An ultra-compact folded inductor based mm-wave rat-race coupler in CMOS

This paper presents the design scheme of a fully integrated folded inductor based mm-wave rat-race coupler within only one inductor footprint. The detailed circuit analysis and design equations are presented. A proof-of-concept design example at 70 GHz is implemented in a 130nm CMOS process. Compared with other reported rat-race couplers at 60–70GHz, the proposed design achieves a more than 3× size reduction and occupies a core chip area of only 0.037mm2. Based on the measurements, the proposed rat-race coupler design achieves an insertion loss of 4.1dB, i.e., an additional passive loss of 1.1dB at 70GHz, and the return loss is better than 15dB with 22GHz bandwidth (31.4%). The phase difference is within 180°±5° from 62GHz to 78GHz, and the amplitude mismatch is within 1dB from 67GHz to 81GHz. To the best of our knowledge, this is the first demonstration of a folded inductor based rat-race coupler.