Predicting the performance of a 26 GHz transconductance modulated downconversion mixer as a function of LO drive and DC bias

The dependency of RF performance on the local oscillator (LO) drive amplitude and DC bias is an important topic for RF mixers, especially as carrier frequency increases and generation of RF power thus becomes more complex. The prediction of mixer performance, without initial reliance on full circuit simulations, can provide important insights. In this work, mathematical models without the prior use of circuit simulation are developed, leading to a strategy to predict the conversion gain (Gc), DC current, 1 dB input compression point (IP1dB) and third order input intercept point (IIP3) for a SiGe bipolar transistor transconductance mixer. The models show the possibility to trade-off LO RF power and DC bias to achieve a desired performance. The concepts allow a prediction of the necessary DC bias required to support a chosen LO level and desired conversion transconductance or linearity. The mathematical model results, circuit simulation results, and measured hardware results from a 26 GHz prototype of a single-ended mixer are presented and compared, showing good agreement. In a lab-measured example, LO power reduction from +10 dBm to +3 dBm resulted in only a 1 dB reduction in conversion gain, by modifying the DC bias as predicted. The peak conversion gain predicted by the models is within 2.0 dB of circuit simulation and 2.5 dB of measured PCB results. The RMS error for predicted DC current, compared to circuit simulation, is 1.9 mA or better.