Quasi-elliptic ultra-wideband bandpass filter with super-wide stopband

Abstract

This paper describes a low-loss and compact planar ultra-wideband (UWB) quasi-elliptical bandpass filter, which is based on a via-free multimode resonator (MMR) that requires no defected ground plane. The MMR structure essentially consists of a step impedance transmission-line resonator that is loaded centrally with a folded open-circuited stub. The MMR structure is also loaded with two pairs of short high impedance stubs with their ends folded towards the central stub. The proposed MMR structure excites several resonant modes comprising three even and two odd-modes within the filter’s passband. The resulting transmission response is highly selective with a low-loss (0.4 dB) passband across 3.60–11.3 GHz when fabricated on a conventional dielectric substrate, which is otherwise only possible using high temperature superconductors. In addition, the structure generates transmission zeros to yield a filter that exhibits an extremely wide stopband extending up to 35.5 GHz for S21 ≥ 20 dB. The via-free planar filter structure makes its design and construction easy and economical for mass production. The filter’s performance was verified through measurements. The planar filter structure can be easily manufactured using standard PCB technology.