Synthesis of Ultra-Wideband (UWB) Bandpass Filters by Multistage of Coupled Stepped-Impedance Resonators and Short-Circuited Stubs

摘要

In terms of the transmission-line theory, a synthesis of an ultra-wideband bandpass (UWB) filter prototype composed of multistage stepped-impedance resonators (SIRs) and two short-circuited shunt stubs positioned at input-, and output- ports is presented. Following the derivation of the equivalent circuit and the corresponding Chebyshev-type filtering function, the synthesis procedure is described in detail. Furthermore, a design table is created for practical design of the UWB filters. As an example, a two stage UWB filter with an FBW of 110% is synthesized based on the proposed filter prototype to meet FCC's outdoor mask. The synthesized circuit model are confirmed by the commercial circuit simulator and then optimized by the EM simulator, fabricated in microstrip line and characterized by the network analyzer. The good agreements between the measured and predicted frequency responses validate the effectiveness of newly proposed filter structure and the corresponding synthesis technique. In addition, the designed filter exhibits good characteristics of comparatively low insertion loss, quite sharp skirt, very flat group delay and good stopband (especially in lower one) as well. It should be also highlighted that, compared with the conventional parallel-coupled stepped-impedance-resonator (SIRs) filter, the newly proposed filter structure can reduce the total length of about 3/4λ{sub}g. Moreover, in terms of the presented design technique, the proposed filter prototype can be also used to easily realize the UWB filters with an FBW even greater than 110%.