Nonlinear model for predicting intermodulation distortion in GaAs FET RF switch devices

A nonlinear model for predicting IMD (intermodulation distortion) in GaAs FET RF switch devices has been developed. A novel unified equivalent circuit model of the switch FET for the ON and the OFF states has been proposed. The nonlinearities of the test FET as a function of the gate bias were identified and modeled up to 40 GHz by fitting scattering parameter data. These nonlinearities were combined into a single nonlinear model constructed from a number of elementary linear and nonlinear elements. The nonlinear circuit equations were incorporated as a user-defined model in LIBRA to perform the harmonic balance technique. This model has been used to predict the IMD and phase shifting performance of three of the individual bit chips (11.25, 22.5, and 180 degrees ) of a 6-18-GHz wideband 5-b phase shifter. In this phase shifter, GaAs MESFETs are used as switch devices which configure the bits into high- or low-pass filter structures.<>