A New Method for Minimizing Distortions in Time-Domain Measurements of Communications Signals

A communications system reduces downconverter inaccuracies in time-domain measurements or samples of received microwave communications I and Q complex signals by converting received signal to baseband taking measurements or samples of the I and Q waveforms at differing phase shifts of a demodulating carrier signal for a local oscillator or carrier tracking loop used during downconversion so that I and Q imbalances may be detected and removed by lowpass equivalent averaging for improved characterization of downconverters or for improved signal reception. In the preferred form, the phase shifts are 0 and .pi./2 for a conventional measurement, and then at .theta., and .theta.+.pi./2, with .theta.=.pi./4+m.pi./2 for an integer m for the second measurement where I and Q imbalances and baseband nonlinearities are indicated by differences between the two measured or sampled signals, where .theta. provides for optimum error detection for reducing the errors by averaging the measurements.