说明：  锁频环又称为自动频率控制（AFC）环，其利用反馈回路对输入频率变化进行调整，最后达到对频率的跟踪，广泛应用于雷达，卫星导航等领域对多普勒频率的跟踪。
(Locked loop, also known as automatic frequency control (AFC) loop, the use of feedback loop to adjust the input frequency changes, and finally achieve frequency tracking is widely used in radar, satellite navigation and other fields on the Doppler frequency tracking.)