FM demodulation using a PLL can suffer if capture is poor. In that context, capture is influenced by sufficient frequency deviation and SNR; what is the practical consequence of poor capture?

FM demodulation using a PLL can suffer if capture is poor. In that context, capture is influenced by sufficient frequency deviation and SNR; what is the practical consequence of poor capture?

• A. The signal experiences significant distortion due to loss of lock.
• B. Capture has no effect on the demodulation quality.
• C. The carrier frequency shifts randomly.
• D. The demodulated output becomes completely unrecognizable.

Answer: (D)

In a PLL-based FM demodulator, the output relies on the loop’s ability to lock to and track the instantaneous carrier frequency. When capture is good, the VCO can follow the frequency changes caused by the modulation, and the phase/frequency error signal becomes a faithful representation of that modulation. If capture is poor, the loop cannot acquire or maintain lock. The VCO frequency drifts relative to the input, and the phase detector output becomes dominated by noise and wander rather than the actual frequency deviations in the signal. As a result, the demodulated waveform loses the information content of the modulation and the output becomes essentially unrecognizable.

In a PLL-based FM demodulator, the output relies on the loop’s ability to lock to and track the instantaneous carrier frequency. When capture is good, the VCO can follow the frequency changes caused by the modulation, and the phase/frequency error signal becomes a faithful representation of that modulation.

If capture is poor, the loop cannot acquire or maintain lock. The VCO frequency drifts relative to the input, and the phase detector output becomes dominated by noise and wander rather than the actual frequency deviations in the signal. As a result, the demodulated waveform loses the information content of the modulation and the output becomes essentially unrecognizable.