Analytical Synthesis of Fully Canonical Cascaded-Doublet Prototype Filters

This letter presents an analytical procedure for the synthesis of a low-pass prototype filter constituted by the cascade of doublet blocks coupled by inverters or shunt reactances. The considered doublet is a second-order resonating structure composed of two resonant and two nonresonant nodes which allow the placement of two finite transmission zeros (TZs) in the frequency response. The considered configuration allows realizing compact fully canonical filters with the maximum number of TZs (the doublets are typically implemented through dual-mode cavities). The proposed procedure introduces for the first time an analytical procedure for synthesizing the low-pass prototype of the cascaded-block topology, overcoming the time consuming and inaccurate procedures currently based on numerical optimization. Two synthesis examples are presented to validate the novel synthesis procedure.