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A 10-40 GHz broadband subharmonic monolithic passive mixer using the standard 0.18 mum CMOS process is demonstrated. The proposed mixer is composed of a two-stage Wilkinson power combiner, a short stub and a low-pass filter. Likewise, the mixer utilizes a pair of anti-parallel gate-drain-connected diodes to achieve subharmonic mixing mechanism. The two-stage Wilkinson power combiner is used to excite a radio frequency (RF) and local oscillation (LO) signals into diodes and to perform broadband operation. The low-pass filter supports an IF frequency range from dc to 2.5 GHz. This proposed configuration leads to a die size of less than 1.1 times 0.67 mm 2 . The measured results demonstrate a conversion loss of 15.6-17.6 dB, an LO-to-RF isolation better than 12 dB, a high 2LO-to-RF isolation of 51-59 dB over 10-40 GHz RF bandwidth, and a 1 dB compression power of 8 dBm.