FM quadrature demodulator with two phase comparison circuits

An FM quadrature demodulator for demodulating a pair of FM-modulated signals in mutual phase quadrature, said FM-modulated signals including a modulation signal which is frequency-modulated on a carrier, said FM quadrature demodulator comprising:

• an in-phase input terminal and a quadrature input terminal for receiving said FM-modulated signals;
• a first phase comparison circuit having a first input coupled to said quadrature input terminal, a second input and an output;
• a second phase comparison circuit having a first input coupled to said in-phase input terminal, a second input and an output;
• phase shifting means comprising a polyphase filter having a bandpass characteristic which is symmetrical around the carrier frequency, said polyphase filter having in-phase and quadrature inputs coupled, respectively, to said in-phase and quadrature input terminals, and in-phase and quadrature outputs coupled, respectively, to said second inputs of said first and second phase comparison circuits, said phase shifting means realizing a phase shift which varies with said modulation signal;
• a differential stage having inputs coupled, respectively, to the outputs of said first and second phase comparison circuits, and an output;
• and
• a low-pass filter having an input coupled to the output of said differential stage, an output of said low-pass filter being coupled to an output of said FM quadrature demodulator.