Low-cost multiband compact branch-line coupler design using response features and automated EM model fidelity adjustment

Design closure of compact microwave components is a challenging problem because of significant electromagnetic (EM) cross-couplings in densely arranged layouts. A separate issue is a large number of designable parameters resulting from replacement of conventional transmission line sections by compact microstrip resonant cells. This increases complexity of the design optimization problem and requires employment of expensive high-fidelity EM analysis for reliable performance evaluation of the structure at hand. Consequently, neither conventional numerical optimization algorithms nor interactive approaches (e.g., experience-driven parameters sweeps) are capable of identifying optimum designs in reasonable timeframes. Here, we discuss application of feature-based optimization for fast design optimization of dual- and multiband compact couplers. On one hand, design of such components is difficult because of multiple objectives (achieving equal power split and good matching and port isolation for all frequency bands of interest). On the other hand, because of well-defined shapes of the S-parameter responses for this class of components, feature-based optimization seems to be well suited to control multiple figures of interest as demonstrated in this work. Two-level EM modeling is used for further design cost reduction. More importantly, we develop a procedure for automated determination of the low-fidelity EM model coarseness that allows us to find the fastest possible model that still ensures sufficient correlation with its high-fidelity counterpart, which is critical for robustness of the optimization process. Our approach is illustrated using two dual-band compact couplers. Experimental validation is also provided.