随机产生一次数据x=randn（1，1024*8） 对这个序列进行FFT然后求功率谱密度 。应用MATLAB画出来横坐标为频率（Frequency(hz)））纵坐标为功率谱密度（Power Spectrum Density） (dB))的图形(Randomly generate a data x=randn (11024*8) to carry out FFT for this sequence and then obtain power spectral density. Drawing the abscissa as the frequency (Frequency (Hz)) and drawing the vertical coordinate as Power Spectrum Density (dB) by MATLAB.)