Synthesis of UWB bandpass filter by multistage of one-wavelength commensurate SIRs

A novel filter prototype composed of multistage of stepped impedance resonators (SIRs) is proposed to realize the ultra-wideband bandpass (UWB) filter. The relation between the modes number and stage number of resonators is firstly discussed, followed by the derivation of the corresponding Chebyshev filtering function. Furthermore, the equivalent circuit model of this new filter prototype is established based on the transmission-line theory for filter synthesis. As an example, a FCC UWB filter is synthesized with the proposed filter prototype to have the passband from 3.1 GHz to 10.6 GHz (fractional bandwidth is about 110%). The synthesized circuit model are further optimized by EM simulator, fabricated in microstrip line and characterized by network analyzer. The good agreement between the measured and predicted frequency responses validates the effectiveness of newly proposed synthesis techniques. In addition, the designed filter exhibits good characteristics of comparatively low insertion loss, quite sharp skirts, very flat group delay and good stopband (especially in lower one) as well.