A novel miniaturized directly coupled UWB BPF

Abstract

A novel ultra-wideband (UWB) bandpass filter (BPF) structure with directly coupled I/O ports is presented in this paper. The presented structure consists of a two-end short stepped-impedance resonator with a pair of stepped-impedance open stubs (SIOSs) and one uniform-impedance open stub (UIOS) symmetrically loaded on its high-impedance section. The presented structure has multiple resonant modes. The directly coupled configuration, the SIOSs and UIOS can provide three transmission zeros on its upper stopband, which not only improve the skirt of the upper transition band but also extend the upper stopband. As an example, an UWB BPF is designed, fabricated, and measured. The measured results show that the fabricated filter has a less than 0.7 dB insertion loss, better than 14 dB return loss and wide stopband up to 18.5 GHz. The measurement agrees well with the simulation.