说明：  二阶锁相环Matlab仿真代码，如入两路信号和信噪比，输出锁相以后的信号。可以仿真初始频差，和频率斜升的情况
(second-order PLL Matlab simulation code, such as two-way signals and signal to noise ratio, the output signal after the lock-in. Simulation can initial frequency difference, and frequency ramp-up of)