Communication Systems — 50 MCQs

Important summary — Communication Systems

Communication systems transfer information from a transmitter to a receiver through a channel. Key topics: analog/digital signals, modulation (AM/FM/PM) to shift baseband to passband, bandwidth requirements (AM bandwidth = 2f_m), multiplexing (FDM/TDM), antennas & radiation patterns, noise types (thermal, shot), SNR, Nyquist & Shannon limits, sampling theorem, PCM, detection & demodulation, and modern digital modulation (ASK/FSK/PSK/QAM). Practice bandwidth calculations, modulation index, and SNR/capacity relations.

1Which modulation varies the amplitude of carrier according to message signal?

A) Amplitude Modulation (AM) ✅

B) Frequency Modulation (FM)

C) Phase Modulation (PM)

D) Pulse Modulation

2In FM, information is carried in:

A) Amplitude of carrier

B) Frequency deviation of carrier ✅

C) Phase only

D) Time delay

3The bandwidth of an AM signal with maximum modulating frequency f_m is approximately:

A) f_m

B) 2 f_m ✅

C) f_m/2

D) 4 f_m

4Carrier frequency is generally much higher than message frequency because:

A) To allow efficient radiation and smaller antennas ✅

B) To reduce message bandwidth

C) To avoid modulation

D) For better noise

5Shannon–Hartley theorem gives channel capacity C = B log₂(1+SNR). Here B is:

A) Bandwidth of channel in Hz ✅

B) Bit rate

C) Signal power

D) Noise power

6Which multiplexing technique separates channels by frequency?

A) TDM

B) FDM (Frequency Division Multiplexing) ✅

C) CDMA

D) PCM

7In amplitude modulation, modulation index m > 1 leads to:

A) Overmodulation and distortion ✅

B) Better reception

C) Reduced bandwidth

D) No change

8Which parameter quantifies the strength of a signal relative to noise?

A) Signal-to-Noise Ratio (SNR) ✅

B) Bandwidth

C) Modulation index

D) Carrier frequency

9FM is preferred over AM for radio broadcasting because:

A) FM has better noise immunity and fidelity ✅

B) FM uses less bandwidth

C) FM transmitters are cheaper

D) AM cannot carry audio

10AM demodulation (envelope detector) fails when there is:

A) High noise and distortion causing envelope crossing ✅

B) No carrier

C) Proper AGC

D) Low modulation index

11Which modulation is used in digital cellular systems for better spectral efficiency?

A) AM

B) PSK / QAM (Phase Shift Keying / Quadrature Amplitude Modulation) ✅

C) FM

D) PWM

12Nyquist criterion for zero intersymbol interference defines maximum data rate for a noiseless channel as:

A) 2 B (symbols per second) ✅

B) 2 B log₂(1+SNR)

C) B log₂(1+SNR)

D) B/2

13Which antenna parameter describes the directional concentration of radiated power?

A) Antenna gain (or directivity) ✅

B) Impedance

C) Bandwidth

D) Polarization

14Which noise is thermal in origin and is present in all resistive components?

A) Shot noise

B) Thermal (Johnson–Nyquist) noise ✅

C) Flicker noise

D) Burst noise

15Pulse Code Modulation (PCM) includes which steps?

A) Sampling, quantization, and encoding ✅

B) Modulation only

C) Demodulation only

D) Filtering only

16Bandwidth required by an FM signal approximately follows Carson's rule: B_T ≈ 2(Δf + f_m). Here Δf is:

A) Frequency deviation of carrier ✅

B) Modulating frequency

C) Carrier frequency

D) Sampling frequency

17Which detection method is commonly used for AM signals?

A) Frequency discriminator

B) Envelope detector ✅

C) Phase-locked loop

D) Correlator

18In communication, attenuation refers to:

A) Increase in signal power

B) Reduction in signal amplitude/power over distance ✅

C) Distortion of waveform

D) Addition of noise

19Which multiplexing is time-based and assigns time slots to channels?

A) FDM

B) TDM (Time Division Multiplexing) ✅

C) SDM

D) WDM

20A matched filter maximizes _____ for a known signal in additive white Gaussian noise (AWGN):

A) Bit rate

B) Signal-to-Noise Ratio (SNR) at sampling instant ✅

C) Bandwidth

D) Modulation index

21Which modulation technique is angle modulation?

A) AM

B) FM and PM ✅

C) PCM

D) ASK

22In digital communication, bit error rate (BER) depends mainly on:

A) SNR and modulation scheme ✅

B) Antenna gain only

C) Bandwidth only

D) Carrier amplitude only

23An ideal low-pass filter has which impulse response?

A) Delta function

B) sinc function (sin x / x) ✅

C) Step function

D) Exponential decay

24Which of these is an example of analog modulation?

A) AM and FM ✅

B) PCM

C) ASK only

D) NRZ coding

25For AM signal s(t) = A_c[1 + m(t)]cos(ω_c t), if m(t)=0.5cos(ω_m t), the modulation index is:

A) 0.5 ✅

B) 1

C) 0.25

D) 2

26Which device converts analog signals to digital by sampling and quantization?

A) DAC

B) ADC (Analog-to-Digital Converter) ✅

C) Mixer

D) Modulator

27FM stereo broadcasting uses what to transmit left and right channels?

A) Single mono signal

B) Multiplexing with pilot tone and L+R & L−R signals ✅

C) Two separate FM stations

D) Time division only

28Which is NOT a source of noise in communication systems?

A) Thermal agitation

B) Intentional jamming ✅

C) Intermodulation

D) Multipath fading

29Coaxial cable is preferred over twisted pair for high frequency because:

A) Lower attenuation and better shielding from interference ✅

B) Cheaper

C) More flexible

D) No bandwidth limits

30Which modulation results in constant amplitude and varying phase?

A) AM

B) PSK (Phase Shift Keying) ✅

C) FM

D) ASK

31In a superheterodyne receiver, incoming RF is converted to a fixed lower frequency called:

A) Baseband

B) Intermediate Frequency (IF) ✅

C) Local Oscillator frequency

D) Nyquist frequency

32Spread spectrum techniques (like CDMA) are used for:

A) Narrowband only

B) Resistance to interference and multiple access ✅

C) Increasing thermal noise

D) Reducing signal power

33Multipath fading occurs due to:

A) Single direct path only

B) Multiple propagation paths causing constructive and destructive interference ✅

C) Low SNR only

D) Antenna failure

34Which sampling theorem defines minimum sampling rate f_s ≥ 2 f_max to avoid aliasing?

A) Nyquist sampling theorem ✅

B) Shannon theorem

C) Parseval theorem

D) Fourier theorem

35Bandwidth of an ideal low-pass channel used for transmitting baseband signals is:

A) Infinite

B) Finite B Hz ✅

C) Always equal to carrier frequency

D) Zero

36Which coding reduces redundancy and error probability in digital transmission?

A) Source coding and channel coding (e.g., Huffman, CRC) ✅

B) AM modulation

C) Analog filtering only

D) No coding

37Optical fiber communication primarily offers advantage of:

A) High attenuation

B) High bandwidth and low loss ✅

C) Easy wireless propagation

D) High interference

38Which device mixes two signals to produce sum and difference frequencies?

A) Filter

B) Mixer (nonlinear device) ✅

C) Oscillator

D) Antenna

39Guard bands are used between adjacent channels to:

A) Increase data rate

B) Prevent interference between channels ✅

C) Reduce power

D) Improve modulation

40Which modulation is resistant to amplitude noise but sensitive to phase errors?

A) ASK

B) FM

C) PSK ✅

D) AM

41Guard interval in OFDM helps to mitigate:

A) Thermal noise

B) Multipath intersymbol interference (ISI) ✅

C) Bandwidth

D) Doppler shift

42Doppler shift in mobile communication is caused by:

A) Relative motion between transmitter and receiver ✅

B) Antenna polarization

C) Multiplexing

D) Time division

43Which is used to increase range of wireless link by matching impedance between antenna and transmitter?

A) Balun / Matching network ✅

B) Mixer

C) Filter only

D) ADC

44FSK (Frequency Shift Keying) represents digital data by:

A) Different amplitudes

B) Different frequencies ✅

C) Different phases

D) Time slots

45Which device regenerates and retimes signals in long-distance digital links?

A) Antenna

B) Repeater / Regenerator ✅

C) Mixer

D) Modulator

46Which modulation is primarily used for satellite TV audio?

A) AM

B) FM

C) PCM

D) QPSK/8PSK (digital modulation) ✅

47Link budget in a communication system accounts for:

A) Gains and losses including transmitter power, antenna gains, path loss and system losses ✅

B) Only transmitter power

C) Only noise

D) Only bandwidth

48Which of the following improves SNR at receiver?

A) Increasing noise power

B) Increasing signal power or using better antennas/filters ✅

C) Reducing bandwidth only

D) Increasing transmitter impedance

49Which term describes unwanted copies of the signal arriving at different times causing fading?

A) Multipath ✅

B) Modulation

C) Sampling

D) Filtering

50Shannon capacity doubles if SNR increases by:

A) 1 (in linear scale)

B) 3 dB approximately (doubling power) ✅

C) 10 dB

D) 20 dB