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 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 a fractional bandwidth (FBW) of 110%, with amplitude imbalance under 0.2-dB from 0.8 to 10.47 GHz (171.6% FBW) and phase difference within 2° from 0.8 to 13.59 GHz (177.8% FBW). Port isolation better than 45-dB within 10-dB return loss is from 1.65 to 6.49 GHz (118.9% FBW). The core size is an exceptionally compact 0.000099λ02 at 3.5 GHz. This work exhibits the smallest footprint and the broadest bandwidth, alongside state-of-the-art amplitude and phase balance performance.