BiCMOS Rat-Race Coupler Based on Slow-Wave CPS Transmission lines for 120 GHz Applications

In this paper, a new on-chip rat-race coupler, based on slow-wave coplanar stripline transmission lines, is proposed for 120 GHz applications. High quality factor transmission lines were employed. A parametric analysis targeting the geometrical variables of slow-wave is presented. A 19.6% area reduction for the core of the coupler is achieved thanks to slow-wave technique manifested in conductive ribbons placed under transmission lines. Simulation results show that isolation and reflection coefficients are preserved under 15 dB. Insertion loss value is 4.3 dB for the SW-CPS version at 120 GHz. Maximum amplitude imbalance is 0.6 dB reported at 140 GHz. While, phase imbalance difference for the proposed structure doesn't exceed 2.5° for the whole bandwidth of interest.