電路直接替代法用於改善步階低通濾波器之頻率響應斜度

Direct Circuit Replacement Method Enhanced Skirt Response of Stepped-Impedance Low-Pass Filter

本文提出電路直接替代法以指叉低通結構取代奇數階步階式阻抗低通濾波器之中間開路殘段，利用指叉低通結構使步階式阻抗低通濾波器於接近3dB截止頻率響應近乎陡直，有效改善頻率響應之斜度。指叉低通結構之電路特性除了以集總元件求取其等效電路外並使用電磁模擬軟體IE3D進行分析。文中並以本法介紹兩種步階式阻抗低通濾波器之設計使其3dB截止頻率附近達到極陡直的斜度，濾波器3dB截止頻率均設計在2.45 GHz。由實際測量與模擬結果顯示本法的可行性與適用性。

An interdigital low-pass structure directly replacing the center open stub of an odd-order stepped-impedance low-pass filter to achieve sharp skirt response is proposed. The characteristics of this structure were first analyzed by lumped element equivalent circuits and investigated by means of a full-wave microwave circuit simulator, IE3D. These simulated results then were applied as a guideline for the replacement. Two stepped-impedance low-pass filters were designed to enhance the skirt response near the 3dB cutoff frequency. The measured results show very good improvement on skirt response and in the rejection level of the stop band.