Non-linear AC/DC mixed-mode RF simulation to estimate EVM temperature drift of a GaAs pHEMT wideband IQ modulator IC

The author developed a GaAs wideband IQ modulator IC, which is utilized in RF signal source instruments with direct-conversion architecture. For applications requiring the precision of electric instrumentation, temperature drift is highly critical. In the author´s previous studies on temperature drift mechanism of RC-CR phase shifter, equivalent parasitic capacitances are the dominant factor to cause temperature drift of phase error. The analysis was conducted by mathematical simulation where the FET as variable resistances in RC-CR phase shifter was modeled as a parallel pair of variable re- sistance and simplified parasitic capacitance while the analysis did not explain well frequency dependence of the temperature drift on the measurement. For more accurate simulation, this paper reports non-linear circuit simulation using empirically temperature-dependent non-linear FET models, where DC and non-linear AC operations are simultaneously simulated. Utilizing this method, the simulation result showed a better match with the measurement in terms of non-linear temperature dependence.