Intermodulation analysis of dual-gate FET mixers

A detailed intermodulation analysis of dual-gate FET (DG-FET) mixers is presented. The analysis method is based on a large-signal/small-signal analysis using time-varying Volterra-series methods. The analysis program allows one to probe the internal nodes of DG-FETs to evaluate the nonlinear current components. Therefore, it helps physical understanding of intermodulation distortion (INM) mechanisms in DG-FET mixers. The program was used to identify the major sources of MM generation. It was found from the analysis that the nonlinearities due to the output conductance (G(d3) and G(d2)) of the lower common-source FET were most responsible for INM generation. The impact of the upper common-gate FET on IMD generation was also found to be nonnegligible, especially at high local oscillator (LO) power levels. The analysis also predicted the presence of IMD "sweet spots" using bias optimization, which was experimentally proved by the fabricated mixers at X- and Ka-bands. The optimized X-band hybrid mixer showed measured intermodulation characteristics (OIP3 similar to 13.6 dBm) comparable to those of the resistive mixers (OIP3 similar to 15.3 dBm) with low LO and dc power conditions. Very good agreement between the measurement and simulation was found for the two mixers using different types of transistors, which validates our analysis method and supports the general adequacy of our approach. This paper presents a comprehensive IMD analysis and design methodology for DG-FET mixers, and shows that well-designed DG-FET mixers are very promising for modern monolithic applications requiring high linearity.