抑制二階諧波的平行耦合線帶通濾波器

Abstract

在平行耦合線帶通濾波器的設計中，本文利用輸入、輸出級耦合線長度適當的增加與重疊，在上截止頻帶產生兩個傳輸零點，以抑制濾波器的寄生二階諧波，增加截止帶的衰減率、並改善通帶的對稱性。這種重疊架構的耦合線，會讓奇模的路徑長度大於偶模的路徑長度，因為奇模的相速比偶模的相速來得大，此結構剛好可以針對奇模、偶模相速不一致的情形作補償產生零點。本論文利用這一個特點，將兩個傳輸零點放在最佳的位置，達到最好的二階諧波抑制效果。本文有數個實作電路，其模擬與量測結果相當一致。

An overlap scheme at both end stages of parallel coupled microstrip filters is investigated to improve the passband symmetry and suppress the parasitic passband at twice the center frequency. The overlap scheme extends the phase length for the odd mode so that the electric lengths of the entire filter for both odd and even modes can be identical, so that a transmission zero can be located near twice the passband frequency. Simulation results show that the required overlap length may change as the filter specification or the substrate is changed. It is found that the spurious response at twice the passband frequency can be suppressed down under –40 dB. Good agreement between measured and simulated results shows that the idea works very well.