Demodulation of single-sideband (SSB) signals Which one of the following circuits cannot demodulate an SSB signal into intelligible audio?

Difficulty: Easy

Correct Answer: Phase discriminator (FM detector)

Explanation:


Introduction / Context:
SSB signals have no carrier and just one sideband. Demodulation requires a locally generated carrier (or a product detector) to translate the SSB spectrum back to audio. FM detectors are designed for frequency variations, not for amplitude/phase-coherent detection needed by SSB.



Given Data / Assumptions:

  • SSB reception requires carrier reinsertion or coherent detection.
  • Receiver has access to a beat-frequency oscillator (BFO) or product detector.
  • Phase discriminator is an FM detector type.


Concept / Approach:
A product detector multiplies the incoming SSB with a locally generated sinusoid near the original carrier frequency to shift the spectrum to audio. An FM phase discriminator responds to instantaneous frequency/phase deviations and does not recover amplitude-encoded SSB audio.



Step-by-Step Solution:
Identify detectors viable for SSB: product detector; mixer plus BFO; synchronous detectors.Recognize that an FM phase discriminator requires a frequency-modulated input to produce audio.Since SSB is not FM, a phase discriminator cannot demodulate it properly.



Verification / Alternative check:
Communications receivers implement SSB modes via BFO/product detectors; FM detectors are disabled in SSB/AM modes, confirming the incompatibility.



Why Other Options Are Wrong:
Product detector / BFO + mixer: Standard SSB demodulation methods.


Balanced modulator as synchronous detector: When driven by a reinserted carrier, it functions as a coherent detector.


Synchronous envelope detector: With a locked carrier, it will demodulate SSB/AM.



Common Pitfalls:
Assuming any “detector” works for all modulation types; detectors are modulation-specific.



Final Answer:
Phase discriminator (FM detector)

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