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This paper presents an integrated 2.4 GHz ultra low power CMOS frequency down-converting active mixer based on a single balanced Gilbert-cell resistor-loaded topology where all MOS transistors of mixer core are optimally biased in subthreshold region. At only 500 muW DC power consumption under 1.0 V power supply and small LO power of -9 dBm, this mixer has a measured power conversion gain of 15.7 dB and DSB noise figure of 18.3 dB. At same DC power dissipation and LO power, -9 dBm IIP3 and -28 dBm input PldB are obtained while the input return losses at RF and IF ports are better than -20 dB and -8 dB, respectively. Measured LO to IF and LO to RF isolations are more than 22 dB and 33 dB, respectively. With a help of a newly defined figure of merit that contains the effects of conversion gain, noise figure, linearity, LO power, DC power consumption and operating frequency, it is shown that the subthreshold mixer presented here is superior to conventional CMOS mixer designs reported in the literature.