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说明：  络检波法，也可以用同步解调法，现在我们来使用同步解调法，即将已调信号和一个与载波信号同频率的信号相乘，在通过一个理想低通滤波器就可以得到原始信号(Envelope detection method, you can also use synchronous demodulation method, and now we use synchronous demodulation method, is about modulated signal and a signal is multiplied with the same frequency of the carrier signal, through an ideal low-pass filter can get the original signal)