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DRDO MCQs

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

Question 401
Communication SystemsCommunication Fundamentals

The primary purpose of modulation in a communication system is to?

  • ATranslate a baseband signal to a suitable carrier frequency
  • BEliminate all noise completely
  • CMake information independent of bandwidth
  • DConvert every signal to DC
Report an error
Correct Answer: A. Translate a baseband signal to a suitable carrier frequency
Explanation: Modulation shifts information onto a carrier, enabling practical transmission, frequency allocation and efficient antennas.
Reference: Standard communication systems: communication fundamentals
Question 402
Communication SystemsCommunication Fundamentals

A basic electronic communication system normally consists of?

  • ATransmitter, channel and receiver
  • BOnly an oscillator and resistor
  • CRectifier, filter and regulator only
  • DMotor, generator and transformer
Report an error
Correct Answer: A. Transmitter, channel and receiver
Explanation: The basic communication chain contains a transmitter, a transmission medium or channel, and a receiver.
Reference: Standard communication systems: communication fundamentals
Question 403
Communication SystemsCommunication Fundamentals

The information signal before modulation is commonly called the?

  • ACarrier signal
  • BBaseband or message signal
  • CLocal oscillator signal
  • DIntermediate-frequency signal only
Report an error
Correct Answer: B. Baseband or message signal
Explanation: The original information-bearing signal is called the message or baseband signal.
Reference: Standard communication systems: communication fundamentals
Question 406
Communication SystemsDSB-SC & SSB

In a DSB-SC system, which component is suppressed?

  • ABoth sidebands
  • BCarrier
  • CMessage signal
  • DReceiver local oscillator
Report an error
Correct Answer: B. Carrier
Explanation: Double-sideband suppressed-carrier modulation transmits both sidebands while suppressing the carrier component.
Reference: Standard communication systems: dsb-sc & ssb
Question 409
Communication SystemsDSB-SC & SSB

One important advantage of SSB over conventional AM is?

  • ATwice the bandwidth requirement
  • BReduced bandwidth and improved power efficiency
  • CNeed for a much larger carrier
  • DComplete immunity to all noise
Report an error
Correct Answer: B. Reduced bandwidth and improved power efficiency
Explanation: SSB transmits one sideband and usually suppresses the carrier, saving both bandwidth and transmitted power.
Reference: Standard communication systems: dsb-sc & ssb
Question 410
Communication SystemsAM Demodulation

An envelope detector is most directly suitable for demodulating?

  • AProperly modulated conventional AM with carrier
  • BDSB-SC with no carrier under all conditions
  • CSSB-SC without carrier
  • DBPSK only
Report an error
Correct Answer: A. Properly modulated conventional AM with carrier
Explanation: A simple envelope detector can recover the message from conventional AM when a sufficiently strong carrier is present and overmodulation is avoided.
Reference: Standard communication systems: am demodulation
Question 411
Communication SystemsDemodulation

Coherent detection requires the receiver to have?

  • AA locally generated carrier with correct frequency and phase reference
  • BNo oscillator
  • COnly a diode and capacitor
  • DAn infinite bandwidth antenna
Report an error
Correct Answer: A. A locally generated carrier with correct frequency and phase reference
Explanation: Synchronous or coherent detection multiplies the received signal by a locally generated carrier synchronized in frequency and phase.
Reference: Standard communication systems: demodulation
Question 413
Communication SystemsAmplitude Modulation

In conventional amplitude modulation, which carrier parameter is varied according to the message signal?

  • AAmplitude
  • BFrequency
  • CPhase only
  • DPropagation velocity
Report an error
Correct Answer: A. Amplitude
Explanation: In AM, the carrier amplitude varies in accordance with the instantaneous message signal while carrier frequency and phase remain nominally constant.
Reference: Standard communication systems: amplitude modulation
Question 415
Communication SystemsAmplitude Modulation

For a single-tone AM wave, modulation index m is commonly equal to?

  • AMessage amplitude divided by carrier amplitude
  • BCarrier amplitude divided by message amplitude
  • CCarrier frequency divided by message frequency
  • DMessage frequency multiplied by carrier frequency
Report an error
Correct Answer: A. Message amplitude divided by carrier amplitude
Explanation: For standard sinusoidal AM, m=Am/Ac when the modulating and carrier amplitudes are expressed consistently.
Reference: Standard communication systems: amplitude modulation
Question 420
Communication SystemsFrequency Modulation

A major advantage of FM over conventional AM in many analog radio applications is better?

  • ANoise immunity
  • BDC power conversion
  • CCarrier suppression
  • DAntenna elimination
Report an error
Correct Answer: A. Noise immunity
Explanation: Because information is carried in frequency rather than amplitude, FM can provide improved rejection of amplitude noise and better noise performance under suitable conditions.
Reference: Standard communication systems: frequency modulation
Question 422
Communication SystemsFrequency Modulation

Pre-emphasis and de-emphasis in FM systems are used primarily to improve?

  • AHigh-frequency noise performance
  • BDC supply regulation
  • CAntenna mechanical strength
  • DCarrier power efficiency to 100%
Report an error
Correct Answer: A. High-frequency noise performance
Explanation: Pre-emphasis boosts higher message frequencies before transmission and de-emphasis restores the response at the receiver while reducing high-frequency noise.
Reference: Standard communication systems: frequency modulation
Question 425
Communication SystemsAmplitude Modulation

In conventional AM, the carrier component itself contains?

  • AAll of the message information
  • BNo message information by itself
  • COnly the highest audio frequency
  • DOnly digital data
Report an error
Correct Answer: B. No message information by itself
Explanation: The unmodulated carrier does not itself contain the message; the information resides in the sidebands.
Reference: Standard communication systems: amplitude modulation
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Question 426
Communication SystemsSampling

Aliasing occurs when?

  • ADifferent frequency components become indistinguishable after inadequate sampling
  • BThe carrier amplitude becomes zero
  • CA signal is amplified linearly
  • DQuantization levels become infinite
Report an error
Correct Answer: A. Different frequency components become indistinguishable after inadequate sampling
Explanation: Sampling below the required rate can cause spectral replicas to overlap, making different analog frequencies appear identical in the sampled data.
Reference: Standard communication systems: sampling
Question 427
Communication SystemsSampling

An anti-aliasing filter is normally placed?

  • ABefore the sampler
  • BAfter the final loudspeaker only
  • CInside the DC supply
  • DAcross the antenna mechanically
Report an error
Correct Answer: A. Before the sampler
Explanation: A low-pass anti-aliasing filter limits the input bandwidth before sampling so unwanted high-frequency components do not alias into the desired band.
Reference: Standard communication systems: sampling
Question 432
Communication SystemsSampling

If a 4 kHz sinusoid is sampled at 6 kHz, the sampling theorem is?

  • ASatisfied exactly
  • BSatisfied with large margin
  • CNot satisfied
  • DIndependent of sampling rate
Report an error
Correct Answer: C. Not satisfied
Explanation: For fm=4 kHz, the minimum Nyquist rate is 8 kHz. Sampling at 6 kHz can cause aliasing.
Reference: Standard communication systems: sampling
Question 434
Communication SystemsNoise & SNR

Signal-to-noise ratio is defined as the ratio of?

  • ASignal power to noise power
  • BNoise power to signal power
  • CSignal voltage to bandwidth only
  • DCarrier frequency to message frequency
Report an error
Correct Answer: A. Signal power to noise power
Explanation: SNR compares desired signal power with noise power, S/N.
Reference: Standard communication systems: noise & snr
Question 439
Communication SystemsPCM

PCM stands for?

  • APulse Code Modulation
  • BPhase Carrier Multiplication
  • CPulse Current Mixing
  • DPeriodic Code Measurement
Report an error
Correct Answer: A. Pulse Code Modulation
Explanation: PCM means Pulse Code Modulation, in which sampled amplitudes are quantized and represented by binary code words.
Reference: Standard communication systems: pcm
Question 441
Communication SystemsPCM

The main operations in a basic PCM transmitter are?

  • ASampling, quantization and encoding
  • BRectification, regulation and inversion
  • CMixing, filtering and mechanical switching only
  • DDemodulation, decoding and amplification only
Report an error
Correct Answer: A. Sampling, quantization and encoding
Explanation: A basic PCM transmitter samples the analog signal, quantizes each sample and encodes the quantized value into a binary word.
Reference: Standard communication systems: pcm
Question 447
Communication SystemsPCM

Companding in PCM telephony is mainly used to?

  • AImprove effective quantization performance for low-level signals
  • BEliminate sampling completely
  • CIncrease antenna height
  • DSuppress every binary zero
Report an error
Correct Answer: A. Improve effective quantization performance for low-level signals
Explanation: Nonuniform quantization implemented through companding gives finer effective resolution to weaker signals and improves their quantization SNR.
Reference: Standard communication systems: pcm
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Question 452
Communication SystemsDigital Modulation

In frequency-shift keying (FSK), binary symbols are commonly represented by?

  • ADifferent carrier frequencies
  • BDifferent antenna resistances
  • COnly different DC supply voltages
  • DDifferent sampling periods with no carrier
Report an error
Correct Answer: A. Different carrier frequencies
Explanation: FSK maps different digital symbols to different carrier frequencies.
Reference: Standard communication systems: digital modulation
Question 454
Communication SystemsDigital Modulation

How many distinct carrier phase states are used in QPSK?

  • A2
  • B4
  • C8
  • D16
Report an error
Correct Answer: B. 4
Explanation: Quadrature phase-shift keying uses four phase states, allowing each symbol to represent two bits.
Reference: Standard communication systems: digital modulation
Question 455
Communication SystemsDigital Modulation

Each QPSK symbol carries how many bits?

  • A1
  • B2
  • C3
  • D4
Report an error
Correct Answer: B. 2
Explanation: Four possible symbols correspond to log2(4)=2 bits per symbol.
Reference: Standard communication systems: digital modulation
Question 457
Communication SystemsInformation Theory

According to Shannon’s capacity formula for an AWGN channel, channel capacity is?

  • AC=B log2(1+S/N)
  • BC=2B only
  • CC=B(S/N) without logarithm
  • DC=S/N divided by B
Report an error
Correct Answer: A. C=B log2(1+S/N)
Explanation: Shannon's formula gives the theoretical maximum reliable information rate C=B log2(1+S/N) for an ideal band-limited AWGN channel.
Reference: Standard communication systems: information theory
Question 461
Communication SystemsNoise Figure

The noise factor of an ideal noiseless receiver is?

  • A0
  • B1
  • C2
  • DInfinite
Report an error
Correct Answer: B. 1
Explanation: Noise factor F=SNRin/SNRout. An ideal noiseless system does not degrade SNR, so F=1, corresponding to 0 dB noise figure.
Reference: Standard communication systems: noise figure
Question 462
Communication SystemsNoise Figure

In a cascade of receiver stages, the overall noise performance is especially sensitive to the noise characteristics of the?

  • AFirst stage
  • BLast stage only
  • CPower supply only
  • DAntenna cable length only regardless of loss
Report an error
Correct Answer: A. First stage
Explanation: Friis' formula shows that noise added by early stages is most important because later-stage noise contributions are divided by preceding gain.
Reference: Standard communication systems: noise figure
Question 463
Communication SystemsReceiver Fundamentals

A low-noise amplifier at the front end of a receiver is mainly used to?

  • AProvide gain while adding as little noise as practical
  • BSuppress the desired signal
  • CConvert all signals to DC
  • DMake channel bandwidth infinite
Report an error
Correct Answer: A. Provide gain while adding as little noise as practical
Explanation: An LNA amplifies weak received signals with minimal added noise, helping preserve receiver SNR.
Reference: Standard communication systems: receiver fundamentals
Question 464
Communication SystemsDigital Receivers

The main purpose of a matched filter in a digital receiver is to?

  • AMaximize output SNR at a specified sampling instant for a known signal in AWGN
  • BEliminate the need for synchronization
  • CMake all symbol waveforms identical
  • DGenerate carrier power
Report an error
Correct Answer: A. Maximize output SNR at a specified sampling instant for a known signal in AWGN
Explanation: For a known signal in additive white Gaussian noise, the matched filter maximizes the signal-to-noise ratio at the decision sampling instant.
Reference: Standard communication systems: digital receivers
Question 465
Communication SystemsDigital Receivers

Intersymbol interference (ISI) occurs when?

  • AOne transmitted symbol interferes with neighboring symbols
  • BNoise power becomes exactly zero
  • COnly one symbol is transmitted
  • DCarrier frequency equals zero
Report an error
Correct Answer: A. One transmitted symbol interferes with neighboring symbols
Explanation: Channel dispersion or bandwidth limitations can spread pulses so that adjacent symbol intervals overlap, causing ISI.
Reference: Standard communication systems: digital receivers
Question 466
Communication SystemsDigital Receivers

The Nyquist zero-ISI criterion is concerned primarily with?

  • APulse shaping to avoid interference at symbol sampling instants
  • BIncreasing thermal noise
  • CGenerating AM sidebands
  • DReducing antenna length
Report an error
Correct Answer: A. Pulse shaping to avoid interference at symbol sampling instants
Explanation: Nyquist pulse-shaping conditions allow neighboring pulses to have zero contribution at the desired sampling instants, ideally eliminating ISI.
Reference: Standard communication systems: digital receivers
Question 468
Communication SystemsError Performance

For binary signaling, bit error rate (BER) is defined as?

  • ANumber of erroneous bits divided by total transmitted or received bits considered
  • BNumber of correct bits divided by bandwidth
  • CCarrier power divided by noise power
  • DSymbols per second divided by bits per symbol
Report an error
Correct Answer: A. Number of erroneous bits divided by total transmitted or received bits considered
Explanation: BER is the fraction of bits detected incorrectly over the observation interval.
Reference: Standard communication systems: error performance
Question 473
Communication SystemsDigital Communication Fundamentals

The term baud refers to the number of?

  • ABits transmitted per second in every case
  • BSymbols transmitted per second
  • CBytes transmitted per second
  • DCarrier cycles per bit only
Report an error
Correct Answer: B. Symbols transmitted per second
Explanation: Baud is the symbol rate. It equals bit rate only when each symbol represents one bit.
Reference: Standard communication systems: digital communication fundamentals
Question 474
Communication SystemsError Performance

Which digital modulation among coherent BPSK and coherent QPSK has the same theoretical BER versus Eb/N0 in ideal AWGN when Gray mapping and coherent detection are used?

  • AQPSK can have the same BER as BPSK
  • BQPSK must always have twice the BER
  • CBPSK always has zero BER
  • DNeither can operate in AWGN
Report an error
Correct Answer: A. QPSK can have the same BER as BPSK
Explanation: Coherent Gray-coded QPSK can be viewed as two orthogonal BPSK channels, giving the same theoretical bit-error performance versus Eb/N0 in ideal AWGN.
Reference: Standard communication systems: error performance
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Question 476
Communication SystemsReceivers

In a superheterodyne receiver, the intermediate frequency is produced by a?

  • AMixer
  • BPower amplifier only
  • CEnvelope capacitor only
  • DAntenna reflector
Report an error
Correct Answer: A. Mixer
Explanation: The mixer combines the RF input and local-oscillator signal, producing sum and difference frequency components; one is selected as the IF.
Reference: Standard communication systems: receivers
Question 483
Communication SystemsInformation Theory

For a binary symmetric channel with crossover probability p=0, its capacity is?

  • A0 bit per channel use
  • B0.5 bit per channel use
  • C1 bit per channel use
  • D2 bits per channel use
Report an error
Correct Answer: C. 1 bit per channel use
Explanation: With no crossover errors, a binary input can convey one bit of information per channel use.
Reference: Standard communication systems: information theory
Question 484
Communication SystemsReceivers

The main purpose of automatic gain control (AGC) in a receiver is to?

  • AMaintain a more nearly constant output level despite changes in received signal strength
  • BIncrease carrier frequency continuously
  • CEliminate the local oscillator
  • DCreate intentional aliasing
Report an error
Correct Answer: A. Maintain a more nearly constant output level despite changes in received signal strength
Explanation: AGC automatically varies receiver gain so large variations in received signal strength produce smaller variations in output level.
Reference: Standard communication systems: receivers
Question 486
Communication SystemsFM Receivers

In FM receivers, a limiter stage is used primarily to?

  • ARemove unwanted amplitude variations before frequency detection
  • BIncrease AM modulation index
  • CGenerate PCM codes
  • DReduce IF to zero in every design
Report an error
Correct Answer: A. Remove unwanted amplitude variations before frequency detection
Explanation: Since ideal FM information is carried in frequency, amplitude variations caused by noise can be clipped or limited before demodulation.
Reference: Standard communication systems: fm receivers
Question 487
Communication SystemsFM Receivers

The capture effect is most strongly associated with?

  • AFM reception
  • BIdeal PCM quantization
  • CPure DSB-SC generation
  • DTDM framing only
Report an error
Correct Answer: A. FM reception
Explanation: In FM, when two signals at nearly the same frequency are present, the stronger signal can dominate receiver output; this is called the capture effect.
Reference: Standard communication systems: fm receivers
Question 491
Communication SystemsAngle Modulation

For a sinusoidal message in phase modulation, increasing the message frequency while keeping message amplitude and phase sensitivity fixed causes the peak frequency deviation to?

  • ADecrease
  • BRemain zero
  • CIncrease
  • DRemain independent of frequency
Report an error
Correct Answer: C. Increase
Explanation: In PM, instantaneous frequency deviation is proportional to the time derivative of the message, so for a sinusoid its peak value increases with message frequency.
Reference: Standard communication systems: angle modulation
Question 496
Communication SystemsReceivers

A superheterodyne receiver converts a selected incoming RF signal to a fixed?

  • AAudio frequency only
  • BIntermediate frequency
  • CZero frequency in every receiver
  • DSampling frequency
Report an error
Correct Answer: B. Intermediate frequency
Explanation: A superheterodyne receiver mixes the selected RF signal with a local oscillator to produce a fixed intermediate frequency for convenient filtering and amplification.
Reference: Standard communication systems: receivers
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