In a PLL, which component is responsible for turning phase error into a control voltage that tunes the VCO?

• A. Phase detector monitors the output clock and synchronizes it to a reference
• B. Loop filter is optional and can be omitted in a PLL
• C. Phase detector compares the reference with the VCO; loop filter shapes the error signal; VCO changes frequency; feedback closes the loop to lock
• D. The reference oscillator alone fixes the output frequency without feedback

Answer: (C)

In a PLL, the phase detector compares the reference signal with the VCO output and produces an error signal that represents how far out of phase they are. The loop filter then takes that error signal and shapes it into a smooth, appropriate control voltage. This voltage is what actually tunes the VCO, nudging its frequency so the output more closely matches the reference. The feedback path from the VCO back to the phase detector closes the loop, so as the VCO locks in, the phase error goes to zero (within the loop’s limits). So the essential idea is the chain: phase error is detected, converted into a usable control voltage by the loop filter, and that voltage drives the VCO to achieve and maintain lock.