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A wideband Gilbert upconversion micromixer with a phase-inverter rat-race coupler is demonstrated in this paper using 0.35-μm SiGe heterojunction bipolar transistor (HBT) technology. By utilizing a crossover phase inverter with the coplanar stripline (CPS) realization, the size of a rat-race coupler is highly reduced. This rat-race coupler employed at the RF port combines differential mixer outputs to single-ended while the common-mode LO leakage is absorbed by the isolated port of the coupler. Consequently, the demonstrated upconverter has the conversion gain of -8 dB while the OP1dB and OIP3 are around -7 dBm and 5 dBm, respectively, within the RF 3-dB bandwidth from 11 to 19 GHz. Additionally, the LO-to-RF isolation is over 50 dB within the RF bandwidth.