仿真不同频率的组合信号通过滤波器后作FFT，得到滤波后信号的fft频谱及fft求模，并找出最大值及索引，非常实用。-The combination of simulation of different frequency signal through a filter made after FFT, the filtered signal the fft spectrum and fft modulo, and find out the maximum and the index is very practical.