共平面波導饋入之直交分合波器

Abstract

摘要 本論文是研究關於直交分合波器(Branch-line coupler)的原理與特性。在論文中利用立體耦合架構(3-D coupling structure)來設計雙層結構之直交分合波器。在實作上，由於四分之ㄧ波長的要求，分枝線分合波器的頻寬多限制於10%-20%之間。不過，本論文所設計出之分合波器不同於傳統架構之分合波器，該結構利用額外增加的耦合量改變中心頻率附近的阻抗匹配，而有著較大的寬頻(Wideband)響應，且只需ㄧ級就能達到寬頻的效果。 首先在單層板中利用共平面波導(Coplanar waveguide)饋入微帶線(Microstrip)的方式，實現了立體耦合架構(3-D coupling structure)。將原本在單層板中實現的平面電路設計推展到立體設計。並依此設計直交分合波器。此分合波器有著寬頻(Wideband)的優點，且頻帶內十分平坦(Flatness)，並可以完全保證低頻阻隔。 由於上述之直交分合波器所使用的微帶線(Microstrip)其色散現象(Dispersion phenomenon)還有輻射損失(Radiation loss)嚴重，然而帶線架構(Stripline)卻沒有這些缺點。所以接下來亦利用共平面波導饋入帶線(Stripline)的方法，實現立體架構(3-D structure)。其一是利用連通柱(Via)直接電性接觸的方式傳遞訊號；其二是利用四分之ㄧ波長共振器(Resonator)在中心頻率時候共振，將電磁能量從共平面波導饋入至帶線(Stripline)的方式。據此設計出兩種雙層板材的直交分合波器，以改進插入損耗(Insertion loss)及輻射損耗(Radiation loss)。以上可以在量測結果中得知，在同樣的中心頻率下，相較於微帶線架構之直交分合波器，帶線架構之分合波器的確有較好的效果。

Abstract In the beginning of this thesis, the basic theory and characteristics of quadrature hybrid coupler, which is also known as branch-line couplers are introduced. Afterwards, we make use of 3-D coupling structures to realize branch-line couplers in multi-layered PCB. In practice, due to the quarter-wave length requirement, the bandwidth of a branch-line coupler hybrid is limited to 10-20%. But, the branch-line coupler we designed is different from the conventional structure. The broadband branch-line coupler has been developed to enlarge operation bandwidth by enhanced impedance matching design at port junctions. Hence, we achieve wideband performance by only one section. At first, we make use of CPW-fed microstrip method to implement the 3-D coupling structures in a single-layered PCB. Using the 3-D coupling structures, the uniplanar circuit design implemented in a single layered PCB can be extended to multi-layered design. This kind of branch-line coupler has the advantages of wideband, flat response on transmission band, and DC current blocking. Since the microstrip lines used in the CPW-fed microstrip branch-line coupler possess serious dispersion phenomenon and radiation loss. One can use strip lines to avoid these disadvantages. Therefore, we further use the method of CPW-fed strip lines to realize the 3D coupling structures to modify the branch-line coupler. There are two main techniques for the transition between a microstrip and CPW. One is the electrical contact, and the other is the electromagnetic coupling. The former usually uses via hole for the electrical contact. The mechanism of the proposed transition for the later is base on the electromagnetic coupling between stripline quarter-wavelength open-stub and CPW quarter-wavelength short-stub. This transition design is capable of inducing strong coupling between CPW and stripline around the resonance frequency. Accordingly, we design two kinds of CPW-fed branch-line coupler in multi-layered PCB to improve insertion loss and radiation loss. Observed from the measurement data, the insertion loss of stripline type is better than that of microstirp type at the same center frequency.