FET upconverter design using load dependent mixing transconductance

A FET upconverter design procedure using load-dependent mixing transconductance in the FET equivalent circuit is reported. The approach relies on a combination of radio-frequency measurements and equivalent circuit representations with a frequency-domain computer-aided design (CAD) tool such as Touchstone to create a simple but realistic mixer model simulating the local oscillator (LO) dynamic loading, thereby allowing real-time circuit analysis and synthesis. The design optimizes the drain network for acceptable match at the selected sideband and desired LO rejection while avoiding impedance values in the LO frequency range which would otherwise cause severe degradation in conversion loss. This method allows simulations of the conversion gain over a specific bandwidth using few microwave measurements and simple equivalent circuits. From a more general point of view, this work demonstrates the acute importance of the LO drain termination of a gate mixer in determining conversion efficiency. It has been shown that neither the short-circuit nor the open-circuit drain termination at the LO frequency will cause conversion gain extrema. The results should apply to downconverters as well, since the same first-order mixing process is involved.<>