寬頻帶八埠前向式方向耦合器暨四路徑寬頻相移器

Abstract

本論文以週期性結構之觀念為基石，提出八埠雙對稱前向式耦合器之設計，微型化及寬頻帶的特性為此論文之兩大目標。並利用新式之寬頻八埠耦合器，設計出寬頻四路徑相移器，可應用於建構寬頻八乘八巴特勒矩陣之相移電路，並達到縮小化的目的。在此，一個四模態分析技術首次提出，用於解析雙對稱八埠網路的散射參數矩陣。舉凡八埠元件具雙對稱之結構者，可適用此四模態分析技術，將八埠網路問題簡化為解雙埠網路。 一具有兩對耦合線並於接地平面嵌入週期性缺陷設計之結構被提出，此結構由三層金屬疊構所組成。此八埠耦合線結構具有雙對稱結構，因此存在偶-偶、奇-奇、偶-奇和奇-偶等四種傳播模態。就偶-偶模態而言，其電氣長度會因缺陷結構之等效電感的影響而呈現非線性增加的現象；另外其他三個模態之電氣長度仍舊保持隨頻率線性遞增的變化。偶-偶模態和其他三個模態之相位差距，也因為偶-偶模態電氣長度改變而增加。此外，偶-偶模態之特性阻抗並非設計於50Ω，而是透過調整偶-偶模態電氣長度之方式，達到輸入阻抗的匹配。並藉由此調整電氣長度之方式可設計出不同耦合量的八埠方向耦合器。設計之6-dB方向耦合器，其長與寬各為0.57波長及0.25波長，較現有許多其他八埠方向耦合器之尺寸為小。為更進一步微型化，於兩對耦合線上，周期性地並聯一段0.25波長的殘段，將使單位長度之相位差增大，進而縮減實際長度；另外將接地平面上缺陷結構做蜿蜒處理，亦有助達到微型化之目的。微型化結構之長與寬可再縮小，分別為0.33波長及0.19波長。 前面所提兩個八埠前向式耦合器之設計，其頻寬並非相當大。因此，一新式寬頻帶八埠前向式耦合器被提出。該耦合器亦為雙對稱結構，其四模態分析皆可達到相當寬頻帶的阻抗匹配。不同前述兩種耦合電路，此結構之偶-偶模態和其他三個模態間的相位差距，在某一頻帶間會近乎常數值。根據寬頻阻抗匹配與常數相位差距此兩項特性，可設計不同之功率輸出。其中6-dB及2-dB輸出耦合器可以達到47%的比例寬頻。 將上述寬頻帶八埠前向式耦合器設計為0-dB功率輸出，並加上四條相位延遲傳輸線，形成可同時提供四對不同相位差之寬頻相移器。因為此四路徑寬頻相移電路為三層金屬結構，透過重新安排八乘八巴特勒矩陣中3-dB耦合器之擺放位置於上、下金屬層，八乘八巴特勒矩陣中第二階的相移網路，將可單由此四路徑相移器來實現。不僅避免設計複雜的交叉線路，也大幅減少八乘八巴特勒矩陣尺寸。 在目前微波及毫米波系統趨向多埠輸入，多埠輸出情形下，預期所設計之八埠方向耦合器及四路徑寬頻相移電路，將有更多實際的相關應用。

We propose several eight-port, forward-wave direction couplers (FWDCs) for the purpose of compact sizes and broadband performances, based on periodic structures. We also design a broadband four-way phase shifter. This four-way phase shifter can be applied to the broadband 8 × 8 Butler matrix as phase correction network. Here, the four-mode analysis of dually symmetric networks is proposed for the first time. This technique can be used to simplify the eight-port problem to a two-port problem An eight-port coupler, composed of two pairs of microstrip coupled lines and etched periodical structures on the middle ground plane, is proposed. There are two planes of symmetry in this structure. Such a dually symmetric structure supports four distinct propagation modes, comprising the even-even (ee) mode, odd-odd (oo) mode, even-odd mode (eo) and odd-even (oe) mode. For the even-even mode, the electrical length dramatically increases with corresponding increases in frequency due to the slow-wave effect. The electrical lengths are linearly dependent on frequencies for the other three modes. Besides, the electrical length difference between even-even and the other three modes is also enlarged. Input impedance matching for the four propagation modes can be achieved by tailoring its dispersion characteristics through the patterned ground plane. An eight-port, forward-wave coupler designed with arbitrary coupling levels can be performed by adjusting the electrical length of even-even mode. Compared to some other eight-port couplers, the total size of the proposed 6-dB couplers (0.57 λg x 0.25 λg ) is more compact. Furthermore, an eight-port, forward-wave directional coupler by using open stubs and defected ground structure (DGS) is also proposed to reduce coupler size. The electrical lengths of the odd-odd, even-odd and odd-even modes are larger than the previous structure because of the capacitive enhancement. Owing to the longer DGS, the electrical length of the even-even mode is more enhanced as well. Thus, the phase difference between the even-even mode and the other three modes will increase. By using these techniques, we can obtain an eight-port 6-dB coupler with the most compact size (0.33 λg x 0.19 λg ). The aforementioned works concentrate on the design of size reduction. Next, a novel eight-port coupler is developed with periodic H-shape structures on the top and bottom layers for broadband applications. The broadband impedance matching for the four modes can be achieved and the phase difference between even-even mode and the other three modes can keep at an almost constant value over a broad bandwidth. 6-dB coupler, for example, is designed with the fractional bandwidth (FBW) of 47%. We utilize this broadband coupler with 0 dB through level to design a novel, broadband, four-way phase shifter by adding four delay transmission lines. It can support four pairs of phase-shifting routes designed with same or various phase differences in wild frequency range simultaneously. The errors of the phase imbalances for every pair of output ports are smaller than ±6° from 4.7 to 6.5 GHz, and the corresponding FBW is 36%. Finally, we propose a modified broadband 8 × 8 Butler matrix with three layers to miniaturize the size by using the proposed three-layer, four-way phase shifter. This broadband four-way phase shifter can simplify the design of the phase correction network and reduce the number of components and total size of the 8 × 8 Butler matrix.