function [y_FM_demodulated] = FM_IQ_Demod(y)
%This function demodualtes an FM signal. It is assumed that the FM signal
%is complex (e.g. an IQ signal) centered at DC and occupies less than 90%
%of total bandwidth. 
%filter design!
b = firls(30,[0 .9],[0 1],'differentiator'); %design differentiater
d=y./abs(y);%normalize the amplitude (i.e. remove amplitude variations)
rd=real(d); %real part of normalized siganl.
id=imag(d); %imaginary part of normalized signal. 
y_FM_demodulated=(rd.*conv(id,b,'same')-id.*conv(rd,b,'same'))./(rd.^2+id.^2);%demodulate!
end