Abstract: An analyzer for measuring the response of an electronic device (DUT 206) to an RF input signal from a signal generator (240a) is described. An active load pull circuit (201) is connected to the DUT 206, which receives an output signal from the DUT 206 and then feeds a modified signal back to the DUT 206. The signal is modified by a signal processing circuit (237) in view of input signals x, y to control the magnitude gain and phase change effected by the feedback circuit (237). Thus, positive feedback loops are avoided and better control of the analyzer is permitted. A network analyzer, or other signal measuring device (242), logs the waveforms (from which s-parameters derived) observed at ports of the DUT 206, thereby allowing the behaviour of the DUT 206 under various load conditions to be analyzed.