Which of the following best describes the basic components and function of a phase-locked loop?

• A. Phase detector monitors the VCO output only; loop filter and feedback are optional
• B. Loop filter acts as the sole frequency-determining element
• 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)

Phase-locked loops rely on a closed feedback path to align the VCO output to a stable reference in frequency and phase. The phase detector compares the reference signal with the VCO output and produces an error signal that reflects their phase difference. The loop filter processes that error signal, shaping its amplitude and response so the system remains stable and responds appropriately to changes. The VCO then adjusts its frequency according to the control voltage coming from the loop filter. The feedback path sends the VCO output back to the phase detector, closing the loop so the system can continuously correct itself and achieve lock. When locked, the VCO frequency tracks the reference (often with a defined multiplication/division factor), maintaining a steady phase relationship. This description captures all the essential components and how they interact to produce a locked condition. The other statements misrepresent the roles of the components or ignore the necessity of the feedback loop for locking.