中文说明：
雷达LFM信号及其回波信号频谱，雷达信号检测到目标时候，反射后得到雷达回波信号由于多普勒效应，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。
English Description：
Radar LFM signal and its echo signal spectrum, when the radar signal detects the target, the radar echo signal is obtained after reflection. Due to the Doppler effect, the frequency of the signal is offset. This source code simulates the echo signal of the target range information detected by the radar.