锁相环是一种反馈电路，其作用是使得电路上的时钟和某一外部时钟的相位同步。PLL通过比较外部信号的相位和由压控晶振（VCXO）的相位来实现同步的，在比较的过程中，锁相环电路会不断根据外部信号的相位来调整本地晶振的时钟相位，直到两个信号的相位同步。-PLL is a feedback circuit, and its role is to make the clock circuit and a phase of external clock synchronization. PLL signal by comparing the external voltage-controlled crystal phase and the (VCXO) to achieve synchronization phase, in comparison, the phase-locked loop circuit will continue in accordance with external signals of local oscillator phase to adjust the clock phase, until Phase synchronization of two signals.