Frequency demodulator having circuit cancelling undesired signal components

DETAILED DESCRIPTION An object of the present invention is to provide a frequency demodulation circuit which is capable of cancelling moire components generated from the lower sideband components of an FM signal.
Another object of the present invention is to provide a frequency demodulation circuit which is capable of cancelling an undesired cross-modulation moire component generated from a cross-modulation signal contained in an FM signal.
To achieve the above objects, a frequency demodulation circuit of the present invention comprises: a conventional frequency demodulator for demodulating an input FM signal to obtain a demodulated signal; a moire component generator which generates an undesired moire component from either the demodulated signal or the input FM signal; and a subtractor for subtracting the generated undesired moire component from the demodulated signal so as to thereby obtain a demodulated signal from which the undesired moire component is eliminated.
Generally, an FM signal obtained by modulating a carrier frequency f.
sub.
c with a signal frequency f.
sub.
s contains an nth-order lower sideband component f.
sub.
n =f.
sub.
c -nf.
sub.
s, where n is a natural number.
A demodulated signal obtained by demodulating such an FM signal by an m-times-multiplier pulse counting demodulator contains an m-multiplied carrier frequency mf.
sub.
c, where m is a natural number, the signal frequency f.
sub.
s and an nth-order lower sideband component J.
sub.
-n =mf.
sub.
c -nf.
sub.
s.
This means that the nth-order lower sideband component J.
sub.
-n of the demodulated signal can be calculated as J.
sub.
-n =mf.
sub.
c -nf.
sub.
s by using the m-multiplied carrier frequency mf.
sub.
c and the signal frequency f.
sub.
s which can be obtained from the demodulated signal.
Alternatively, since the signal frequency f.
sub.
s is expressed as f.
sub.
s =f.
sub.
c -f.
sub.
n, the component J.
sub.
-n can be expressed as J.
sub.
-n =mf.
sub.
c -n(f.
sub.
c -f.
sub.
n)=nf.
sub.
n -(n-m)f.
sub.
c.
This means that the nth-order lower sideband component J