Digital FM demodulator utilizing uncorrelated clock reference signals

#This invention relates to an FM demodulator, especially to an FM demodulator which yields the modulated object signal directly in digital word string form.

#In FM demodulating circuits which follow the final section of the IF amplifiers in radio communication equipment, etc, a standard practice is to obtain the modulated object signal in the form of an analog signal, even when the communication equipment itself actually needs it in the form of digial word string, and a demodulated analog signal is obtained after it is A/D (analog-to-digital) converted after appropriate processing as necessary.

#On the other hand, there has been used a pulse-counting type FM demodulator which counts the FM carrier directly by use of a counter, over a given duration. Actually, in this case, conventional digital output can be obtained directly. However, the output of the IF amplifier still preserves the narrow band nature of the signal significantly, and the object signal appears merely in a small deviation over a fixed count number over each count duration. Also resolution and data rate can not be optimized; hence, the foregoing characteristics prevent this method from being widely used.

#The sole FIGURE is an exemplary block diagram of a preferred embodiment of the invention. The members are given below.

#(1) . . . input terminal.

#(2) . . . Quadrature tank circuit, or any frequency dependent phase difference generating circuit.

#The embodiment essentially is an FM demodulator comprising

#a frequency-dependent phase difference generating means, such as a quadrature tank circuit, delay line, injection lock oscillator, phase lock loop (PLL), etc, to which the input FM signal to be demodulated is supplied;

#a phase comparator means which compares the phases of the signals, one passing through the frequency dependent phase difference generating means and the other coming directly; means for generating a binary, unaveraged, pulse width modulated signal from the output of the phase comparator means;