抑制多重高階諧波之平行耦合微帶線帶通濾波器設計

Abstract

本論文提出過耦合的方法壓抑帶通濾波器之多重諧波，其中以四分之波長耦合級可用以壓抑二階諧波，而六分之波長耦合和八分之波長耦合，分別壓抑三階諧波和四階諧波，以改善通帶的對稱性和延伸上截止帶。電路設計的方法和步驟可根據傳統帶通濾波器設計，僅需稍做修改即可，所設計的二個電路，模擬和實做量測結果相當一致。 本文第二部分將上述過耦合觀念應用於鋸齒耦合微帶線結構，四分之波長耦合級可設計同時壓抑二階和四階諧波；六分之波長耦合可設計同時壓抑三階和六階諧波；七分之波長耦合可壓抑七階諧波；若於輸入輸出端使用擇定鐀入產生的零點，壓抑五階諧波。如此可達到多重高階諧波的抑制，並大幅延伸截止帶的頻寬看增加截止帶中的衰減量。 本文第三部分，提出以週期性步階式阻抗共振腔設計壓抑多重諧波之平行耦合微帶線帶通濾波器，一段四分之波長耦合級由週期性的高低阻抗組合而成，適當的調整每個週期性步階式阻抗共振腔的阻抗比，將傳輸零點放在特定的頻率以抑制諧波。所設計的電路也可延伸截止帶的頻寬和增加截止帶中的衰減量。

The inherent zero of a microstrip coupled stage near twice the design frequency (2fo) is found tunable by varying its coupling length. This zero is used to suppress the unwanted response of paralle-coupled line filters at this frequency. The above idea is extended to design over-coupled middle stages of the filter for the spurious |S21| peaks at 3fo and 4fo, so that the upper stopband can be greatly enhanced to 5fo. The passband preserves a response as good as the traditional design. Measured results have a good agreement with simulation data and show that the idea works very well. Corrugated coupled lines are also devised to design microstrip bandpass filters with multispuriois suppression. Quarter-wave corrugated stages are tuned to allocate inherent transmission zeros at 2fo and 4fo. Stages with proper coupling lengths are arranged to cancel the unwanted peaks at 3fo, 6fo and 7fo, and the tapped input/output scheme is employed to tackle that at 5fo. The measured data of designed bandpass filters show rejection levels better than 30dB in the upper stopband. Three circuits are fabricated and measured to demonstrate the idea. Periodic stepped-impedance resonators (PSIRS) are proposed to design bandpass filters for multispurious suppression. Denoted as PSIRN, a PSIR of □/2 long at design frequency fo consists of N periods of hi-Z and low-Z sections. A PSIRN coupled section shows transmission zeros at various frequencies. The zero can be tuned by changing the impedance ratio of the hi-Z and low-Z sections, and be adopted to suppress the spurious peaks. Responses of the PSIR filters show good rejection in the upper stopband.