Rapid hierarchical simulation-driven design of compact multi-section branch-line couplers

In this paper, we present a methodology for expedited hierarchical simulation-driven design of miniaturized multi-section branch-line couplers (BLCs). The design process starts from optimization of elementary cells of a slow-wave structure, followed by a fast tuning of their cascade as well as the tuning of the entire coupler. The last two stages exploit surrogate-assisted optimization with the underlying low-fidelity models constructed using local response surface approximations and circuit-theory-based connection of lower-level components (e.g., elementary cells for cascade tuning, etc.). This results in an overall low computational cost of the design optimization process, which is less than three EM simulations of the entire microwave component at hand. The proposed methodology is demonstrated through the design of a novel compact two-section BLC for 0.85 GHz to 1.15 GHz frequency range.