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Coherent detection of DSB-SC requires a local carrier matched in

  • A.Amplitude only
  • B.Frequency only, phase being irrelevant to the product
  • C.Neither, provided the intermediate frequency is high enough
  • D.Frequency and phase

Answer: D

Both must match: a frequency error produces a beat and a phase error scales the output by its cosine, falling to zero when the reference is in quadrature with the signal. That quadrature null is the reason a phase-tracking loop is needed rather than a free-running oscillator. Amplitude of the reference affects only the output level. And no choice of intermediate frequency removes the need for a coherent reference. Keeping the carrier is a decision about the receiver, and keeping the second sideband a decision about the channel.

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