Quadrature compensation for orthogonal signal channels

Abstract

A system for processing inphase and quadrature data channels, such as a Costas loop QPSK demodulator, employs an additional feedback loop for adjustment of phase offset between carrier reference signals, this loop being in addition to the Costas error loop for control of frequency and phase of an oscillator which provides the regenerated carrier signal. The additional loop employs cross-channel products of demodulated inphase and quadrature data signals as does the Costas loop. The regenerated carrier is applied via an adjustable phase-offset unit to provide quadrature carrier reference signals to phase detectors of inphase and quadrature data channels. The phase offset unit includes a 90 degree hybrid circuit energized by the carrier signal at a main input port plus an adjustable fraction of the carrier power applied to an auxiliary input port. Adjustment of the relative magnitudes of the signals at the two input ports of the hybrid circuit in accordance with the sum of the cross-channel products provides a desired amount of phase offset between the two carrier reference signals in correspondence with the deviation from orthogonality of the components of the composite input signal. This removes crosstalk resulting from the deviation in orthogonality of the components of the composite input signal.