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A 17-GHz Gilbert single-quadrature down-converter (SQDC) with polyphase filters is successfully demonstrated using 0.15-μm pseudomorphic high electron mobility transistor (pHEMT) technology for the first time to the best of our knowledge. Thanks to the semi-insulating GaAs substrate and an advanced technology, the image rejection Gilbert mixer works well at high frequencies. A polyphase filter at the LO stage is employed to generate precisely differential quadrature signals and a four-stage polyphase filter at the IF stage can filter out the unwanted image signal to achieve image rejection. From 10-MHz to 90-MHz IF frequencies, a 30-dB image rejection ratio is achieved when RF and LO frequencies are designed around 17 GHz. The Gilbert SQDC has a 5.5-dB conversion gain, 0-dBm IP1dB, and 16-dBm IIP3.