Rapid Design Tuning of Miniaturized Rat-Race Couplers Using Regression-Based Equivalent Network Surrogates

A simple technique for fast design tuning of compact rat-race couplers is presented. Our approach involves equivalent circuit representation, corrected by nonlinear functions of frequency with coefficients extracted through nonlinear regression. At the same time, the tuning process connects two levels of coupler representation: EM simulation of the entire circuit and re-optimization of the coupler building blocks (slow-wave cells and folded transmission lines) with specifications obtained by appropriate manipulation of the corrected network model. The latter is driven by the necessity of accounting for EM cross-couplings within the coupler that result in performance degradation. Demonstration examples of a miniaturized rat-race coupler indicate that rapid and reliable design closure (at a cost of two or three coupler simulations) can be accomplished even for strong cross-couplings within the structure.