Synthesis Design of Multiband Bandpass Filters Employing Multimode Bandstop Resonators With Star-Like Topology

Multimode resonators (MMRs) can be utilized to build multiband bandpass filters (MBPFs). Each individual mode serves as the operating mode for one of the passbands in a well-known coupling topology. However, several difficulties are involved with the classical design method. In this article, we present a novel star-like topology to design MBPFs based on MMRs. Differently from the classical use, in the proposed novel topology, they are so regarded as bandstop resonators and provide multiple controllable transmission zeros. To explain the proposed concept and the implementation method, two bandstop MMRs - a classical dual-mode stub loaded resonator (DSLR) and a new tri-mode dual-stub loaded resonator (TDSR) - are investigated with detailed design formulas and diagrams. Then, a tri-band MBPF with equal individual sub-bands, a quad-band and two quint-band MBPFs with unequal individual sub-bands are synthesized and designed using the proposed bandstop MMRs. Additionally, simulations and experiments are used to verify the efficiency of all prototypes. Good agreements with the theoretical calculations are observed for all the MBPFs.