This paper demonstrates a high-frequency 0.35-μm SiGe BiCMOS sub-harmonic downconverter with reactive in-phase and quadrature-phase (I/Q) generators. Two reactive I/Q generators are integrated in our work to provide differential quadrature RF and LO signals. The I/Q sub-harmonic downconverter with single-ended LO, RF, and IF (I/Q) ports has a conversion gain of 1 dB, IP1dB of -10 dBm, IIP3 of 0 dBm and IIP2 of 21 dBm at 16.4 GHz. The magnitude and phase errors between the I and Q channels are 1.34% and 0.6°, respectively. The dc power consumption of this I/Q mixer without output buffers is approximately 5 mW.