寬頻四乘四巴特勒矩陣之設計

Abstract

由於時代的進步以及智慧型手機的出現，人們對於無線通訊系統的品質以及需求量有爆炸性的成長。因此智慧型天線被提出並應用於第一、二、三代無線通訊系統中來改善訊號的品質，並增加資料的傳輸速度。其中一種智慧型天線為波束切換式天線，它是利用一個經過特殊設計的電路來為天線陣列產生特定的饋入訊號，使天線的主波束可以指向特定的方向，並可藉由選擇電路不同的輸入端埠來改變主波束的方向。巴特勒矩陣便是其中一種可以達到此種目的的電路，且因為巴特勒矩陣的使用元件數較少、損耗較小、以及較便宜等優點，所以它很快的成為一種非常熱門的技術。然而在不久的將來，只傳遞聲音、文字、圖案已經無法滿足人們對於無線通訊的要求，大量的資訊如影片等資訊都必須列入考慮。在此種情況下，普通的第一、二、三代無線通訊系統已經無法負荷。所以寬頻的第四代無線通訊系統勢必成為未來重點發展的技術，也因此智慧型天線必須一同進化為寬頻的智慧型天線。有鑑於此，本論文主要致力於寬頻的四乘四巴特勒矩陣之設計。 首先，在研究過四乘四的巴特勒矩陣後，可以由研究結果了解到四乘四的巴特勒矩陣可以切割成兩個部份的電路分開設計，此兩電路分別為：方向耦合器以及中間電路。方向是耦合器主要是用來分波，而中間電路則是用來調整各個出口端之間的相位差來產生合適的訊號。因此對於寬頻的巴特勒矩陣而言最重要的部分在於做出寬頻的方向耦合器，以及寬頻的中間電路。 對於方向耦合器，本論文提出一種新的寬頻前向式耦合器。此種耦合器是利用週期性結構來設計。在奇偶模態的分析方面，此結構在兩種模態中都能達到寬頻的阻抗匹配；更重要的是，藉由調整結構的物理參數，可以使兩模態的相位差在一個寬頻帶內維持是一固定值。根據以上兩種特點，此種耦合器可以用來設計寬頻的前向式耦合器。此外此耦合器特意使用三層結構來達到縮小化，以及簡化中間電路。在分析方面，本論文提出了一簡單的等效電路來描述此新型耦合器在奇偶模態下的傳播特性，同時還以此等效電路來建立一套完整的設計流程。 中間電路是由45度相移器以及交叉耦合器組成，由於耦合器的特殊三層結構可以使此中間電路在設計上有大幅的簡化，此簡化對於整體大小的縮減有很大的幫助。對於中間電路，本論文藉由簡單的架構來使各個輸出端可以在一個寬頻帶內維持相等的相位差。本論文也建立了等效電路來描述中間電路，並以此等效電路來建立一套設計流程。藉由適當的合併本論文提出的寬頻前向式耦合器以及中間電路就可以得到一寬頻的四乘四巴特勒矩陣及其完整的設計流程，此電路大小僅有1.02 λg × 1.17 λg且在正負6度的誤差限制之下可以達到47%的比例頻寬。與文獻中同樣為寬頻的四乘四巴特勒矩陣設計比較起來，這種新型的結構不論是在大小或是在頻寬上都有較好的表現。

Due to the progress of time and the emergence of smart phones, the desire for wireless communication with high quality and high transmission rate increases explosively. Therefore, the concepts of smart antenna have been proposed and used in the first, second, and third generation wireless communication systems to improve the quality of the signals and increase the data transmission rate. One type of the smart antennas is called the switch-beam antenna. For this type of smart antennas, a beam-forming matrix is used to generate a series of feeding signals and they are sent to the antenna array to create a specific radiation pattern. The main beam of the radiation pattern can be directed to a particular direction. Besides, the main-beam direction can be switched to different directions by using different input port of the beam-forming matrix. The Butler matrix is one of the beam-forming matrices and it becomes a popular technique due to the small loss. Nevertheless, transmission of voices, messages, and figures can no longer satisfy people’s desire for wireless communication systems soon in the future. Much larger information such as videos has to be taken into consideration. Under this circumstance, the invention of broadband fourth generation systems will certainly play an important role in the future technologies. Therefore, the smart antennas should evolve into broadband smart antennas together and that’s the reason why this thesis dedicates to the design of broadband 4 × 4 Butler matrices. The design of the 4 × 4 Butler matrix can be divided into two parts, 3-dB couplers and a middle network. The 3-dB couplers are used to split the power and the middle network is used to adjust the phase imbalances between output ports. Hence, the key points to design a broadband Butler matrix are broadband 3-dB couplers and a broadband middle network. For the 3-dB couplers, a novel broadband forward-wave directional coupler is proposed in this thesis. It is designed based on the periodic structure. The broadband impedance matching of the even and odd modes can be achieved. Besides, the phase difference between two modes can keep at an almost constant value over a broad bandwidth. Because of these special properties, the proposed coupler can be expected to be a broadband coupler. Equivalent models are proposed to describe the propagating characteristics of both modes. Furthermore, a detailed design procedure is also proposed based on the equivalent models. Last but not least, this coupler is fabricated using a three-layer structure to miniaturize the size and simplify the design of the middle network. The middle network is composed of two 45° phase shifters and a crossover. A middle network with a broadband constant phase difference between the phase shifter and the crossover is proposed. The equivalent models of the proposed middle network are also constructed and used to form a design procedure. A broadband Butler matrix and its design procedure can be obtained by combining the proposed broadband coupler and middle network. The size of the proposed broadband Butler matrix is 1.02 λg × 1.17 λg. Moreover, the errors of the phase imbalances between output ports are smaller than ±6° from 1.96 GHz to 3.15 GHz, and the corresponding fractional bandwidth is 47%. Compared with the broadband Butler matrix designs in previous works, the proposed Butler matrix can achieve a broader fractional bandwidth while using a more compact size.