Filter design for folded canonical topologies based on equivalent circuit segmentation

This paper presents a methodology to design filters with folded canonical topologies, which implement cross couplings between non-adjacent resonators. The technique is based on segmenting the traditional coupling matrix in a step-by-step fashion. At each step, a subset of the whole physical structure is optimized to match the response of the corresponding segment of the coupling matrix. In the context of this design technique, in this paper we propose an efficient segmentation methodology of the coupling matrix based on multiport networks. The use of multiport networks allows to generate at each step several goal functions, which are simultaneously used during the optimization of the corresponding physical segment. These multiport networks allow to efficiently monitor the different paths of the signal, present in folded canonical topologies. It is shown that this strategy leads to a fast convergence of the step-by-step segmentation technique for the design of this type of coupling topologies. We apply the proposed methodology to the design of two filters using the quartet topology. The first filter has two transmission zeros placed at the real frequency axis, and the second one has two complex transmission zeros intended for group delay equalization. The results indicate that the proposed methodology is effective for the design of this type of coupling topologies, leading to initial dimensions for the filters that typically have less than 1% of error when they are compared with those obtained from a final global optimization. (C) 2019 Published by Elsevier GmbH.