Band-notched ultra-wideband bandpass filter design using combined modified quarter-wavelength tri-section stepped-impedance resonator

This study presents a new band-notched ultra-wideband (UWB) bandpass filter (BPF) that is designed using a new structure-shared multi-mode resonator (MMR). The MMR is formed by combining two quarter-wavelength (lambda/4) tri-section stepped-impedance resonators (TSSIRs) in a proper fashion to exhibit both lambda/4 and half-wavelength (lambda/2) types of resonance. With the first two resonant frequencies of the inherent lambda/4 TSSIR, the first two of the parasitic lambda/2 SIR and the coupling peaks of the input/output interdigital parallel-coupled lines properly located in the UWB passband, a six-transmission-pole UWB BPF can be realised. In this filter design, besides obtaining a uniform in-band transmission response, a notched band centred around 5.5 GHz is created by embedding L-shaped open-end slots in the TSSIR and loading the TSSIR with closely coupled winding-type resonators (WTRs) to filter out the 5-GHz wireless local-area network radio signal interferences. In addition, spur-lines are embedded in the TSSIR to sharpen the passband skirts for selectivity improvement.