以多模共振腔技術發展高頻率選擇性之帶通濾波器

Abstract

本計畫第一年 (2008/08-2009/07) 將研究具有寬頻、寬止帶以及高頻率選擇度之多模 共振腔帶通濾波器。電路設計將利用步階式阻抗共振腔的多模共振特性，整合寬邊耦合 與開路殘段等技術，單一諧振元件具有多階，以達到高頻率選擇度。 本計畫第一步先行研究具有多諧振模特性之步階阻抗共振腔的設計。根據傳輸線理 論，具有特殊長度比以及阻抗比的步階阻抗共振腔可以被設計為單一元件多共振模，用 於濾波器設計時，一個共振模貢獻一階，於是得到單元件高階響應。這種元件與傳統矩 型（或圓形）波導管多模元件不同的地方在於後者之多模諧振頻率均相同，本計畫所提 多模之諧振頻率各不相同。本計畫先以比例頻寬為 50%，設計並製作四至九階（四至 九傳輸極點）之準柴比雪夫帶通濾波器，此寬頻電路之輸入/輸出耦合量與外部品質因素 之間的關係也有詳細的討論。運用相同的設計概念，本計畫也擬挑戰符合超寬頻系統 （UWB）所使用的帶通濾波電路，並具有高頻率選擇性。 若想要增加通帶的頻率選擇度，可以增加電路的階數，也可以創造傳輸零點。本計畫 提出了搭配兩個四分之一波長開路殘段的半波長共振腔，以實現具備高頻率選擇度之三 模以及五模帶通濾波器。由於通帶兩側具有零點，此濾波器擁有相當銳利的邊帶之外， 也維持著良好的通帶介入損耗。初步結果證實，此電路的實做結果與模擬相當一致。 本計畫的第二年 (2009/08-2010/07) 將探討具有多重傳輸零點、寬截止帶以及高階微 小化之多模環形帶通濾波器。首先提出一具備寬截止帶之週期性步階式阻抗微型化環形 帶通濾波器，先以步階式阻抗共振環取代全波長環形共振腔，達到使用面積微型化以及 大幅提高第二諧振頻率目的，再利用輸入/輸出的饋入機制所產生的額外零點，進一步壓 制第二諧振頻率，達成寬截止帶的設計效果。接著使用四分之一波長寬邊帶耦合線來取 四分之三波長的微帶線使得電路面積可以更進一步的縮小。此組電路微型化的效果相當 良好，約為傳統環形濾波器的 19.4%。 上述的電路具備兩個傳輸極點與零點，當階數增加時，極點與零點的數目才會增加。 本計畫也提出了利用雙環形結構的四模帶通濾波器。此濾波器不但擁有四個傳輸極點 外，也同時具備了四個傳輸零點。由於通帶兩側擁有零點，此濾波器擁有比傳統環形濾 波器電路更好的邊帶，且保持良好的介入損耗。電路的實際量測將與模擬比對。

In the first year (2008/08 - 2009/07), this proposal aims at developing bandpass filters based on the multimode property of the stepped-impedance resonators (SIRs). The circuit structure incorporates broadside-coupled and proper shunt open stubs to realize broadband responses with a sharp transition band and a wide upper stopband. First, planar broadband bandpass filters of order up to nine are synthesized based on the multimode property of the stepped-impedance resonators (SIRs). The resonant frequencies of the SIRs are calculated based on the impedance and length ratios of its hi-Z and low-Z segments. Using different numbers of dual- or triple-mode SIRs, quasi-Chebyshev filters of orders from four to nine are synthesized with a fractional bandwidth Δ = 50%. Then, ultra-wideband (UWB) bandpass filter of order nine is contrived based on the multimode SIRs. To achieve sufficient coupling between adjacent SIRs, broadside-coupled structure is employed. The coupling levels are subject to allocating the nine transmission poles in the passband from 3.1 to 10.6 GHz based on the mode split graph. Finally, a bandpass filter of order five is developed based on a single multimode resonator. Using half-wavelength resonator tapped at its center with two open stubs in the vertical direction, a triple-mode bandpass filter can be achieved. By properly adjusting the length of open stubs, two zeros can be placed at upper and lower sides of the passband. A triple-mode resonator is tapped with additional two side stubs to design a pentad-mode resonator bandpass filter with quasi-elliptic function response. A quintuple-mode bandpass filter with Δ = 35% is fabricated. In the second year (2009/08 - 2010/07) of this proposal, multimode bandpass filters by means of single and coupled-ring resonators will be investigated. Dual-mode and quadruple-mode bandpass circuits with miniaturized area, sharp transition band and wide upper stopband will be realized. First, a periodic stepped-impedance ring resonator (PSIRR) is proposed to design dual-mode bandpass filters with a minimal area. The resonant frequencies of a PSIRR with different numbers of impedance steps are analyzed by the transmission line theory. For a realizable impedance ratio, the proposed bandpass filter not only has its first spurious response be pushed up to 3fo, but also a 55% size reduction as compared with a conventional dual-mode ring resonator circuit. Next, a microstrip-to-CPW (coplanar waveguide) broadside-coupled section is used to replace a 3λ/4-section of the conventional dual-mode 1λ−ring resonator filter for circuit area miniaturization. It is believed that it is a new idea to incorporate the broadside-coupled section into a ring resonator filter for size reduction. In experiments, two realized circuits show only 19.4% of the area of a uniform ring. Finally, quadruple-mode coupled-ring resonator is used to design quasi-elliptic function bandpass filters with a relatively wide bandwidth. The resonator consists of a pair of coupled rings with two open stubs symmetrically tapped to the inner ring. The stubs are used to move a transmission zero near the center frequency to its lower transition band. It is believed that it is the first ring resonator bandpass filter based on the coupled-ring to create four transmission poles and four transmission zeros. Two filters with relative bandwidths of 10% and 15% are fabricated and measured. 表