| This dissertation presents a generalized theoretical and experimental treatment of GaAs MESFET mixers, which is adequate to determine conversion gain and input/output impedances for upconverters, downconverters, harmonic mixers, and modulators. The local oscillator (LO) excitation may be applied to the gate or drain, the intermediate frequency need not be small compared to the LO, and the FET embedding impedances may be specified arbitrarily. The FET is described by a lumped element, quasistatic nonlinear equivalent circuit, and a new and highly accurate model for determining its parasitic capacitances and current voltage characteristic is presented.;The theory is verified experimentally by the design of two FET mixers. One exhibits 11 dB gain, 5.5 dB noise figure, and 15 dBm third order intercept point. The other has 6 dB gain, 4.3 dB minimum noise figure, 12 dBm intercept point, and 500 MHz bandwidth.;The basic approach is to use a large-signal nonlinear analysis to determine the time waveforms for the FET equivalent circuit elements, followed by a small-signal, linear, time-varying analysis. The large-signal analysis uses a highly efficient harmonic balance numerical algorithm. It is an extension of one originally proposed for diode mixers. In the small-signal analysis the circuit elements are described by conversion matrices, which can be treated analogously to scalar impedances. |
