Design of a Compact Ultra-wideband Bandpass Filter Based on Improved Multi-mode Resonator

This paper proposes an improved multi-mode resonator (MMR) composed by a folded third order stub-step impedance resonator (SIR) with four open-circuit stubs and a center short-circuit stub. Analysis and design procedure of the novel structure are presented in details. For demonstration, a compact UWB Bandpass filter with operating frequencies from 3.1 to 10.6 GHz is designed, fabricated and tested. With total size of 0.974$\lambda_{g}\times 0.186$ $\lambda_{g}$, the filter has advantages of low insertion loss and extremely broad stopband region up to 25 GHz. Good agreements between the simulation and measurement results primely verify the design method.