In a PLL-based FM demodulator, the output of the phase detector is proportional to _______.

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RRB JE ECE 22 Apr 2025 Shift 1 CBT 2 Official Paper

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Answer 1

In a Phase-Locked Loop (PLL)-based FM demodulator, the phase detector is a key component that:

Compares the input FM signal with the output of a Voltage-Controlled Oscillator (VCO).
Produces an output voltage proportional to the phase difference between the two signals.

FM signal → fed into the phase detector.
Phase detector compares it with VCO output.
The output of the phase detector reflects the instantaneous phase difference.
This varying voltage corresponds to the modulating signal (since frequency variations in FM lead to phase changes).
After low-pass filtering, it gives the original message signal.

Option	Why It's Incorrect
1) Carrier frequency	The VCO tracks the input frequency; the detector works on phase, not frequency directly.
2) Amplitude	PLL FM demodulation is amplitude-insensitive.
3) Frequency deviation	While the output of the entire PLL relates to frequency deviation, the phase detector output is directly proportional to phase difference.