A digitally implemented demodulator. A frequency-modulated signal is applied to a limiting amplifier such that the signal amplitude is fixed at a constant level. The signal is undersampled and quadrature demodulated. The demodulator generates the cross-product of the baseband complex envelope to recover the original modulating signal. A digital Frequency Shift Keyed signal can be further recovered by applying the recovered signal to a data slicer to square-up the signal, and a matched filter for improved error resistance.