FM and FSK detection using a subtractor filter

FM detection is a classic subject in all analog communication systems textbooks. The standard techniques include FM slope detection, balanced frequency discriminator and Foster-Seeley phase shift discriminator. Another technique widely used nowadays to demodulate FM is a phase locked loop (PLL). This technique depends on generating a local wave using a voltage controlled oscillator (VCO). This generated wave is compared with the FM wave using a phase detector. The output of the phase detector is zero if the two frequencies are equal, otherwise the output of the subtractor low pass filter (LPF) is a signal of a frequency equal to the difference between that of the VCO and the input FM wave. The LPF output is applied to the input of the VCO so that the frequency of the VCO approaches the input signal.