说明：  实现FFT变换的函数。调用形式为y=bianhuan(xr,n) 其中，xr为要进行傅立叶变换的采样信号为输入,y为输出，n为信号xr的长度。
(A function of the realization of FFT transform. Call the form of y = bianhuan (xr, n) of them, xr Fourier transform in order to carry out the sampling signal for the input, y is output, n is the length of the signal xr.)