A Fully Differential Ultra-Compact Broadband Rat-Race Coupler Using Folded Inverted Coupled-Lines in 180-nm CMOS

This paper presents an ultra-compact and broadband fully-differential rat-race coupler in 180-nm CMOS process, utilizing a novel folded inverted coupled-line (FICL) structure. By tuning the coupling coefficient k of the FICL, the phase response of the differential transmission line is significantly tailored, enabling a drastic reduction in the electrical length required to achieve a 90° phase shift while simultaneously expanding the operational bandwidth. The prototype achieves a measured insertion loss below 1-dB from 1.93 to 6.65 GHz, with amplitude imbalance under 0.2-dB from 0.8 to 10.47 GHz and phase difference within 2° from 0.8 to 13.59 GHz. Port isolation better than 45-dB is from 1.65 to 6.49 GHz. The core size of the proposed coupler is only 0.774 mm × 0.874 mm, corresponding to an exceptionally compact 0.000099λ₀² at 3.5 GHz. To the best of our knowledge, this work exhibits the smallest footprint and the broadest bandwidth.