串接短平行耦合線低通濾波器設計

A Cascaded Short Coupled Line Low Pass Filter Design

本文提出一種以短平行耦合線與並聯開路殘段組成低通濾波器之設計方法，且由開路殘段產生的傳輸零點增加3 dB 截止頻率附近之頻率響應斜率，同時增加截止頻寬。電路設計以耦合線pi 型等效電路為基礎，並遵循一般濾波器設計流程達到要求之規格。實際電路製作取五階與七階柴比雪夫頻率響應，截止頻率設為2.45 GHz，漣波值為0.01 dB，並以電磁模擬軟體MicrowaveOffice和IE3D 進行模擬，電路實作以FR-4板材雕刻。實測與模擬結果顯示具有良好的一致性。

We have reported a design procedure for a compact low-pass filterusing short parallel coupled-line and parallel open stub. By introducing thetransmission zero contributed by the connected open stubs, the frequencyselectivity and bandwidth of the low-pass filter were significantlyimproved. To have a systematic design, the circuit was synthesized basedon the traditionally used network synthesized method incorporating thePi-type network corresponding to the equivalent circuit of a coupled-linestructure. A 5-th order and a 7-th order Chebyshev low-pass filter weredemonstrated to have a ripple level around 0.01 dB in the pass-band region.The excellent agreement between the measured result and that of thenumerical simulation using IE3D and Microwave Office validates thedesign concept and equivalent circuit of the structure under consideration.