a time - domain baseband measurement method measures modulated microwave signals typically used in communication systems by converting microwave signals to baseband before measurement for improved accuracy compared to direct measurement at the microwave frequency . a downconverting receiver is first characterized using a prior characterization method and then the modulated microwave signal is applied to the downconverting receiver and the response of the downconverting receiver is removed to provide an accurate characterization of the modulated microwave signal . such an accurate measurement of the modulated microwave signal can be used for communications system performance verification as well as for characterizing communications devices and systems . one particular application is the measurement of input / output characteristics of nonlinear power amplifiers using such modulated microwave signals .