本論文中將提出具有高選擇度之平衡-非平衡濾波器設計，電路架構為結合平衡-非平衡轉換器與帶通濾波器的創新設計，藉由可控制的傳輸零點對選擇度與頻寬做調整，本次設計將以數位廣播之L頻段(1452 MHz~1492 MHz)作為設計實例;平衡-非平衡轉換器設計上利用適當的集總元件使得耦合線的電子長度小於或等於λ/4，且串聯於耦合線之間的電容可有效控制諧波處零點的位置；接著考量與濾波器電路架構結合時特性上的變化，將對非對稱饋入之平衡-非平衡轉換器提出探討；另外為了增加外頻的選擇度，本論文使用髮梳式帶通濾波器的架構整合於上述的平衡-非平衡轉換器結構中，並且經由電路分析後，設計者可以自由選擇零點位置而達成高選擇度的設計需求，在此架構下所使用之平衡-非平衡轉換器與帶通濾波器會共用部分的耦合線段，因此當濾波器階數增加時亦不會大幅增加電路的使用面積，故此架構乃是一個具高選擇度、微型化與具彈性化之設計。電路的製作以ARLON 25N 0.46 mm的板材完成，介電常數為3.38、介質損耗為0.0025，並且由量測的結果在平衡端大小的不平衡性小於1 dB而相位不平衡性則在6°以內，而此結果與模擬比較後具有良好的一致性。 In the thesis, the design of the Balanced-to-unbalanced filters with high selectivity is presented. The proposed circuit configuration is built by combining a balun and bandpass filters in which the selectivity and bandwidth can be adjusted by controlling the position of transmission zeros. The proposed filters are designed in L-band (1452 MHz~1492 MHz). By adding lumped elements in the balun circuit, the electric length of coupled lines can be shorter than λ/4. Moreover, a transmission zero which is controlled by a series capacitor between the coupled lines of the balun can be applied to restrain the undesired second harmonic. In order to improve the selectivity, the filters will be integrated into the above-mentioned balun. Using circuit analysis, the designer can set transmission zeros at anywhere to get great selectivity. The balun and the bandpass filters will be merged together by using common coupled lines, so the miniaturization could be achieved. The effect of combining the balun and bandpass filters, and the characteristic of asymmetrical balun will be discussed. The proposed balanced-to-unbalanced filter has advantages regarding selectivity, circuit compactness, and design flexibility. All circuits are fabricated on substrate, ARLON 25N. The relative permittivity, loss tangent, and thickness of the substrate are 3.38, 0.0025, and 0.46 mm, respectively. The proposed balanced-to-unbalanced filters achieved 0.5 dB amplitude imbalance and 6° phase imbalance. The measured results are in good agreement with the simulation.
