说明：  线性调频信号是一种雷达常用的电磁波信号，当雷达信号检测到目标时候，反射后得到雷达回波信号，此时由于多普勒效应，信号的频率发生偏移，这个源码仿真出雷达检测到目标距离信息的回波信号。
(LFM signal is a commonly used electromagnetic radar signal, when the radar signal detection to the target time, after reflection of radar echo signal, at this time due to Doppler effect, the signal)