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 3/4λg. 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%.%小型超広帯域バンドパスフィルタを実現するため、伝送線路理論に基づいて、入力および出力端に終端短絡スタブ付き多段平行結合ステップインピーダンス共振器型フィルタおよびその合成方法を提案した．新提案のフィルタにおいて、等価回路を構成しそれに対応する近似伝達関数を導出した後、等価回路における各回路パラメータを求める方法を変数分離法により導出し具体的設計のための設計表を求める．最後に提案した設計理論を確かめるため、実際例として、FCCの規定（帯域：3．10－10．6GHz、比帯域：110％）に準拠した2段フィルタの合成、シミュレーション、試作を行い、その特性を示す．測定結果は理論、シミュレーション結果と良好な一致を見ることができ、設計理論の妥当性が確かめられた．理論的に設計されたUWBフィルタは低挿｛損失、急峻なスカ－ト特性、平坦な群遅延特性、海に良好な低域側の阻止域特性を呈することが確認されている．更に、新提案のフルタの全長は、従来のスタブなし多段平行結合ステップインピーダン云共振器型フィルタと此較して、約3／4波長の短縮を実現している．また、捷案したフィルタの構造及び合成理論を用いることにより、任意の比帯域を持つフィルタ（比帯域が110％より広帯域となるフィルタでさえ）を実現することも可能である．