说明：  一般来说载波频率我们是知道的，为什么我们不直接在接收机产生这个载波频率呢?原因是实际的原因，不可避免的有频率偏差，我们需要用一个跟踪器跟踪这个细小的差值，锁相环就是这样的功能模块。
(Generally speaking, we know the carrier frequency, why do we not produce this carrier frequency directly in the receiver? The reason is the actual reason, the inevitable frequency deviation, we need to use a tracker to track the small difference, the PLL is such a function module.)