Novel circular dual-mode filter with both capacitive and inductive source–load coupling for multiple transmission zeros

In this paper, a novel miniaturized bandpass filter (BPF) using a circular dual-mode resonator with both capacitive source–load (S–L) coupling and inductive S–L coupling is presented for the first time. The proposed filter consists of a circular open stub-loaded dual-mode resonator and two meandering feed lines with both capacitive and inductive S–L coupling to obtain seven transmission zeros and good harmonic suppression. Compared with the other two BPFs using two conventional feeding styles, i.e. only capacitive S–L coupling and only inductive S–L coupling, this type of BPF can not only possess a dual-mode response, but also generate more transmission zeros to suppress an undesired harmonic passband. Moreover, the transmission zeros can be also controllable to meet the required filter selectivity. The simulated and measured results are presented and show good agreement.