中文说明：
用MATLAB仿真的数字下变频程序，变频，滤波未用matlab函数，而是用代码完成，便于移到DSP或FPGA上。供参考。
English Description：
The digital down conversion program simulated with MATLAB does not use matlab function for frequency conversion and filtering, but is completed with code, which is easy to move to DSP or FPGA. For reference