高品質無線通訊雙頻操作被動元件之研發(I)

Abstract

本計畫第一年 (2007/08~2008/07) 探討如何設計具有多重傳輸零點、寬截止帶、以及高階微小化 的雙頻帶濾波器。我們先前的研究結果顯示，當雙頻帶濾波器的階數增加時，可實現的頻寬範圍會 變小。因此，期望發展具有橢圓函數響應的雙頻帶濾波器。如此將可無需增加階數，也能改善通帶 的選擇度。同時，適當地安排傳輸零點的頻率也可用來抑制高階膺頻，增加高止帶的頻寬。 計畫中使用新型的堆疊式步階阻抗諧振腔濾波器設計雙頻帶濾波器。初步的分析顯示，若利用 斜對稱平行耦合線饋入，即可在兩個通帶的兩側產生零點。研究發現必須適當地選擇耦合線的長度 以及間距，才能在兩個通帶都得到良好的折返損失。在初步的實驗中，只需要使用二個步階阻抗諧 振腔，即可設計出橢圓函數響應的雙頻帶濾波器。這比我們之前須使用四個步階阻抗諧振腔的設計 更小也更簡易。此外，如何利用傳輸零點抑制高階膺頻的方案也將在此計劃中研究。 本計畫第二年 (2008/08~2009/07) 研究具有雙頻帶操作的方向耦合器（90o 與 180o 分合波器） 及其微小化設計。先將 180o 分合波環周邊以步階阻抗傳輸線取代，以達到微小化與骯頻操作的目 的。每一段 λ/4 週邊均以兩段低阻抗及一段高阻抗傳輸線段串接之傳輸線電路取代，所得之分合波 器所佔用面積為傳統設計之 49％，初步研究成果中以操作於 900/1800 MHz 雙頻電路之實作與量測 為例。 在進一步的成果中，利用步階阻抗傳輸線並接開路殘段，取代標準設計結構中四分之波長傳輸 線段。步階阻抗傳輸線段以及並接殘段的特性阻抗與長度等幾何參數，均需先由理論推導出相關公 式，再將以繪圖或解非線性方程式得到。在初步成果中，如果只使用開路殘段而不使用步階阻抗， 所設計的雙頻帶180o 耦合器，其周長只有傳統單頻帶設計的32%。如果進一步使用步階阻抗，環的 周長更可達傳統單頻帶設計的25%以內，實作電路之測量頻率響應與設計模擬相當一致。在本計劃 中，亦將利用上述方法設計微小化的雙頻直交（90o）耦合器。具有抑制高階諧波，以達到寬截止帶， 並兼具雙頻與微小化的設計將一併研究。

In the first year (2007/08~2008/07) of this proposal, dual-band bandpass filters with plural transmission zeros, wide upper stopband and compact size will be studied. In our previous study of stacked SIR bandpass filters, it is found that the higher the dual-band filter order is, the smaller the realizable bandwidth will be. Therefore, it is desirable to design a dual-band BPF exhibiting quasi-elliptic function response at two designated bands, so that the selectivity and stopband performance can be improved and not to resort to the increase of filter order. The stepped-impedance resonator (SIR) in a stacked configuration is designed to exhibit two designated bands. Plural transmission zeros on both sides of the two passbands are generated by using skew-symmetric coupled-line feed. By adjusting the geometry of the feed lines, the return losses within two bands can be improved. Microstrip filters were measured to verify the design. Further, how to suppress higher-order harmonics by transmission zeros will also be developed in this proposal. In the second year (2008/08~2009/07) of this proposal, a planar dual-band rat-race coupler is miniaturized by equipping the ring with stepped-impedance sections. Each λ/4 on the circumference of the ring is replaced by a cascade of two low-Z sections with a hi-Z section in between. The area of the experimental circuit has 49% of that of the traditional design with uniform impedance. An experimental coupler with dual-band operation at 900/1800 MHz is fabricated and measured to validate the design. Furthermore, microstrip couplers are designed to have a dual-band characteristic and miniaturized circuit area. For the rat race coupler, it is realized by attaching open stubs to both ends of a stepped-impedance line section to replace a λ/4-section in the traditional 6λ/4-ring. The circuit geometric parameters are obtained by solving simultaneous nonlinear equations formulated based on the transmission line theory. In our prototype design using uniform transmission line sections, the circumference of the 180o coupler uses only 32% of the conventional circuit designed at lower band. When stepped-impedance sections with an impedance ratio R = 4 are used, the circumference can be further reduced to be less than 25% of the conventional design. Microstrip rat race ring hybrids operating at 2.45/5.2 GHz are fabricated and measured. Measured results have good agreement with the design and simulation. In this proposal, the above approach is also employed to design dual-band branch-line couplers with circuit miniaturization. Circuit design with suppression of spurious harmonics will also be investigated.