封裝微帶線電路之雜散模抑制 : 分析與設計

Abstract

本論文旨在研究幾種可行的微、毫米波電路的封裝結構，用以阻絕或抑 制封裝微帶線中雜散模(即高階模)的傳播。這些結構設計包括︰(1)在封 裝微帶線的上蓋加上一金屬隔板，用以阻絕入射的雜散模;(2)利用兩金屬 隔板組成一個部分封裝之電路結構，在封裝結構內可再加上吸收體，用以 抑制雜散模對電路的干擾;(3)在封裝微帶線中微帶線的兩側對稱地放置兩 片金屬片，以阻絕雜散模的傳播。對於這些電路結構的分析，論文中將提 出兩種高效率的分析方法來求得各個結構的電磁特性。第一種為結合線方 法與模式匹配法的數值分析技巧。此方法利用線方法本身的優點，亦即在 二維分析中，其電磁場僅在一個維度做切割取點近似，而在另一維度則以 解析函數求得，因此對模式匹配法中面積分的處理，可提供簡捷的運算過 程。同時此方法僅需利用到在不同介質層界面上的電磁場，因此不論在計 算時間上或記憶體需求上，都具有高效率性。第二種分析方法為一改良式 的有限元素法，稱之為〝遞迴式有限元素法〞。利用此法，我們可以很有 效率地分析一有限週期數的週期性結構，或是一極大的均勻區域中的電磁 現象。論文中將利用兩個實例來驗證此法的效率性。同時，論文中也將適 當地結合線方法、模式匹配法、與有限元素法，來分析一連結式的電路系 統。 對於雜散模的處理，我們將先分析一金屬隔板在封裝微帶線中的散 射效應。論文中將先比較封裝微帶線中各個傳播模(包括主模與雜散模)的 場型分佈，接著再分析金屬隔板的厚度與深度對屏蔽雜散模效應的影響。 我們發現在適當的設計下，金屬隔板可完全反射雜散模，而且對主模的傳 播沒有影響。 利用金屬隔板的特性，論文中接著提出一由兩金屬隔板所 組成的部分封裝之電路結構，此封裝結構內可再加上一吸收體，以吸收並 抑制封裝結構內所可能產生的共振現象。我們在此封裝電路內放置兩種電 路元件，一為短電流源(以模擬一主動元件)，另一為微帶線間隙(以模擬 一被動元件)，以分析此結構對電路元件的影響。部分封裝結構對雜散模 的抑制效應及其對主模的可能干擾，將藉由比較電路元件在有無此電路構 裝屏蔽下的電磁特性來加以分析。由論文所得結果，我們發現利用此部分 封裝(吸收性)之電路結構，雜散模對電路的影響將可被有效的抑制，同時 主模仍可正常傳播而不受此構裝的影響。 最後，論文中將提出一簡單的 結構設計，即在封裝微帶線中微帶線的兩側對稱地放置兩片金屬片，以阻 絕雜散模的傳播。金屬片可與微帶線電路同時製造，因此製做相當簡單。 我們將先研究側邊金屬片對封裝微帶線中各個傳播模的傳播常數及場型分 佈的影響。由此我們將發現在適當的金屬片寬度設計之下，微帶線中雜散 模的場型將會因金屬片的介入而被完全改變，進而使得這些雜散模可被金 屬片大量反射。接著側邊金屬片在封裝微帶線中的散射效應，將藉由改變 金屬片的寬度與長度來加以研究分析。由所得結果可知，利用大小適當的 側邊金屬片，我們可以有效地阻絕雜散模的傳播，同時不干擾到主模的正 常運作。 In this thesis, we propose and analyze several practical designs for shielding or suppressing the spurious (higher-order) modes in a packaged microstrip line circuit. These designs include (1) using a metal diaphragm attached to the top cover of the packaged microstrip line to reflect the spurious modes; (2) using a partially sealedpackage formed by two metal diaphragms, with or without an absorber affixed inside the package, to suppress the influence of the spurious modes; and (3) using two metal patches symmetrically deposited at the two sides of the center microstrip to chock off the spurious modes. To analyze these structures, two efficient techniques will be presented in the thesis, that is, the Method of Lines (MoL) Coupled with the Mode-Matching Method (MMM), and the Recursive Finite Element Method (RFEM). In the analysis of the method of lines coupled with the mode-matching method, it will be shown that, with the inherent advantages of the MoL that the field solutions are discrete only in one direction while are analytic in the other one, the mode-matching solution can be then formulated in a very concise and effective way. Moreover, only the tangential field components at discrete points on the interface between media are needed in this analysis, leading to that only a small amount of computation time and computer memory are required. In the analysis of the recursive finite element method, we will show that the electromagnetic problem involving a finite periodic structure with a large period number can be solved very efficiently using this technique. Also, a hybrid method combining the method of lines, the mode-matching method, and the finite element method, has been used to analyze a cascaded system. The scattering effects of a diaphragm will be first analyzed using the method of lines coupled with the mode- matching method. Based on a comparison on the modal field distributions, the shielding effect of the diaphragm will be studied by varying the depth and thickness of the diaphragm. It will be found that, with a suitable design, the diaphragm can mostly reflect the powers of the spurious modes while has little influence on the dominant mode. Knowing the effectiveness of the diaphragm, a partially sealed package formed by two metal diaphragms, with and without an absorber affixed inside the package, is then proposed for suppressing the influence of the spurious modes on the circuits. For a specific analysis, an electric current filament and a microstrip line gap are chosen to simulate, respectively, an active circuit element and a passive circuit element to be packaged. The suppression effect of the package on the spurious modes and the influence on the dominant mode are fully studied by comparing the excitation and scattering characteristics of the circuit elements with and without the package. It will be shown that this package design can effectively suppress the excited or scattered spurious modes while has little influence on the propagation of the dominant mode. Finally, a simple structure formed by two metal patches symmetrically deposited at the two sides of the center microstrip is proposed and analyzed for blocking the spurious modes in a packaged microstrip line. The variations (with the patch width) of the effective dielectric constants and field distributions, of the modes in the packaged microstrip line with infinitely long side patches, will be first investigated. The results suggested that there exist a range of patch widths at which the field distributions of the higher-order modes are totally different from those of the microstrip line without side patches. The scattering of the patches, as a function of the patch length and width, will be then studied using the recursive finite element method.It will be found that, by simply choosing appropriate patch sizes, the spurious modes can be reflected without sacrificing the normal propagation of the dominant mode.