Phase imbalance compensation in SAR/ISAR quadrature demodulation

I/Q channel imbalance in quadrature demodulation is one of the major concerns in the design of synthetic aperture radar (SAR) and inverse SAR (ISAR) receivers. The amplitude and phase mismatches result in the additional mirror signal, which acts as a correlated interference superimposed on the desired signal and, hence, degrades the whole system´s performance significantly. In this paper, we first analyze the effects of I/Q channels imbalance in detail, and focus on the relationship between the mirror signal and imbalance effect. Then a new digital method for phase imbalance compensation is proposed. The phase mismatch is adaptively estimated and compensated by optimizing the matched filtering results without requiring any testing signals or measurement circuits. The method is based on the principle that when the phase mismatch is compensated perfectly, the entropy of the range-resolved image will reach the minimum or the contrast will reach the maximum. In addition, the simulation results indicate that the I/Q phase mismatch can be effectively compensated during the normal operation of the SAR/ISAR receivers, even in the rapidly changing case, as long as a linear system model for the imbalance is valid.