Minimum-delay frequency-shift-compensating complex demodulator with arbitrary phase adjustment

Abstract

A solid-state gyroscope in a closed flight-control loop produces a DSSC QAM body angular-rate output signal and a drive current, which is converted to a demodulation reference signal. The frequency of the demodulation reference signal drifts slowly. A digital complex AM demodulator must demodulate the rate-output signal and correct the phase shift with as little delay as possible. This demodulator employs three elemental Hilbert transformers (EHT), one for direct current (d.c.) blocking and two for phase shifting. The gains of these Hilbert transformers shift with the drive frequency. In order to hold signal-path delays to a minimum, the gains of the Hilbert transformers are held to unity by means of a simple Least Mean Squared (LMS) adaptive loop whose output feeds the gain multiplier of all three Hilbert transformers. Automatic phase-shift correction is also included by adaptively rotating the demodulator reference phase in order to decorrelate the outputs of the demodulator.