Synthesis-Based Design of Quasi-Inline Filters With Strongly Coupled Resonators Quadruplets

A comprehensive design procedure for quasi-inline filters using strongly coupled resonator quadruplets (SCRQs) is presented in this article. Just recently proposed, the SCRQ is a novel building block structure that allows for the generation of two transmission zeros and three passband poles in the response of microwave filters. This block belongs to the category of strongly coupled resonators, whose adoption has the following advantages: 1) reduced overall filter size (quasi-inline configuration can be realized) and 2) avoids the usage of capacitive couplings (only positive couplings are required). After a brief summary recalling the concept behind the working principles of the SCRQ, a suitable model for this new block is introduced, which is then used in an original synthesis procedure of higher order filters composed by an arbitrary number of cascaded SCRQs. In addition to several numerical synthesis examples, the design technique here introduced is validated through a manufactured prototype exhibiting a response with 11 poles and four transmission zeros.