27 real Communication Systems questions from the ECE Core bank, as asked in Indian campus drives and tech interviews. Every question has a verified answer and an AI-tutor explanation on placd — free to start.
1. What is AM modulation index?
Junior
A.FM bandwidth is approximately twice the sum of peak frequency deviation and highest modulating frequency, giving about 180 kHz for broadcast FM with 75 kHz deviation
B.Nyquist filter that gives zero intersymbol interference at sampling instants with bandwidth (1 + α)Rs/2, where roll-off α trades bandwidth against timing tolerance
C.ratio of message amplitude to carrier amplitude; values above 1 cause over-modulation and envelope distortion, and at 1 the sidebands carry one-third of total power
D.modulation carrying two bits per symbol on four carrier phases, giving twice the spectral efficiency of BPSK with the same bit error rate for a given Eb/N0
2. Which term means: "ratio of message amplitude to carrier amplitude; values above 1 cause over-modulation and envelope distortion, and at 1 the sidebands carry one-third of total power"?
A.AM modulation index — modulation carrying two bits per symbol on four carrier phases, giving twice the spectral efficiency of BPSK with the same bit error rate for a given Eb/N0
B.AM modulation index — average information per symbol in bits, maximised at log2 M when all M symbols are equally likely, and setting the lower bound on lossless code length
C.AM modulation index — ratio of message amplitude to carrier amplitude; values above 1 cause over-modulation and envelope distortion, and at 1 the sidebands carry one-third of total power
D.AM modulation index — maximum error-free bit rate of a channel equals bandwidth times log2(1 + S/N), so capacity grows linearly with bandwidth but only logarithmically with power
A.FM bandwidth is approximately twice the sum of peak frequency deviation and highest modulating frequency, giving about 180 kHz for broadcast FM with 75 kHz deviation
B.cascade noise figure F1 + (F2 − 1)/G1 + …, showing the first stage dominates, which is why a low-noise amplifier is placed right after the antenna
C.architecture mixing the RF signal with a local oscillator down to a fixed intermediate frequency such as 455 kHz, so selectivity is set once and an RF stage rejects the image
D.modulation carrying two bits per symbol on four carrier phases, giving twice the spectral efficiency of BPSK with the same bit error rate for a given Eb/N0
5. Which term means: "FM bandwidth is approximately twice the sum of peak frequency deviation and highest modulating frequency, giving about 180 kHz for broadcast FM with 75 kHz deviation"?
A.Carson's rule — FM bandwidth is approximately twice the sum of peak frequency deviation and highest modulating frequency, giving about 180 kHz for broadcast FM with 75 kHz deviation
B.Carson's rule — smallest number of bit positions in which two valid codewords differ; a code detects dmin − 1 errors and corrects floor((dmin − 1)/2)
C.Carson's rule — ratio of message amplitude to carrier amplitude; values above 1 cause over-modulation and envelope distortion, and at 1 the sidebands carry one-third of total power
D.Carson's rule — architecture mixing the RF signal with a local oscillator down to a fixed intermediate frequency such as 455 kHz, so selectivity is set once and an RF stage rejects the image
A.FM bandwidth is approximately twice the sum of peak frequency deviation and highest modulating frequency, giving about 180 kHz for broadcast FM with 75 kHz deviation
B.ratio of message amplitude to carrier amplitude; values above 1 cause over-modulation and envelope distortion, and at 1 the sidebands carry one-third of total power
C.maximum error-free bit rate of a channel equals bandwidth times log2(1 + S/N), so capacity grows linearly with bandwidth but only logarithmically with power
D.modulation carrying two bits per symbol on four carrier phases, giving twice the spectral efficiency of BPSK with the same bit error rate for a given Eb/N0
8. Which term means: "maximum error-free bit rate of a channel equals bandwidth times log2(1 + S/N), so capacity grows linearly with bandwidth but only logarithmically with power"?
A.Shannon-Hartley capacity — modulation carrying two bits per symbol on four carrier phases, giving twice the spectral efficiency of BPSK with the same bit error rate for a given Eb/N0
B.Shannon-Hartley capacity — cascade noise figure F1 + (F2 − 1)/G1 + …, showing the first stage dominates, which is why a low-noise amplifier is placed right after the antenna
C.Shannon-Hartley capacity — maximum error-free bit rate of a channel equals bandwidth times log2(1 + S/N), so capacity grows linearly with bandwidth but only logarithmically with power
D.Shannon-Hartley capacity — average information per symbol in bits, maximised at log2 M when all M symbols are equally likely, and setting the lower bound on lossless code length
A.modulation carrying two bits per symbol on four carrier phases, giving twice the spectral efficiency of BPSK with the same bit error rate for a given Eb/N0
B.ratio of message amplitude to carrier amplitude; values above 1 cause over-modulation and envelope distortion, and at 1 the sidebands carry one-third of total power
C.maximum error-free bit rate of a channel equals bandwidth times log2(1 + S/N), so capacity grows linearly with bandwidth but only logarithmically with power
D.average information per symbol in bits, maximised at log2 M when all M symbols are equally likely, and setting the lower bound on lossless code length
11. Which term means: "average information per symbol in bits, maximised at log2 M when all M symbols are equally likely, and setting the lower bound on lossless code length"?
A.Entropy of a source — average information per symbol in bits, maximised at log2 M when all M symbols are equally likely, and setting the lower bound on lossless code length
B.Entropy of a source — ratio of message amplitude to carrier amplitude; values above 1 cause over-modulation and envelope distortion, and at 1 the sidebands carry one-third of total power
C.Entropy of a source — maximum error-free bit rate of a channel equals bandwidth times log2(1 + S/N), so capacity grows linearly with bandwidth but only logarithmically with power
D.Entropy of a source — Nyquist filter that gives zero intersymbol interference at sampling instants with bandwidth (1 + α)Rs/2, where roll-off α trades bandwidth against timing tolerance
A.ratio of message amplitude to carrier amplitude; values above 1 cause over-modulation and envelope distortion, and at 1 the sidebands carry one-third of total power
B.Nyquist filter that gives zero intersymbol interference at sampling instants with bandwidth (1 + α)Rs/2, where roll-off α trades bandwidth against timing tolerance
C.architecture mixing the RF signal with a local oscillator down to a fixed intermediate frequency such as 455 kHz, so selectivity is set once and an RF stage rejects the image
D.maximum error-free bit rate of a channel equals bandwidth times log2(1 + S/N), so capacity grows linearly with bandwidth but only logarithmically with power
14. Which term means: "architecture mixing the RF signal with a local oscillator down to a fixed intermediate frequency such as 455 kHz, so selectivity is set once and an RF stage rejects the image"?
A.Superheterodyne receiver — cascade noise figure F1 + (F2 − 1)/G1 + …, showing the first stage dominates, which is why a low-noise amplifier is placed right after the antenna
B.Superheterodyne receiver — ratio of message amplitude to carrier amplitude; values above 1 cause over-modulation and envelope distortion, and at 1 the sidebands carry one-third of total power
C.Superheterodyne receiver — modulation carrying two bits per symbol on four carrier phases, giving twice the spectral efficiency of BPSK with the same bit error rate for a given Eb/N0
D.Superheterodyne receiver — architecture mixing the RF signal with a local oscillator down to a fixed intermediate frequency such as 455 kHz, so selectivity is set once and an RF stage rejects the image
A.maximum error-free bit rate of a channel equals bandwidth times log2(1 + S/N), so capacity grows linearly with bandwidth but only logarithmically with power
B.modulation carrying two bits per symbol on four carrier phases, giving twice the spectral efficiency of BPSK with the same bit error rate for a given Eb/N0
C.ratio of message amplitude to carrier amplitude; values above 1 cause over-modulation and envelope distortion, and at 1 the sidebands carry one-third of total power
D.average information per symbol in bits, maximised at log2 M when all M symbols are equally likely, and setting the lower bound on lossless code length
17. Which term means: "modulation carrying two bits per symbol on four carrier phases, giving twice the spectral efficiency of BPSK with the same bit error rate for a given Eb/N0"?
A.QPSK — modulation carrying two bits per symbol on four carrier phases, giving twice the spectral efficiency of BPSK with the same bit error rate for a given Eb/N0
B.QPSK — ratio of message amplitude to carrier amplitude; values above 1 cause over-modulation and envelope distortion, and at 1 the sidebands carry one-third of total power
C.QPSK — architecture mixing the RF signal with a local oscillator down to a fixed intermediate frequency such as 455 kHz, so selectivity is set once and an RF stage rejects the image
D.QPSK — Nyquist filter that gives zero intersymbol interference at sampling instants with bandwidth (1 + α)Rs/2, where roll-off α trades bandwidth against timing tolerance
A.modulation carrying two bits per symbol on four carrier phases, giving twice the spectral efficiency of BPSK with the same bit error rate for a given Eb/N0
B.cascade noise figure F1 + (F2 − 1)/G1 + …, showing the first stage dominates, which is why a low-noise amplifier is placed right after the antenna
C.maximum error-free bit rate of a channel equals bandwidth times log2(1 + S/N), so capacity grows linearly with bandwidth but only logarithmically with power
D.Nyquist filter that gives zero intersymbol interference at sampling instants with bandwidth (1 + α)Rs/2, where roll-off α trades bandwidth against timing tolerance
20. Which term means: "cascade noise figure F1 + (F2 − 1)/G1 + …, showing the first stage dominates, which is why a low-noise amplifier is placed right after the antenna"?
A.Friis noise formula — FM bandwidth is approximately twice the sum of peak frequency deviation and highest modulating frequency, giving about 180 kHz for broadcast FM with 75 kHz deviation
B.Friis noise formula — average information per symbol in bits, maximised at log2 M when all M symbols are equally likely, and setting the lower bound on lossless code length
C.Friis noise formula — cascade noise figure F1 + (F2 − 1)/G1 + …, showing the first stage dominates, which is why a low-noise amplifier is placed right after the antenna
D.Friis noise formula — architecture mixing the RF signal with a local oscillator down to a fixed intermediate frequency such as 455 kHz, so selectivity is set once and an RF stage rejects the image
A.average information per symbol in bits, maximised at log2 M when all M symbols are equally likely, and setting the lower bound on lossless code length
B.Nyquist filter that gives zero intersymbol interference at sampling instants with bandwidth (1 + α)Rs/2, where roll-off α trades bandwidth against timing tolerance
C.FM bandwidth is approximately twice the sum of peak frequency deviation and highest modulating frequency, giving about 180 kHz for broadcast FM with 75 kHz deviation
D.cascade noise figure F1 + (F2 − 1)/G1 + …, showing the first stage dominates, which is why a low-noise amplifier is placed right after the antenna
23. Which term means: "Nyquist filter that gives zero intersymbol interference at sampling instants with bandwidth (1 + α)Rs/2, where roll-off α trades bandwidth against timing tolerance"?
A.Raised-cosine pulse shaping — Nyquist filter that gives zero intersymbol interference at sampling instants with bandwidth (1 + α)Rs/2, where roll-off α trades bandwidth against timing tolerance
B.Raised-cosine pulse shaping — smallest number of bit positions in which two valid codewords differ; a code detects dmin − 1 errors and corrects floor((dmin − 1)/2)
C.Raised-cosine pulse shaping — ratio of message amplitude to carrier amplitude; values above 1 cause over-modulation and envelope distortion, and at 1 the sidebands carry one-third of total power
D.Raised-cosine pulse shaping — architecture mixing the RF signal with a local oscillator down to a fixed intermediate frequency such as 455 kHz, so selectivity is set once and an RF stage rejects the image
A.cascade noise figure F1 + (F2 − 1)/G1 + …, showing the first stage dominates, which is why a low-noise amplifier is placed right after the antenna
B.modulation carrying two bits per symbol on four carrier phases, giving twice the spectral efficiency of BPSK with the same bit error rate for a given Eb/N0
C.Nyquist filter that gives zero intersymbol interference at sampling instants with bandwidth (1 + α)Rs/2, where roll-off α trades bandwidth against timing tolerance
D.smallest number of bit positions in which two valid codewords differ; a code detects dmin − 1 errors and corrects floor((dmin − 1)/2)
26. Which term means: "smallest number of bit positions in which two valid codewords differ; a code detects dmin − 1 errors and corrects floor((dmin − 1)/2)"?
A.Minimum Hamming distance — smallest number of bit positions in which two valid codewords differ; a code detects dmin − 1 errors and corrects floor((dmin − 1)/2)
B.Minimum Hamming distance — average information per symbol in bits, maximised at log2 M when all M symbols are equally likely, and setting the lower bound on lossless code length
C.Minimum Hamming distance — architecture mixing the RF signal with a local oscillator down to a fixed intermediate frequency such as 455 kHz, so selectivity is set once and an RF stage rejects the image
D.Minimum Hamming distance — cascade noise figure F1 + (F2 − 1)/G1 + …, showing the first stage dominates, which is why a low-noise amplifier is placed right after the antenna
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