用FFT计算信号的功率谱密度时，需要先计算FFT，再计算双边谱密度，然后计算单边谱密度。在matlab里注意FFt要除以采样点数，计算双边谱时要注意除以频率分辨率。

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When calculated using the FFT power spectrum density signal, you need to calculate the FFT, and then calculate the bilateral spectral density, and then calculate the unilateral spectral density. In matlab in Note FFt divided by sampling points should be noted when calculating the bilateral spectrum divided by the frequency resolution.