具延伸截止帶之雙通帶微帶線濾波器

Abstract

本論文提出一個新型雙通帶濾波器的設計，具有延伸上截止帶。此電路由兩個部分組成，其一是半波長的耦合線主電路，其二是饋入於半波長耦合線主電路中點部分的子電路。 本文以傳輸線理論分析電路，並提供雙通帶響應的相關設計圖，以說明傳輸極點與零點的位置、兩通帶之間的相對位置以及兩通帶的頻寬。文中討論子電路對於延伸截止帶的效果，適當地設計子電路為短路步階阻抗殘段(shorted□stepped-impedance stub)之阻抗比例和電氣長度來控制傳輸零點，以實現具有寬截止帶的雙通帶濾波器。為了改善濾波器截止帶的響應，採取兩種方法，折疊耦合線成為雪夫曼區段來補償奇偶模波速的不同；並在輸入端埠和輸出端埠加上並聯開路殘段(shunt open stub)，在適當的頻率產生傳輸零點。本文實作一個電路，並加以量測，以驗證所提之設計。

A new circuit structure is proposed for design of dual-band bandpass filters with extended upper stopband. This circuit consists of two parts: one is the main circuit having a coupled-line section of long, and the other is a sub-circuit feed-in the middle of the main circuit, where is the wavelength at arithmetic mean frequency of the two passbands. Analysis of the circuit is formulated by the transmission line theory, and design curves for dual-band response circuit are provided to allocate the positions of transmission poles and transmission zeros, the ratio of two passband frequencies, and the fractional bandwidths of two passbands. The effects of stopband extension by sub-circuit are discussed. The impedance and length ratios of the short stepped-impedance stub will be adjusted to control transmission zeros for stopband extension. For enhancing the filter performance in the stopband, two methods are adopted. The coupled lines are folded and become Schiffman sections for compensating the phase velocity difference between the even and odd modes, and the insertion of the shunt open stubs at the input and output ports to create the transmission zeros. One circuit is fabricated and measured to validate the idea.