Rapid surrogate-assisted design optimization of minimum-size broadband branch-line couplers with variable topology

This work discusses simulation-driven design of miniaturized wideband branch-line couplers with a variable topology. Size reduction is enabled here by replacing uniform transmission lines of the original coupler with slow-wave structures in the form of cascaded compact cells and meander lines. The primary goal is to determine a number of cells in the cascade and particular cell dimensions for which the minimum size of the coupler as well as its required operating conditions are ensured. To this end, we employ a surrogate-assisted technique involving a trust-region gradient search framework. Computational efficiency of the design process stems from estimating the Jacobian of circuit responses at the level of a low-fidelity model of the cascade. The latter is composed in a circuit simulator from duplicated EM-evaluated data blocks of a single cell and is well correlated with the corresponding high-fidelity model. The key advantage of this work is the utilization of a reconfigurable, cheap, and well-aligned low-fidelity model. The proposed approach is demonstrated through design of a minimum-size two-section branch-line coupler with quasi-periodic dumbbell-shaped cells and meander lines. Excellent circuit performance as well as its small size showcase the reliability and usefulness of the presented method. Experimental verification is also provided.