The present invention relates to an FM demodulator for demodulating a signal which has been frequency modulated by a digital signal and, more particularly, to an FM demodulator suitable for an integrated circuit.
An FM demodulator in an integrated circuit configuration may be implemented by a so-called direct conversion system. The system is such that a received signal is directly frequency converted to a baseband signal to allow filtering and other kinds of processing to be achieved in the baseband. Signal processing in the baseband may be accomplished by a well known method which employs differentiation and multiplication. While the differentiation and multiplication scheme is capable of demodulating even a signal which has been modulated by an analog signal, it is difficult to be implemented by an integrated circuit due to the need for an automatic gain control circuit and an analog multiplier.
An example of prior art FM demodulators of the type using the direct conversion system is disclosed in U.K. Patent Application GB No. 2 106 359A. The disclosed FM demodulator, as shown in FIG. 11a of the drawings, has a demodulator construction furnished with digital circuit portions and an analog circuit portion which comprises a switch, resistors, a low-pass filter and other elements. The problem encountered with such a prior art demodulator is that the analog circuit portion is not suitable for an integrated circuit implementation and minute pulse noise (spike noise) appears in an output of the demodulator due to a delay difference between two inputs to a second Exclusive-OR gate (15).