設計微小化與寬拒帶之超寬頻濾波器

Abstract

本篇論文將介紹一個簡易的方法來設計一個超寬頻的集總(lumped)濾波器。本篇所提出的方法，是利用傳輸極點(transmission poles)來設計頻帶內的特性,以及利用傳輸零點(transmission zeros)來改善拒帶的響應。 最後利用史密斯圖的匹配方法求出電路中被動元件的值。另一方面，因為此架構只由八個被動元件所組成而且同時利用上層與下層金屬來設計電路架構，所以具有微小的電路面積。而最後量測結果顯示所設計的濾波器具有以下特性，3dB 頻寬為128% (2.8GHz ~ 11.4GHz)，頻帶內返回損耗最小值為 0.3dB，在12.4 GHz 到 23.7 GHz的頻帶內，其拒帶之嵌入損耗皆大於 20dB，具有非常微小的電路尺寸 0.23 λ × 0.31 λ，其中 λ 是在微帶線結構中中心頻率為7.1 GHz的導波波長。此外由量測資料我們也可以求的其濾波器其群延遲在整個頻帶內變動範圍為0.32 ns 到 0.46 ns。

This paper proposes a simple methodology for designing a wideband compact LC-filter. According to the proposed methodology, given the transmission zeros which can construct the in-band characteristic and the transmission zeros which can improve stopband rejection, the filter’s component values can be obtained graphically on the Smith chart. Additionally, because the filter only consists of eight lumped components, which is implemented at both top and bottom layer, it has compact size. The measured results shows that the filter prototype has a measured 3-dB fractional bandwidth of 128% from 2.8 GHz to 11.4 GHz, minimum insertion loss of 0.3 dB within the pass-band, superior 20 dB stop-band rejection from 12.4 GHz to 23.7 GHz, and very compact circuit size of 0.23 λ × 0.31 λ, where λ is the guided wavelength of the microstrip structure at the center frequency f0 = 7.1 GHz. Moreover, from the measured S-parameters, we obtain that the developed filter with the feeding lines exhibits flat group-delay response ranging from 0.32 ns to 0.46 ns over the whole passband.