2017 Volume 14 Issue 4 Pages 20161259
A generalized circuit topology and synthesis method for multiband bandpass filters with controllable center frequencies and bandwidths are proposed based on a semi-hidden multi-mode coupling element. The so-called semi-hidden multi-mode coupling element is a Π-shape multi-mode J inverter constructed by a transmission line and two multi-mode capacitors at two ends, and the multi-mode capacitors are merged or hidden into nearby multi-mode resonators, modifying the equivalent LC circuits of the multi-mode resonators at the passbands. The multi-mode J inverter gets simplified and only a connecting line needs to be implemented, which simplifies high-order multi-band bandpass filter design. For validating the filter topology and synthesis method, the circuit and design example of triple-band bandpass filter are presented by using triple-stub triple-mode resonators, and a genetic algorithm is used for extracting the parameters of the triple-mode resonators. Good features including controllable bandwidths, high isolations are achieved.