Trellis coded modulation uses
a) Non binary method
b) Uses redundant bits
c) No expansion of bandwidth
d) All of the mentioned

View Answer

Answer: d [Reason:] Trellis coded modulation is a achieves error performance improvements without expansion of bandwidth. It uses redundant non binary modulation in combination with finite state machines.

4. In finite state machines, output is predicted using
a) Past output
b) Present input
c) Past output & Present input
d) None of the mentioned

View Answer

Answer: c [Reason:] In finite state machines, the set of possible future outputs are predicted using present input and past output.

5. In TCM the signals are decoded using
a) Soft decision decoders
b) Hard decision decoders
c) Soft & Hard decision decoders
d) None of the mentioned

View Answer

Answer: a [Reason:] In trellis coded modulation the noisy received signals are decoded using soft decision decoders.

6. Trellis coded modulation scheme
a) Has memory
b) Is memory-less
c) Needs more bandwidth
d) None of the mentioned

View Answer

Answer: a [Reason:] Trellis coded modulation scheme has memory ( a finite state machine) such as the convolutional code.

7. In TCM channel capacity can be increased.
a) True
b) False

View Answer

Answer: a [Reason:] In trellis coded modulation increase in channel capacity is achieved by signal set expansion.

8. Transmission bandwidth depends on
a) Rate of signalling
b) Density of signal points
c) Both of the mentioned
d) None of the mentioned

View Answer

Answer: a [Reason:] Transmission bandwidth of non orthogonal signalling depends on rate of signalling and not on density of signal points in the constellation.

9. Which determines the error performance in TCM?
a) Reduced distance
b) Free distance
c) Bandwidth
d) None of the mentioned

View Answer

Answer: b [Reason:] Free distance which is the minimum distance between members of set of allowed code sequences determines the error performance.

10. The TCM decoder uses
a) Minimum likelihood detector
b) Maximum likelihood detector
c) Does not use redundant bits
d) None of the mentioned

View Answer

Answer: b [Reason:] The task of convolutional decoder is to estimate the path that the message had traversed through the encoding trellis using maximum likelihood detector.

Communications MCQ Set 2

1. A stationary stochastic process has
a) Finite energy signal
b) Infinite zero signal
c) Zero energy signal
d) None of the mentioned

View Answer

Answer: b [Reason:] A stationary stochastic process is an infinite energy signal and hence its fourier transform does not exist.

2. The power spectral density function of stochastic process is
a) Real
b) Odd
c) Real & odd
d) None of the mentioned

View Answer

Answer: a [Reason:] The power spectral density function of a stochastic function is real and even.

3. For a periodic function, the spectral density and auto correlation functions form
a) Fourier transform pair
b) Laplace transform pair
c) Hilbert transform pair
d) Z transform pair

View Answer

Answer: a [Reason:] For a periodic function, the spectral density and auto correlation function forms fourier transform pair.

4. The capacity of a channel is given by
a) Number of digits used in coding
b) Volume of information it can take
c) Maximum rate of information transmitted
d) Bandwidth requires for information

View Answer

Answer: c [Reason:] The capacity of channel is given by maximum rate of information transmitted.

5. In a communication system, a process in which statistical averages and time averages are equal is called as
a) Stationary
b) Ergodic
c) Gaussian
d) Poisson

View Answer

Answer: b [Reason:] In ergodic process statistical averages and time averages are equal.

6. A rectangular pulse of duration T is applied to a matched filter. The output of the filter is a
a) Rectangular pulse of duration T
b) Rectangular pulse of duration 2T
c) Triangular pulse
d) Sine function

View Answer

Answer: c [Reason:] The output of matched filter when rectangular pulse of duration T is applied is triangular pulse.

7. The line code has a zero dc component for pulse transmission of random binary data is
a) NRZ
b) RZ
c) Alternate mark inversion
d) None of the mentioned

View Answer

Answer: c [Reason:] The line code has a zero dc component for pulse transmission of random binary data is alternate mark inversion.

8. The auto-correlation of white noise is
a) A delta function
b) A constant
c) Gaussian
d) None of the mentioned

View Answer

Answer: a [Reason:] The auto-correlation of white noise is a delta function.

9. Ionospheric communication can take place in
a) High frequency band
b) Very high frequency band
c) Ultra high frequency band
d) Super high frequency band

View Answer

Answer: a [Reason:] Ionospheric communication takes place in high frequency band.

10. Satellite communication takes place in
a) Ultra high frequency band
b) Super high frequency band
c) Ultra & Super high frequency band
d) None of the mentioned

View Answer

Answer: c [Reason:] Satellite communication takes place in ultra high and super high frequency bands.

11. The weighing matrix is
a) Positive quantity
b) Inverse of covariance matrix of the input vector
c) Both of the mentioned
d) None of the mentioned

View Answer

Answer: c [Reason:] W is a positive definite weighing matrix and it is inverse of covariance matrix of input vector.

12. Non uniform tree _____ bit rate.
a) Increases
b) Decreases
c) Does not affect
d) None of the mentioned

View Answer

Answer: a [Reason:] Non uniform tree for binary search increases the bit rate.

13. Two clock with rates T1 and T2 are incommensurate if its ratio is
a) Rational
b) Irrational
c) Equal
d) None of the mentioned

View Answer

Answer: b [Reason:] Two clock with rates T1 and T2 are incommensurate if its ratio is irrational.

14. Possible timing error detection methods are
a) Early late
b) Zero crossing
c) Early late & Zero crossing
d) None of the mentioned

View Answer

Answer: c [Reason:] Methods for timing error detections are early late timing error detection and zero crossing timing error detection method.

15. Which has minimum power loss during transmission?
a) Twisted Copper cable at 1KHz
b) Optical fiber
c) Twisted copper cable at 100KHz
d) Wave guide

View Answer

Answer: a [Reason:] Twisted copper wire at 1KHz have low power loss.

16. Voltage generated by lightning is an example for
a) Energy signal
b) Power signal
c) Energy & Power signal
d) None of the mentioned

View Answer

Answer: a [Reason:] Voltage generated by lightning is an example for energy signal and it has very high power.

Communications MCQ Set 3

1. Channel coding relates to area such as
a) Waveform coding
b) Structured sequence
c) Waveform coding & Structured sequence
d) None of the mentioned

View Answer

Answer: c [Reason:] Channel coding can be partitioned into two area of studies, one is waveform coding or signal design and the other is structured sequence or structured redundancy.

2. Examples of structured sequences are
a) Block codes
b) Convolutional codes
c) Turbo codes
d) All of the mentioned

View Answer

Answer: d [Reason:] Some examples of structured sequence or structured redundancy codes are block codes, convolutional codes and turbo codes.

3. Antipodal signals are
a) Mirror images
b) 1800 apart
c) One is negative of the other
d) All of the mentioned

View Answer

Answer: d [Reason:] Antipodal signals are mirror images, one is negative of the other and they are 1800 apart.

4. Orthogonal vectors are
a) Perpendicular to each other
b) Their dot product must be zero
c) One signal cannot interfere with the other
d) All of the mentioned

View Answer

Answer: d [Reason:] Orthogonal signals are perpendicular to each other, the inner or dot product between those two signals must equal zero, one vector has zero projection over the other because they do not share same signal space.

5. In orthogonal signalling as k increase there is
a) Improved error performance
b) Degraded error performance
c) Improved bandwidth
d) None of the mentioned

View Answer

Answer: a [Reason:] In orthogonal signalling as k increases there is improved error performance or a reduction in required Eb/N0 at the expense of bandwidth.

6. In non orthogonal signalling as k increases there is
a) Degraded error performance
b) Improved bandwidth efficiency
c) Increase in required Eb/N0
d) All of the mentioned

View Answer

Answer: d [Reason:] Non orthogonal signalling manifests improved bandwidth efficiency at the expense of degraded error performance and increase in required Eb/N0.

7. In orthogonal signal, all cross correlation coefficients are
a) One
b) Zero
c) Negative
d) None of the mentioned

View Answer

Answer: b [Reason:] In orthogonal set all cross correlation coefficients can be made zero.

8. The smaller the cross correlation, the ______ is the distance between the signal vectors.
a) More
b) Less
c) Double
d) Half

View Answer

Answer: a [Reason:] The cross correlation is the measure of distance between two vectors. The smaller the cross correlation value the greater the distance between the two vectors.

9. Hadamard matrix is given as
a) H2 = H1 H1
H1 H1
b) H2 = H1 H1
H1’H1
c) H2 = H1 H1
H1 H1′
d) H2 = H1’H1
H1 H1

View Answer

Answer: c [Reason:] We can construct a code-word set Hk of dimension 2k*2k called a Hadamard matrix.

10. Biorthogonal set consists of
a) Orthogonal signals
b) Antipodal signals
c) Combination of orthogonal and antipodal
d) None of the mentioned

View Answer

Answer: c [Reason:] The biorthogonal set consists of a combination of both orthogonal and antipodal signals.

11. Which codes perform better?
a) Orthogonal
b) Biorthogonal
c) Orthogonal & Biorthogonal
d) None of the mentioned

View Answer

Answer: b [Reason:] Since antipodal vector has have better distance property than the orthogonal signals, biorthogonal codes perform better than the orthogonal codes.

12. Biorthogonal codes needs ________ bandwidth as orthogonal codes.
a) Equal
b) Double
c) Half
d) Triple

View Answer

Answer: c [Reason:] Biorthogonal codes perform better than the orthogonal codes and it needs only half the bandwidth as orthogonal codes.

13. Transorthogonal code is obtained by
a) Deleting last digit of each orthogonal codeword
b) Deleting first digit of each orthogonal codeword
c) Doubling each digit of orthogonal codeword
d) Taking negative of each digit of orthogonal codeword

View Answer

Answer: b [Reason:] A code generated from an orthogonal set by deleting the first digit of each codeword is called as transorthogonal code or simplex code.

14. Which code requires minimum Eb/N0?
a) Orthogonal
b) Biorthogonal
c) Simplex
d) All of the mentioned

View Answer

Answer: c [Reason:] A simplex code represents the minimum energy equivalent and it requires minimum Eb/N0 for a specified symbol error rate.

Communications MCQ Set 4

1. A Marconi antenna has length equal to ________
a) ʎ4
b) ʎ2
c) 1
d) ʎ8

View Answer

Answer: a [Reason:] Marconi antenna is like a vertical wire having length one fourth of the working wavelength. It requires a ground plane with very good conductivity.

2. An antenna can be matched to a feed line by using an LC network only.
a) True
b) False

View Answer

Answer: b [Reason:] An antenna can be matched to a feed line by using an LC network. But it can also be matched to feed line by a loading coil.

3. An array with one driven element, a reflector, and one or more directors is called ________
a) Yagi
b) Marconi
c) Stack
d) Isotropic

View Answer

Answer: a [Reason:] A Yagi-Uda antenna is a directional antenna consisting of many parallel elements in a line. It consists of a single driven element which is connected to the transmitter or receiver with a transmission line, and also contains parasitic elements which are not connected to the transmitter or receiver.

4. The energy picked up by a parabolic antenna is concentrated at the ________
a) edges
b) center
c) left corner
d) focus

View Answer

Answer: d [Reason:] Energy picked up by a parabolic antenna is concentrated at the focus.

5. Antennas are often tested in ________
a) an RF reflective chamber
b) a vacuum chamber
c) an anechoic chamber
d) an echo chamber

View Answer

Answer: c [Reason:] An antenna is basically a transducer that converts radio frequency fields into alternating current or vice versa. They can be used for both transmission and reception of radio signals. They are often tested in an anechoic chamber.

6. Which one of the following consists of non-resonant antennas?
a) folded dipole
b) end-fire array
c) rhombic antenna
d) broadside array

View Answer

Answer: c [Reason:] Non-resonant antennas are like non-resonant transmission lines. Non-resonant transmission lines do not have standing waves. Hence they do not have any reflected wave and they have only forward travelling wave.

7. Which one of the following is very useful as a multiband HF receiving antenna?
a) folded dipole
b) log periodic
c) conical horn
d) square loop

View Answer

Answer: b [Reason:] A log periodic antenna is a multi-element, directional, antenna designed to operate over a wide band of frequencies.

8. Which among the following is not a reason for the use of antenna coupler?
a) To make the antenna look resistive
b) To provide the output amplifier with correct load impedance
c) To discriminate against harmonics
d) To prevent re-radiation of the local oscillator

View Answer

Answer: d [Reason:] Antenna coupler is a device connected between a radio transmitter or receiver and its antenna to improve power transfer between them. Antenna coupler is never used to prevent re-radiation of the local oscillator.

9. Which of the following term is not applied to the Yagi-Uda array?
a) folded dipole
b) high gain
c) good bandwidth
d) high reception of signals

View Answer

Answer: b [Reason:] A Yagi-Uda antenna is a directional antenna consisting of many parallel elements in a line. It consists of a single driven element and also contains parasitic elements.

10. Why top loading is used with an antenna?
a) to increase its effective height
b) to increase its beamwidth
c) to increase its bandwidth
d) to increase its input capacitance

View Answer

Answer: a [Reason:] When the length of the antenna is shorter than a quarter wavelength ( ʎ4), top loading is used to increase its effective height.

Communications MCQ Set 5

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

View Answer

Answer: c [Reason:] Aspect ratio is the ratio of width to the height of image (or screen). Most common videographic 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

View Answer

Answer: a [Reason:] Signal to Noise ratio compares signal power to noise power. 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 halves the bandwidth
b) It increases the bandwidth
c) It decreases the bandwidth by 25%
d) It decreases the bandwidth by 37%

View Answer

Answer: a [Reason:] The two sidebands in DSB-SC wave are exactly same, so in SSB-SC one sideband is removed which reduces its bandwidth to half.

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

View Answer

Answer: d [Reason:] 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

View Answer

Answer: a [Reason:] A signal and its Hilbert transform have same amplitude spectrum i.e. they have same energy density spectrum and also correlation spectrum.

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

View Answer

Answer: a [Reason:] Power density of quadrature component is analog-communications-interview-questions-answers-q6

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

View Answer

Answer: d [Reason:] Frequency of DC signals is zero. For zero frequency, capacitive reactance XC is infinite.

8. If a FM signal having modulation index m_f 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

View Answer

Answer: b [Reason:] 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

View Answer

Answer: a [Reason:] 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 whom we called as slope overload and Granularity occurs when step size is too large.

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

View Answer

Answer: b [Reason:] Angular separation between the two half power points are called as beam width. It is usually expressed in metres.