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A new low power doubly balanced sub-harmonic mixer for 5-GHz industrial scientific medical (ISM) band direct conversion - zero IF receiver in an inexpensive 0.25-μm CMOS is presented. The mixer uses a power and silicon real estate efficient LO frequency generation scheme to overcome LO self-mixing problem common in conventional direct conversion receivers (DCR). Simulated with 1% gm mismatch, 0.5% load mismatch and 2° LO phase error, the mixer is able to achieve very good IIP2 of 55.3 dBm, IIP3 of -6.5 dBm, and voltage conversion gain of 8 dB while consuming less than 1.75 mA from a single 3V supply. The mixer also achieves input compression, P1dB of -12 dBm and an overall double side band noise figure of 5.96 dB. Including die pads, the proposed mixer takes up less than 1mm2 of silicon real estate.