A novel microstrip UWB bandpass filter using stub-loaded multiple-mode resonator

A novel microstrip ultra-wideband (UWB) band-pass filter (BPF) using stub-loaded multiple-mode resonator (MMR) is presented in this paper. The MMR is constructed by loading three open stubs in a uniform-impedance resonator, i.e., one uniform-impedance stub loaded at the center and two stepped-impedance stubs loaded at the symmetrical side locations. Four modes, including two odd modes (f_odd1 and f_odd2) and two even modes (f_even1 and f_even2), could be tuned in the desired band, and two transmission zeros are created near the cutoff frequencies by the open stubs and two interdigital coupling feeding lines, leading to the pass-band selectivity greatly. The paper shows modal resonant frequencies against the open-stub-loaded parameters at the center, which can control the even modes flexibly, while the odd modes remain the same. A compact planar UWB BPF is simulated. The simulated results show good band-pass filtering performance and sharp selectivity.