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This paper proposes a microwatt source-driven down-conversion mixer with broadband asymmetrical broadside-coupled baluns in a 90-nm CMOS low-power process. The forward body biased (FBB) technique reduces the threshold voltage (VTH) and supply voltage for operation in the near weak inversion region in millimeter-wave mixer designs. To effectively reduce the size of the chip, an asymmetrical broadside-coupled balun is developed with a bandwidth of 103 GHz (from 34 to 137 GHz) with a low insertion loss of 3.66 dB (3 dB for an ideal balun) at 58 GHz. The chip area of the balun is 0.016 mm2. The proposed FBB mixer has a 4.2-dB peak conversion gain and a 14.3-dBm input third-order intercept point at 55 GHz under a 2-dBm local-oscillator power. The dc power of the FBB mixer core is only 139 W, while it draws a 278-μA dc current from a 0.5-V supply. The fabricated FBB mixer, comprising two asymmetrical broadside-coupled baluns, and all of test pads and dummy blocks, occupies an area of 0.72 mm2. An FoMMixer1 that is obtained using the ultra-low power consumption FBB mixer is as high as 23.4.