High-selectivity UWB bandpass filter with a notched band using stub-loaded multi-mode resonator

In this paper, a novel type of stub-loaded multiple-mode resonator (MMR) with electrical coupling is developed for exploration of a single band-notch UWB Bandpass Filter (BPF) with high selectivity. Based on even and odd mode theory, five resonant modes, including two odd modes, three even modes, and a notched band can be properly designed to form the UWB passband. The odd-mode resonant frequencies can be controlled by tuning the coupling strength between two open stubs with no impact on the even modes, which provides a high design freedom of the UWB filter. Moreover, a pair of transmission zeros (TZs) can be also generated at either side of the passbands for sharp skirts and a TZ is also created within the passband for interference signal rejection. The simulated and measured results show that a notched band is obtained at 8.0 GHz, which can be controlled by arranging the parameters of the MMR, sharp attenuation slops at the lower and upper bands can be simultaneously realized, which validates the proposed design flow well.