Multimode Stepped-Impedance Resonator (SIR) with Central Via-Coupling for UWB Filter Design

Abstract

This paper discusses a new type of planar stepped-impedance resonator (SIR) - a structure centrally loaded with a via to the ground. The novel SIR has unique properties which can be used to design wideband and ultra-wideband (UWB) filters. The shunt via provides a strong inductive coupling in the centre of the structure, which determines a new mechanism of coupling the SIR modes. The central via modifies electromagnetic field distribution of the SIR filter creating in its frequency response an additional transmission pole in the passband. It also provides a transmission zero at DC, which allows for a direct contact port coupling, with no need for the more problematic, very narrow gaps to couple the structure with input/output lines. Thanks to this, the new SIR can be successfully designed and fabricated on thin dielectric substrates. The gapless couplings require impedance matching between input/output lines and the SIR. This is realized by stepped-impedance transformer sections. Depending on the filter bandwidth, these port matching sections can be non-resonating, or begin to resonate generating additional transmission poles. These properties are discussed in the paper based on two example microstrip bandpass filters: a 4-pole 3.1 - 4.8 GHz and a 6-pole 3.1 - 10.6 GHz UWB filter.