[[alternative]]Design of Multiband Microstrip Bandpass Filters Using SIRs

碩士電子工程學系[[abstract]]本論文旨在利用步階式阻抗共振器(SIR)來設計新型高性能、縮小化之單頻與多頻微波帶通濾波器。在單頻帶通濾波器方面，我們使用非對稱式 λ/2 SIR (由兩個不同之 λ/4 SIRs組成)之結構設計，藉由其 λ/4 SIR殘段與反平行耦合的機制各自在通帶兩側產生一個零點，並選擇適當之SIR阻抗比(RZ)將第二共振頻率往高頻推至9.5 GHz，使所設計之帶通濾波器具有高選擇性與寬止帶之特性。在雙頻帶通濾波器方面，本研究探討並研製了二階及四階電路，我們以U-型λ/4 SIR為主體，並輔以模擬之結構耦合係數與間距關係圖及Butterworth的低通元件參數來設計。利用SIR可調控第二通帶之特性，使兩個通帶能被設計於所要之頻率點。此外，在雙頻濾波器電路中我們也加入反平行耦合結構來產生額外之傳輸零點，此對改善二階電路之截止帶頻率響應效果很有助益。 最後，本研究提出一個以兩段λ/4 SIRs設計之三頻帶通濾波器結構，其三個通帶可以設計在任意的三個頻率點，而電路的特點則在於部分共振器段為共用、通帶可調與製作容易等。在本設計中，此三頻帶通濾波器之兩段λ/4 SIRs之饋入點至共用接地孔可調整為等距，且提供個別共振時所需之Q值。如此濾波器電路不但可以具有優異的頻率響應，附帶地也可將電路的尺寸縮小。預期本論文所完成之各種帶通濾波器設計成果可直接應用於實際之相關微波電路中，所提出之設計概念則可作為此方面研發人員的有用參考。[[abstract]]In this thesis, we employ stepped-impedance resonators (SIRs) to design new single-band and multi-band microwave bandpass filters (BPFs) with high performance and reduced circuit size. For the single-band BPF, the asymmetric λ/2 SIR structure which consists of two different λ/4 SIRs is used as a building block. Two flexibly tunable transmission zeros are designed to be near the two passband edges to increase the skirt sharpness. One of the transmission zeros is due to the series resonance of the λ/4 open stepped-impedance stub; the other is produced by the anti-parallel coupling between adjacent SIRs. By appropriately choosing the SIR impedance ratio, the second passband (or the first spurious response) is pushed to a frequency of as high as 9.5 GHz. Thus wide stopband characteristic is achieved. For the dual-band BPFs, both second-order and fourth-order circuits are designed and investigated. The λ/4 SIRs which have a smaller circuit size and a larger third resonant frequency than that of the λ/2 SIRs are utilized for circuit design. With the aid of the relation of coupling coefficient and spacing between adjacent SIRs obtained from simulation and the Butterworth low-pass filter prototype parameters, the dual-band BPF is designed to have its two passbands located at the desired frequencies. In addition, the λ/4 SIRs are bent into a U-shape to reduce the circuit size. An anti-parallel coupling structure is also introduced to provide an additional transmission zero that lies between the two passbands. The latter is seen to be beneficial especially in enhancing the stopband rejection of the second-order dual-band BPF. For the last part of this thesis, we propose a new tri-band microstrip BPF with each of the coupling resonators composed of two λ/4 SIRs. The tri-band BPF can have its passbands designed at three arbitrarily useful frequencies and have the favorable features of tunable passbands and easy fabrication. In this design, the feed positions for the two □/4 SIRs are carefully located to have the same distance from the via-hole ground and meanwhile to give the required external quality factors. In such a way, the microstripline sections between the via-hole and each of the feed positions can be combined into one to simplify the filter structure and to reduce the BPF circuit size. Results obtained in this research can be applied to commercially practical RF and microwave systems. The technique presented in this research is expected to serve as a useful reference for microwave filter designers in this and related research fields