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Communication Systems MCQs

Practice Communication Systems MCQs from fundamental concepts through intermediate, advanced and application-oriented questions. Each page contains up to 100 questions with answers and explanations where available.

Question 2
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 3
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 4
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 8
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
Question 10
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 13
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 14
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 15
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 17
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 19
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 24
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
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Question 26
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 29
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 30
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 31
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 38
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 43
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 45
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
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Question 51
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
Question 53
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 55
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 56
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 60
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 61
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
Question 63
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 67
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 68
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 69
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 70
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 71
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 72
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 74
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
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Question 79
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
Question 81
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 88
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 89
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 91
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 92
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 96
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
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