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A broadband distributed sub-harmonic resistive field effect transistor mixer with a novel modified rat-race broadside coupler using a 0.13-mum CMOS foundry process is presented in this letter. Using the broadside coupler, a reduced-size rat-race is fabricated with a compact size of 110 times 80 mum2. From the measurement results, the mixer achieved a conversion loss of 12 dB from 32 to 70 GHz of RF frequency. The passive sub-harmonic mixer has zero power consumption. With the modified rat-race broadside coupler, the mixer achieves a good LO-IF isolation of better than 30 dB and a compact core chip size of 0.55 times 0.4 mm2.