Error analysis of phase-insensitive coherent (homodyne) detectors

The phase and amplitude errors of a phase-insensitive coherent (homodyne) detector are analyzed. The detector consists of two balanced mixers which are fed in quadrature phase by one of the RF inputs, followed by a phase-quadrature combiner. Inherent detection errors due to an unwanted carrier in the double sideband modulated signal are analyzed first. Then errors due to the amplitude and phase unbalance of the two mixers and phase unbalance of the combiner are analyzed. For narrow bandwidths it is shown that all errors except those due to the unwanted carrier can be mitigated with careful design of the combiner, using appropriate trim adjustments.