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RRB MCQs

Practice RRB-oriented Electronics & Telecommunication Engineering MCQs with answers and explanations for technical exam preparation. Questions are arranged in pages of up to 100.

Question 1801
Communication SystemsReceivers

In a superheterodyne receiver, increasing the intermediate frequency generally improves?

  • AImage-frequency rejection for a given RF selectivity
  • BAudio output power automatically
  • CModulation index of the transmitter
  • DSampling resolution
Report an error
Correct Answer: A. Image-frequency rejection for a given RF selectivity
Explanation: A higher IF increases image separation from the desired RF signal, which can make image rejection easier for a given RF front-end selectivity.
Reference: Standard communication systems: IF trade-off
Question 1804
Communication SystemsDSB-SC & SSB

The phase-shift method of SSB generation ideally requires two versions of the message signal differing in phase by?

  • A45 degrees
  • B90 degrees
  • C180 degrees
  • D360 degrees
Report an error
Correct Answer: B. 90 degrees
Explanation: The phase-shift method uses quadrature message components and quadrature carrier components so one sideband cancels while the other adds.
Reference: Standard communication systems: phase-shift SSB generation
Question 1805
Communication SystemsVSB Modulation

A vestigial-sideband signal transmits?

  • ABoth complete sidebands with full carrier only
  • BOne complete sideband plus a vestige of the other
  • CCarrier only
  • DNo sideband components
Report an error
Correct Answer: B. One complete sideband plus a vestige of the other
Explanation: VSB retains one full sideband and a small portion of the other, reducing bandwidth while easing practical filtering.
Reference: Standard communication systems: VSB
Question 1806
Communication SystemsVSB Modulation

VSB modulation is widely associated with television video transmission mainly because it?

  • ARequires zero bandwidth
  • BProvides a compromise between SSB bandwidth saving and practical filtering
  • CEliminates all noise
  • DNeeds no carrier recovery under any implementation
Report an error
Correct Answer: B. Provides a compromise between SSB bandwidth saving and practical filtering
Explanation: Video contains significant low-frequency content, making sharp SSB filtering difficult; VSB offers practical filtering with reduced bandwidth.
Reference: Standard communication systems: VSB application
Question 1809
Communication SystemsFrequency Modulation

For narrowband FM, the modulation index beta is generally?

  • AMuch less than 1
  • BExactly 1 in every case
  • CMuch greater than 1
  • DInfinite
Report an error
Correct Answer: A. Much less than 1
Explanation: Narrowband FM is characterized by a small modulation index, beta<<1, so higher-order sidebands are negligible.
Reference: Standard communication systems: narrowband FM
Question 1810
Communication SystemsAngle Modulation

In an ideal angle-modulated signal, the total transmitted power is essentially independent of?

  • ACarrier amplitude
  • BMessage-induced frequency or phase variation
  • CLoad resistance in every transmitter
  • DAntenna efficiency
Report an error
Correct Answer: B. Message-induced frequency or phase variation
Explanation: FM and PM have constant envelope in the ideal model, so total average signal power does not vary with modulation index when carrier amplitude is fixed.
Reference: Standard communication systems: constant-envelope power
Question 1812
Communication SystemsReceivers

In a superheterodyne receiver, using a lower intermediate frequency generally makes it easier to obtain?

  • ABetter adjacent-channel selectivity
  • BLarger image separation
  • CHigher antenna gain
  • DHigher transmitted power
Report an error
Correct Answer: A. Better adjacent-channel selectivity
Explanation: A lower IF permits narrower practical IF filters and can improve selectivity, although it worsens image-frequency separation.
Reference: Standard communication systems: IF selectivity trade-off
Question 1817
Communication SystemsNoise in Communication Systems

For cascaded stages, Friis’ formula shows that the overall receiver noise performance is strongly influenced by?

  • AThe noise factor and gain of the first stage
  • BOnly the last-stage output power
  • CAntenna color
  • DAudio-frequency volume setting
Report an error
Correct Answer: A. The noise factor and gain of the first stage
Explanation: In Friis' cascade formula, noise contributions of later stages are divided by preceding gains, making a low-noise high-gain first stage especially important.
Reference: Standard communication systems: Friis formula
Question 1822
Communication SystemsInformation Theory

For a discrete memoryless channel, channel capacity is the maximum of mutual information I(X;Y) over?

  • AInput probability distributions
  • BCarrier frequencies only
  • CReceiver temperatures only
  • DSampling phases only
Report an error
Correct Answer: A. Input probability distributions
Explanation: Channel capacity is obtained by maximizing the input-output mutual information over permissible input distributions.
Reference: Standard communication systems: channel capacity definition
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Question 1826
Communication SystemsInformation Theory

The mutual information I(X;Y) is zero when X and Y are statistically?

  • AIdentical in every sample
  • BIndependent
  • CGaussian only
  • DPeriodic
Report an error
Correct Answer: B. Independent
Explanation: For statistically independent random variables, observing Y provides no information about X, so mutual information is zero.
Reference: Standard communication systems: mutual information
Question 1828
Communication SystemsSource Coding

The main objective of source coding is to?

  • AReduce statistical redundancy in the source representation
  • BAdd controlled redundancy for error correction
  • CIncrease receiver thermal noise
  • DCreate image-frequency interference
Report an error
Correct Answer: A. Reduce statistical redundancy in the source representation
Explanation: Source coding represents information efficiently by reducing removable redundancy.
Reference: Standard communication systems: source coding
Question 1829
Communication SystemsChannel Coding

The main objective of channel coding is to?

  • ARemove every redundant bit
  • BIntroduce structured redundancy to improve error detection or correction
  • CSuppress the RF carrier only
  • DReduce sampling rate below Nyquist
Report an error
Correct Answer: B. Introduce structured redundancy to improve error detection or correction
Explanation: Channel coding deliberately adds controlled redundancy so the receiver can detect or correct transmission errors.
Reference: Standard communication systems: channel coding
Question 1830
Communication SystemsSource Coding

A prefix code has the useful property that?

  • ANo codeword is a prefix of another codeword
  • BEvery codeword has exactly one bit
  • CAll source probabilities must be equal
  • DIt cannot be decoded sequentially
Report an error
Correct Answer: A. No codeword is a prefix of another codeword
Explanation: The prefix condition permits instantaneous decoding because a complete codeword cannot be confused with the beginning of another.
Reference: Standard communication systems: prefix code
Question 1831
Communication SystemsSource Coding

Huffman coding assigns shorter codewords generally to symbols with?

  • ALower probabilities
  • BHigher probabilities
  • CZero probability only
  • DLonger analog durations
Report an error
Correct Answer: B. Higher probabilities
Explanation: Huffman coding minimizes average codeword length by assigning shorter words to more probable symbols.
Reference: Standard communication systems: Huffman coding
Question 1835
Communication SystemsChannel Coding

For a systematic linear block code, a codeword contains?

  • AOnly parity bits
  • BOriginal information bits explicitly plus parity bits
  • COnly carrier samples
  • DNo redundancy
Report an error
Correct Answer: B. Original information bits explicitly plus parity bits
Explanation: In systematic form, the information bits appear unchanged in specified codeword positions and parity bits occupy the remaining positions.
Reference: Standard communication systems: systematic block code
Question 1836
Communication SystemsChannel Coding

For an (n,k) block code, the code rate is?

  • An/k
  • Bk/n
  • Cn-k
  • D1/(n+k)
Report an error
Correct Answer: B. k/n
Explanation: A block code maps k information bits into n coded bits, so its rate is R=k/n.
Reference: Standard communication systems: code rate
Question 1839
Communication SystemsChannel Coding

In polynomial representation of cyclic codes, multiplication by x followed by reduction modulo x^n-1 corresponds to?

  • AA cyclic shift
  • BAmplitude clipping
  • CSampling
  • DFrequency translation only
Report an error
Correct Answer: A. A cyclic shift
Explanation: Multiplication by x shifts polynomial coefficients; modulo x^n-1 wraps the end coefficient around, implementing a cyclic shift.
Reference: Standard communication systems: cyclic-code polynomial representation
Question 1840
Communication SystemsChannel Coding

In a trellis representation of a convolutional code, a path through the trellis represents?

  • AA possible encoder state and output sequence over time
  • BOnly receiver bandwidth
  • CA single analog carrier cycle
  • DThermal-noise temperature
Report an error
Correct Answer: A. A possible encoder state and output sequence over time
Explanation: Each trellis path corresponds to a possible sequence of encoder state transitions and associated coded outputs.
Reference: Standard communication systems: convolutional-code trellis
Question 1841
Communication SystemsError Control

Interleaving in a digital communication system is especially useful for combating?

  • ABurst errors
  • BCarrier generation
  • CQuantization itself
  • DAntenna polarization mismatch only
Report an error
Correct Answer: A. Burst errors
Explanation: Interleaving spreads adjacent corrupted bits or symbols across different codewords so a burst can appear as more manageable separated errors to the decoder.
Reference: Standard communication systems: interleaving
Question 1842
Communication SystemsError Control

A burst-error channel corrupts 12 consecutive coded bits. An idealized block interleaver primarily helps by?

  • ASpreading those errors across separated decoder positions
  • BIncreasing the RF carrier power automatically
  • CChanging every 0 into 1
  • DRemoving the need for channel coding
Report an error
Correct Answer: A. Spreading those errors across separated decoder positions
Explanation: The interleaver rearranges transmission order, so consecutive channel errors are dispersed after deinterleaving.
Reference: Standard communication systems: burst-error interleaving
Question 1843
Communication SystemsError Control

Automatic repeat request (ARQ) primarily combines error detection with?

  • ARetransmission requests
  • BAnalog frequency modulation
  • CSource entropy maximization
  • DCarrier suppression
Report an error
Correct Answer: A. Retransmission requests
Explanation: ARQ detects erroneous or missing blocks and requests retransmission rather than relying solely on forward correction.
Reference: Standard communication systems: ARQ
Question 1844
Communication SystemsError Control

Forward error correction (FEC) differs from basic ARQ because FEC aims to?

  • ACorrect errors at the receiver without requiring retransmission for each correctable error
  • BEliminate all redundancy
  • COperate only on analog AM signals
  • DMake channel noise exactly zero
Report an error
Correct Answer: A. Correct errors at the receiver without requiring retransmission for each correctable error
Explanation: FEC adds sufficient coding redundancy for the receiver to correct specified error patterns without requesting retransmission.
Reference: Standard communication systems: forward error correction
Question 1845
Communication SystemsChannel Coding

A convolutional encoder is characterized by the fact that its output depends on?

  • AOnly the present input bit
  • BPresent input and encoder memory state
  • COnly receiver noise
  • DCarrier amplitude alone
Report an error
Correct Answer: B. Present input and encoder memory state
Explanation: Convolutional coding has memory: coded outputs depend on the current input and previous input information stored in the encoder state.
Reference: Standard communication systems: convolutional coding
Question 1846
Communication SystemsChannel Coding

The Viterbi algorithm is widely used for?

  • AMaximum-likelihood sequence decoding of convolutional codes
  • BAM envelope detection
  • CFM generation by reactance modulation
  • DAnalog source compression only
Report an error
Correct Answer: A. Maximum-likelihood sequence decoding of convolutional codes
Explanation: The Viterbi algorithm searches a trellis efficiently for the most likely transmitted state sequence of a convolutional code.
Reference: Standard communication systems: Viterbi decoding