Measurement Technique for Characterization of Rapidly Time- and Frequency-Varying Electronic Devices

LLE Review, Volume 78 105 The conventional method for measuring the transfer function of an electronic device uses Fourier transform theory and convolutions and is, therefore, limited to either time-invariant or frequency-invariant devices. The measurement technique presented here enables the complete characterization of electronic devices having any dynamic temporal and spectral frequency response. A technique presented earlier1 applied the windowing of signals in the time and frequency domains (called time-frequency distributions) to characterize photoconductive switches that vary in time and frequency; however, windowing requires a slowly varying envelope approximation, which limits the allowed rate of temporal and spectral variations. The more general technique allows us to measure the frequency response of the optoelectronic (photoconductive) microwave switches on OMEGA’s pulse-shaping system. Unlike microwave diode switches, photoconductive switches do not have a constant conductive on-state, but rather decay monotonically to the off-state after the illumination ceases. A complete linear model for such a device must incorporate both filtering and modulation into a general time-varying filter (or equivalently, band-limited modulator). Any microwave or millimeter-wave device whose properties vary rapidly requires the application of this technique for complete characterization, including elements that depend on charge-carrier dynamics such as photoconductive attenuators, phase shifters, and directional couplers.