Which of the following statements w.r.t. FSK demodulation using PLL circuit is/are correct?
S1: If the input signal frequency changes, the PLL adjusts its frequency output to match the input frequency.
S2: The phase-locked loop is used to track the changes in the frequency of the modulated signal.

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

Answer 1

The correct answer is: 4) Both S1 and S2

Explanation:

S1: If the input signal frequency changes, the PLL adjusts its frequency output to match the input frequency.

Correct. This is a fundamental characteristic of a Phase-Locked Loop (PLL).
In FSK demodulation, when the frequency of the input signal changes (representing binary data), the PLL dynamically adjusts its voltage-controlled oscillator (VCO) output frequency to lock onto the input signal.

S2: The phase-locked loop is used to track the changes in the frequency of the modulated signal.

Correct. A PLL continuously tracks the instantaneous frequency of the input FSK signal and thus can decode the binary information.