a demodulation circuit for demodulating a frequency diverse complex modulated carrier comprises an a / d converter generating a series of samples representing the modulated carrier , a mixer operating to mix the series of samples with a sine wave of one fourth the sampling frequency represented by a series of sine wave values occurring at the sampling frequency , and a decimation filter operating at a decimation factor equal to the sampling frequency divided by the frequency difference between adjacent sub - spectra for folding the sub - spectra and retaining a portion of the mixed down series of samples .