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...http://www.google.de/patents/US5400363?utm_source=gb-gplus-sharePatent US5400363 - Quadrature compensation for orthogonal signal channels
