以人造微帶耦合線實現微型化與寬頻帶前向式方向耦合器

Abstract

本論文以週期性結構之觀念為基石，進行平面前向式耦合器的設計，微型化以及寬頻帶的設計為此論文之兩大目標。首先，一人造耦合線有著週期性磨菇狀接地平面之結構被提出，此結構由三層金屬疊構所組成。此耦合線結構具有對稱性，因此可以採用奇模態與偶模態的方式分析。對奇模態而言，其電長度仍舊保持著隨頻率線性遞增的變化，但對偶模態而言，其電長度會因為磨菇狀接地結構的影響而呈現陡然增大的現象，而奇模態與偶模態的相位差距，也因為偶模態電長度的改變而增大。更特殊的是，此結構之奇模態與偶模態的特性阻抗，可以同時被設計接近於50 Ω，以滿足此二模態阻抗匹配的條件。以此耦合線結構設計的前向式耦合器，因上述兩特性的關係，使得耦合器的設計更加微小化(約為波長的1.26倍)。相較於微帶耦合器，所提出之耦合電路其尺寸縮減率可以超過80%。 為了生產成本的考量，另一人造耦合線有著週期性缺陷平面之結構被提出，此一結構由雙層金屬疊構所組成。對奇模態而言，其電長度仍舊保持隨頻率線性遞增的變化，但對偶模態而言，其電長度會因結構之等效電感的影響而呈現非線性增加的現象。而奇模態與偶模態的相位差距，也因為偶模態電長度的改變而增加。不同於先前提出的結構，奇模態與偶模態的特性阻抗，不可同時設計於50 Ω，但是可以透過對奇模態與偶模態電長度調整的方式，而達成對此二模態輸入阻抗的匹配。利用此提出之雙層金屬的耦合線結構可以達到0-dB與3-dB耦合器的設計。此0-dB與3-dB耦合器設計之長度分別為0.5波長與0.25波長。此雙層結構設計之耦合器，可以較先前之三層結構之耦合器有更小的電路尺寸。相較於微帶耦合器，所提出之耦合電路其尺寸縮減率可以超過92%。 先前幾個耦合電路都是著重於微型化的設計，其頻寬並非相當大。因此，一新式寬頻帶前向式耦合電路被提出。此一結構之奇偶模態分析，皆可達到相當寬頻帶的阻抗匹配。不同於先前四個耦合電路，此結構之奇模態與偶模態的相位差距，在某一頻帶內部會近乎常數值。根據寬頻阻抗匹配與常數相位差距此兩項特性，0-dB與3-dB耦合器可以被設計出且此二耦合器可以分別達到超過64% 與44% 的比例頻寬。此二電路的尺寸約為1.93波長與0.96波長。

This dissertation focuses on the planar forward-wave directional coupler (FWDC) design by using the concept of periodic structure. Compact sizes and broadband performances are two goals achieved in this thesis. In the beginning, a three-layered FWDC with periodic mushroom-shaped ground plane is developed for the purpose of size reduction. The electrical length of odd-mode is kept linearly dependent on frequency, but the even-mode electrical length is dramatically enlarged due to the capacitive enhancement. The phase difference between odd- and even-mode is significantly increased because the even-mode phase delay is getting higher. Besides the large phase difference, the characteristic impedances of these two modes can be designed to be 50 Ω simultaneously for the impedance matching without using additional matching networks. The proposed coupler is designed with the length of 1.26 λg as an example. Compared to the conventional microstrip forward coupler, the size reduction of the proposed structure is more than 80% due to the slow-wave effect of the even-mode. Next, a two-layered FWDC with periodic defected ground structure (DGS) is proposed to reduce both the cost and coupler size. The odd-mode electrical length of the proposed structure is linearly dependent on frequency, but the even-mode electrical length is enhanced due to the inductive loading. The phase difference is also enlarged because the even-mode phase delay is getting higher. Input impedances of these two modes can be perfectly matched by adjusting the electrical lengths. For example, 0-dB and 3-dB coupler with periodic DGS are designed with the length of 0.55 λg and 0.275 λg, respectively. Compared to the microstrip forward coupler, the size of the proposed structure reduces by more than 92%. The aforementioned works concentrate on the design of size reduction. Finally, a novel forward coupler is developed with periodic H-shaped structure for broadband applications. The broadband impedance matching for even- and odd-modes can be achieved and the phase difference between these two modes can be maintained as constant within a wide frequency range. For example, 0-dB and 3-dB couplers are designed with the fractional bandwidth (FBW) 64% and 44%, respectively. The lengths of them are 1.93 λg and 0.96 λg , respectively. Compared to the conventional works, the proposed H-shaped forward couplers have wider operational bandwidth with the compromise on the size.