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A 5-6 GHz active subharmonic I/Q mixer pair has been designed and fabricated for use in direct conversion receivers for the Unlicensed National Information Infrastructure (U-NII) bands. The design includes I and Q subharmonic mixers and an LO conditioning chain. All circuits are fully differential and are designed to operate from a 3.3 V supply. Each mixer consumes 5 mA of current. A conversion gain of 6 dB, an IIP2 of +29 dBm and an input 1-dB compression point of -5.4 dBm were measured. The 90° phase balance between the I and Q mixer IF outputs was investigated in the presence of injected digital switching noise on the power supply lines; 90° balance was maintained over a wide range of switching waveform frequencies and amplitudes.