Theoretical/synthesis design of compact UWB bandpass filters with very sharp roll-off characteristics

A simple quasi-elliptical-function-based design methodology is proposed for a class of ultra-wideband(UWB) filters consisting of stepped-impedance resonators (SIRs) based on parallel-coupled three-line, short-circuited stubs and two-section stepped-impedance open-circuited stubs. Furthermore, the influence of the location of the loaded stubs and the type of stubs applied to the filter characteristics is investigated. As an example, a UWB BPF with a fractional bandwidth (FBW) of 110% (BW: 3.1~10.6GHz) is then synthesized, simulated, fabricated and measured. The measured frequency responses of the UWB filter compare very well with the predicted ones and successfully satisfy outdoor specifications of FCC, which validates the correctness and effectiveness of the proposed technique. The designed UWB filter exhibits very sharp roll-off characteristics of 90dB/GHz and a compact size of 13.05mm.