Excuse me, the principle of phase-locked frequency discrimination in communication system

Phase-locked loop technology can be understood from the basic knowledge of signal system. Automatic frequency tracking/adjustment (AFT) is a negative feedback closed-loop system with frequency error, and there is residual frequency error in steady state. Phase-locked technology is a negative feedback closed-loop system with phase error. In steady state, there is only residual phase error, while the differential of phase is frequency and the error is zero.

It can be understood that PLL can track the frequency of FM signal without error, that is, it can get the local reference frequency fo related to the center frequency fs of FM signal. By multiplying the fo signal with the FM signal, the signal on the carrier can be obtained.