Design of in-line filters with transmission zeros using strongly coupled resonators pairs

This paper presents an original synthesis-based approach for the complete design procedure of pseudoelliptic in-line filters using strongly-coupled resonators pairs (SCRPs). The proposed synthesis method is based on the derivation of the coupling matrix of a new basic building block that we called SymBox. This new topology is composed of four resonators and five couplings, and allows for the generation of one finite-frequency transmission zero. Among its various properties, the most interesting feature of the SymBox including an SCRP is that all the couplings are positive regardless of the transmission zero position (above or below the passband), thus significantly reducing the complexity of the physical structure of the filter. The proposed design procedure is completed with the application of an efficient full-wave methodology, allowing one to accurately obtain the physical dimensions of the filter starting from the electrical parameters calculated in the previous synthesis phase. The complete design of a filter composed by a cascade of resonators and SymBoxes is presented and illustrated with a practical example. It is shown how the filter realized by Bastioli and Snyder in 2013 (whose design was carried out by means of a lengthy full-wave optimization procedure) can be easily designed with the proposed synthesis technique.