R code for complex demodulation of a time series. Applies complex demodulation to a timeseries twice, once on the signal then once on the reverse of the signal, re-reversing the latter and finally averaging the two to correct for artificial shift induced by the butterworth filter.

complexDemodulation.R

cd = function(s,freq,srate){
	x = s*exp(2 * pi * 1i * freq * (1:length(s)) / srate )
	zR1 = filter( x = Re(x) , filt = butter(3,freq/srate/2) )
	zI1 = filter( x = Im(x) , filt = butter(3,freq/srate/2) )
	z1 = 2*sqrt((zR1^2)+(zI1^2))
	x = rev(x)
	zR2 = filter( x = Re(x) , filt = butter(3,freq/srate/2) )
	zI2 = filter( x = Im(x) , filt = butter(3,freq/srate/2) )
	z2 = 2*sqrt((zR2^2)+(zI2^2))
	z = (z1+rev(z2))/2
	return(as.numeric(z))
}