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In this work, the design and measurement of a new 4x subharmonic mixer circuit is presented using CMOS 0.18 m technology. With an RF input signal at 12.1 GHz, and an LO signal at 3.0 GHz, an intermediate frequency of 100 MHz is produced (fIF = fRF - 4fLO). The mixer uses a modified Gilbert-cell topology with octet-phase LO switching transistors to perform the quadruple subharmonic mixing. Included in the design is an active balun for the RF signal and a circuit that generates an octet-phase LO signals from a differential input. The mixer has a conversion gain of approximately 6 dB, 1-dB compression point of -12 dBm, IIP3 of -2 dBm, and IIP2 of 17 dBm. The circuit also exhibits excellent isolation between its ports (e.g. LO-RF: 71 dB, 4LO-RF: 59 dB).