Discussion
Which of the following is not true in case of phase locked loop ?
Answer: D
The statement that is not true is the VCO input is always zero when locked.
In a phase locked loop, the phase detector compares the input with the VCO output, and the filtered error voltage drives the VCO. When locked, the VCO frequency equals the input frequency and tracks it, and the phases are held in a fixed relationship, so statements A, B and C describe a locked loop.
The VCO control voltage is zero only if the input happens to be at the VCO's free-running frequency. For any other input frequency within the lock range, a non-zero control voltage, produced by a small steady phase error, is exactly what holds the VCO at the input frequency.
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