Phase-angle bandpass

The phase angles 0(t), 2(p t), and 3(p t), correspond to FM waves with carrier frequencies / 2/, and 3/c-Thus if the nonlinear system or channel is followed by a bandpass filter centered at / the filter output is the desired FM wave with a carrier / given by... 

The quality factor of a low-pass filter is much greater than that of a bandpass filter, and so a low-pass filter can be used to eliminate the noise by recovering the desired information in the DC component. Since the value of the amplitude of signal Sr is known and dephase angle cp can be determined, the unknown value of amplitude Aj of the signal can be directly obtained. This principle is familiar during the operation of a cw-EPR spectrometer, in which the receiver is tuned to the field modulation frequency and one adjusts the phase of the receiver to maximize the receiver voltage. 

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