PLL (phase lock loop) with PFD (phase-frequency detector) have been widely used in recent years.Reason for their popularity include extended tracking range, lower tracking time and low cost.

• 摘要锁相环(PLL)是一个闭环相位自动控制系统，能够利用一个精确且稳定的频率产生一系列频率准确的信号，为系统内部的其它模块提供稳定的高频时钟。