Direct Coupling Matrix Synthesis for Filters With Cascaded Singlets

This article proposes a direct coupling matrix synthesis approach for filters with cascaded singlets. A sequence of elementary matrix transformations is applied in the synthesis procedure to obtain the target coupling topology containing many cross couplings and nonresonant nodes. Two filter synthesis examples are presented to demonstrate the novel synthesis technique. To physically realize synthesized coupling matrices, some singlets need to be transformed into their dual forms, which are also singlets but with different coupling coefficients. A 6–4 generalized Chebyshev filter is designed according to the synthesized coupling matrix, where the singlets are realized as oversized rectangular waveguide cavities employing nonresonant modes. Due to the cascaded singlet coupling configuration, the modular design technique can significantly reduce the simulation time. After the waveguide modules are assembled, the model-based vector fitting technique is applied to aid the fine-tuning of the filter. Both simulation and measurement results are in good agreement with theoretical filter responses, validating the proposed coupling matrix synthesis and design technique for filters with cascaded singlets.