Minimizing distortions in the time-domain measurement of microwave communications signals

We present a new method to minimize inaccuracies in time-domain measurements of microwave communications signals. The signal measurements are accomplished by converting the signal to baseband where accurate instrumentation is available. The baseband signal is composed of in-phase (I) and quadrature (Q) components. The I and Q waveform measurements differ by a π/2 phase shift in the local oscillator used in the downconversion. Any I/Q imbalances may be detected and reduced by requiring the complex baseband signal to be invariant (apart from a rotation in the complex plane) with respect to the local oscillator phase. The complex baseband signal is measured twice, where the local oscillator (LO) phase is 0, and π/2 for the first signal measurement, and θ, and θ + π/2 for the second. Any I/Q imbalances and baseband nonlinearities are indicated by differences between the two measured signals. An LO phase shift of θ = π/4 + mπ/2, where m is an integer, is optimum for error detection. The errors may be reduced by averaging the measurements and by optimization of the measurement system. Errors caused by I/Q imbalances have been reduced to an error-to-signal power ratio of less than -56 dB. This accuracy is sufficient for essentially any communications system modeling applications.