这个函数返回n点2dw波段的傅里叶变换的DFT样本序列。通常情况下，以插值点的方式进行放大。-This function returns N point DFT samples of 2dw band of Fourier transform of a sequence. Typically, fft() returns N samples of Fourier transform ranging from 0 to 2pi. This function takes the N-point DFT samples and returns N-point samples corresponding to frequency band wc-dw to wc+dw.