Linearity asymmetry in FET resistive mixers

This paper investigates up/down conversion asymmetry in intermodulation distortion observed in measurements of resistive FET mixers. Symmetric behaviour is intuitively expected of such a topology, so a first principle analysis is carried out to determine the responsible mechanism. Previous analysis of up/down conversion asymmetry has focused on conversion gain in diode based mixers, whereas the effects investigated in this paper are for mixers with symmetry in this aspect. The aim is to fully understand the intermodulation mechanism, so that performance can be enhanced. The approach taken is to consider the mixer as a two-port nonlinear element driven by multiple frequency sources. Mixing performance then becomes a function of the relative frequencies and amplitudes, which is related to the mode of operation as an up or down converter. This investigation is performed with FET models of increasing complexity and physical accuracy. In this manner the effect on intermodulation and other mixer performance parameters can be isolated to those differences introduced with each model change such as the effect of adding higher order even/odd terms to the drain or gate non-linear model independently. The result is an understanding into the FET properties that contribute to intermodulation distortion. This knowledge is useful to designers as it allows educated modifications to mixer topology to obtain improved linearity. Results also give the ability to minimize the asymmetry, reducing the design cost involved in producing a separate solution for each mode. These results can also be used to guide modification of the physical structure of a FET for mixer applications.