说明：  数字下变频器的matlab仿真，包括基带信号的调制，然后经过混频模块、CIC模块、FIR模块最后得到基带信号的过程。很多地方都批有详细的注释，很容易理解。
(Digital down converter matlab simulation, including the modulation baseband signal, and then after mixing module, CIC module, FIR module finally get baseband signal process)