An efficient design method of RF filters via optimized rational-function fitting, without coupling-coefficient similarity transformation

An efficient method is presented to design RF filters without the similarity transform of their coupling coefficient matrix as circuit parameters, which is very tedious due to pivoting and deciding rotation angles needed during the iterations. The transfer function of a filter is directly used for the design and its desired form is derived by the optimized rational-function fitting technique. A 3rd order coaxial lowpass filter and an 8th order dual-mode elliptic integral function response filter are taken as an example to validate the proposed method.