27 Communication Systems questions from the ECE Core bank, written for Indian campus drives and tech interviews. Every question has a verified answer and an AI-tutor explanation on placd.
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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
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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"?
Junior
A.AM modulation index
B.Superheterodyne receiver
C.Entropy of a source
D.Raised-cosine pulse shaping
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3. Which statement is correct?
Junior
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
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4. What is Carson's rule?
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.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
Answer + AI explanation with Pro
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"?
Junior
A.Raised-cosine pulse shaping
B.Minimum Hamming distance
C.Entropy of a source
D.Carson's rule
Answer + AI explanation with Pro
6. Which statement is correct?
Junior
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
Answer + AI explanation with Pro
7. What is Shannon-Hartley capacity?
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.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
Answer + AI explanation with Pro
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"?
Junior
A.Entropy of a source
B.Shannon-Hartley capacity
C.Superheterodyne receiver
D.QPSK
Answer + AI explanation with Pro
9. Which statement is correct?
Junior
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
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10. What is Entropy of a source?
Junior
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
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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"?
Junior
A.Entropy of a source
B.QPSK
C.Friis noise formula
D.Raised-cosine pulse shaping
Answer + AI explanation with Pro
12. Which statement is correct?
Junior
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
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13. What is Superheterodyne receiver?
Mid
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
Answer + AI explanation with Pro
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"?
Mid
A.Superheterodyne receiver
B.AM modulation index
C.Shannon-Hartley capacity
D.Raised-cosine pulse shaping
Answer + AI explanation with Pro
15. Which statement is correct?
Mid
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
Answer + AI explanation with Pro
16. What is QPSK?
Mid
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
Answer + AI explanation with Pro
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"?
Mid
A.Carson's rule
B.QPSK
C.Minimum Hamming distance
D.Shannon-Hartley capacity
Answer + AI explanation with Pro
18. Which statement is correct?
Mid
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
Answer + AI explanation with Pro
19. What is Friis noise formula?
Mid
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
Answer + AI explanation with Pro
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"?
Mid
A.Minimum Hamming distance
B.Carson's rule
C.Superheterodyne receiver
D.Friis noise formula
Answer + AI explanation with Pro
21. Which statement is correct?
Mid
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
Answer + AI explanation with Pro
22. What is Raised-cosine pulse shaping?
Senior
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
Answer + AI explanation with Pro
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"?
Senior
A.AM modulation index
B.Raised-cosine pulse shaping
C.Friis noise formula
D.Entropy of a source
Answer + AI explanation with Pro
24. Which statement is correct?
Senior
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
Answer + AI explanation with Pro
25. What is Minimum Hamming distance?
Senior
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)
Answer + AI explanation with Pro
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)"?
Senior
A.Shannon-Hartley capacity
B.Minimum Hamming distance
C.Friis noise formula
D.Carson's rule
Answer + AI explanation with Pro
27. Which statement is correct?
Senior
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
Answer + AI explanation with Pro
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