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A low flicker noise and high linearity passive mixer in 0.18 μm CMOS is proposed. The mixer, which consists of gm-stage, AC coupled LO switch-pair and low noise trans-impedance amplifier (TIA), features a low flicker noise corner frequency of 5 kHz. The mixer is designed for direct conversion receivers in China Mobile Multimedia Broadcasting (CMMB) system. Simulation results show that the passive mixer achieves a voltage conversion gain (CG) of 21.2 dB, intercept point (IIP3) of +13 dBm, double sideband (DSB) noise figure (NF) of 18.9 dB @ 5 kHz, white DSB NF of 18.1 dB, and output DC offset of less than 3 mV. The mixer dissipates 7.2 mA (including I path and Q path) from a 1.8 V supply.