應用於單頻帶與多頻帶之微帶線多模諧振器帶通濾波器之設計與合成

Abstract

本論文研究數種具有多模步階式阻抗諧振器之單頻與多頻帶通濾波器。在第一部分討論射頻與微波濾波器的基本結構與理論，其中包括了微帶諧振設計、諧振器品質因素與相鄰諧振器之間的耦合係數等。 本論文的第二部分探討具有寬頻、超寬頻、寬截止帶以及高選擇度之多模單頻帶通濾波器。這些濾波器的設計將利用寬邊耦合級與微帶開路殘段等不同的結構來實現。首先是利用多模步階式阻抗諧振器合成平面寬頻帶通濾波器。利用傳輸線理論，此諧振器長度比以及阻抗比的結構參數便可輕易的設計，在此實現五個比例頻寬為 50% 且具有包括四、六、六、八、九個傳輸極點的準柴比雪夫響應帶通濾波電路。除此之外，輸入與輸出耦合量與外部品質因素之間的關係也詳細的討論。運用相同的設計概念，本文另外實現了一組可以使用在超寬頻系統上的高選擇度帶通濾波電路。在這裡，寬邊微帶耦合結構提供中間與輸入輸出級足夠耦合量用以成功的設計出擁有九個傳輸極點且比例頻寬超過 110% 之超寬頻帶通濾波器。 由於上述的電路理論上並不具備任何的傳輸零點，因此，唯有當階數增加，通帶的選擇度才會增加。所以，發展具備有橢圓響應之具多模帶通濾波器成為極重要的工作。在本文中，我們也提出了搭配數個微帶線開路殘段的步階式阻抗諧振器，實現高選擇度五模帶通濾波器。由於通帶兩側的零點，此濾波器擁有相當銳利的頻率選擇度，同時維持良好的介入損耗與輸入輸出反射損耗。電路的模擬與實測結果相當一致。 在本論文的第三個部份則製作了數組多模多頻帶通濾波器。藉由傳輸線理論，我們選擇不同的諧振器長度比以及阻抗比，以設計與實做四組運用單一共振電路之雙模雙頻帶、雙模三頻帶與混成雙/三模雙頻帶通濾波器。除此之外，我們也利用開路殘段以增加兩個頻段間的頻率比。運用相同的設計概念，本文亦實現了一組串接混成雙/三模雙頻高選擇性帶通濾波電路。在此也詳細討論每一個頻帶的頻率比以及頻寬與阻抗比之間的關係。由結果顯示，此電路的理論與實做的響應亦均相當一致。 在本論文的最後一個部分則是分析上述各電路的優、缺點，以及其未來的可能發展。

In this dissertation, several sets of multimode single- and multi-band bandpass filters are realized. In the first part, some background for synthesis and design of RF/microwave filters will be presented. Several structures such as straight and ring resonators, quality factor and coupling coefficient between adjacent resonators will be discussed. In the second part, planar broadband bandpass filters of order up to nine are synthesized based on the multimode property of stepped-impedance resonators (SIRs). Based on the transmission line theory, the modal frequencies of the SIRs are calculated based on the impedance and length ratios of its high-Z and low-Z segments. In the synthesis, the SIR coupling schemes are determined by the split mode graphs. Using one, two, two, three, and three dual- or triple-mode SIRs, quasi-Chebyshev filters with four, six, six, eight, and nine transmission poles, respectively, are synthesized with a fractional bandwidth = 50%. Emphasis is placed not only on designing the input/output (I/O) coupling structures for matching the external Q (Qext) and the circuit bandwidth, but also on matching the resonant peaks of the circuit with the nominal Chebyshev poles. Furthermore, a ninth-order ultra-wideband (UWB) bandpass filter is also contrived based on the similar SIR structure. To achieve sufficient coupling between adjacent SIRs, broadside-coupled microstrip structure is employed. The coupling levels are subject to allocating the nine transmission poles in the passband from 3.1 to 10.6 GHz based on the mode split graph. As a result, a bandpass filter synthesized with bandwidth = 110% is simulated and fabricated. In the last circuit of this part, planar bandpass filters are contrived with a very sharp transition band and a wide stopband. The SIR is tapped with two open stubs at its center and two more stubs at the junctions of the low- and high-impedance sections. By properly trimming the stub lengths, the leading five resonances of the SIR have a close proximity and constitute the passband. Besides, two transmission zeros can be created at both upper and lower sides of the passband. Based on the multimode resonator, a single-resonator bandpass filter with five transmission poles is designed to have a fractional bandwidth of 35%. In addition, by cascading two such resonators, a tenth-order filter can be designed and realized. In the third part, planar multiband bandpass filters are implemented. On the strength of transmission line theory, the resonance spectrum of an SIR is plotted on the basis of the impedance and length ratios of hi-Z and low-Z sections. By properly selecting the structure parameters of an SIR, the multiband multimode resonator is readily realized. Using only a single SIR, dual-mode dual-band, dual-mode triple-band and hybrid dual-/triple-mode dual-band bandpass filters are demonstrated. Emphasis is placed on designing specified frequency ratio and circuit bandwidth on each passband. To improve the flexibility of circuit design, extra short open stubs are also used to adjust the circuit bandwidth and passband ratios. Several sets of experimental circuits such as single and coupled structures are presented. Measured results of all experimental circuits show good agreement with simulated responses. In the last part, we will discuss the advantages and drawbacks of the above-mentioned circuits and their possible future developments