具平坦截止頻帶之新型雙通帶濾波器

Abstract

本文提出一個新型雙通帶濾波器的設計，單一級電路包含兩個電路並聯，其一是四分之一波長的耦合線串聯四分之一波長的傳輸線，其二是其串聯反向電路。將多個單一級電路直接串接即可得到多級電路，不僅增進通帶頻率的選擇度，也增加截止帶的衰減量。本文所提出的電路，在兩通帶之兩側以及截止頻帶中有多個傳輸零點。本文以傳輸線理論分析單一級電路，並提供一級與二級電路雙通帶響應的相關設計圖。在實作方面，將指叉式電容加入耦合線的兩端，補償奇偶模波速不相等的問題，藉以改善截止頻帶中的抑制量。本文提供四個實作電路的模擬與量測比較，以驗證此設計。

A new circuit structure is proposed for design of dual-band bandpass filters. A one-stage circuit consists of two two-port networks in shunt connection; one is a coupled-line section of λ/4 long followed by a transmission line segment of identical length, and the other has the same elements but cascaded in reverse order, where λ/4 is the wavelength at arithmetic mean frequency of the two passbands. Higher-orders circuits can be obtained by directly cascading two or more such stages and show improved frequency selectivity. In addition to on both sides of the passbands, transmission zeros are also created in the rejection bands. Analysis of the one-stage circuit is formulated by the transmission line theory and design curves for one- and two-stage circuits are provided. In realization, interdigital capacitors are incorporated with the λ/4 coupled sections for compensating the effect of unequal modal phase velocities on the filter performance in the rejection band. Two circuits are fabricated and measured to validate the idea.