有無接線時跨越隔離區的信號傳輸特性

Abstract

在印刷電路板的設計裡，為了要防止或抑制RF coupling的發生，常會使用Moat來作隔離（Isolation）或切割（Partitioning），以達到更好的信號品質(signal quality)及功能整合度(functional integrity)，如果兩隔離區需做某程度的連接時，就會利用一條或數條跨越Moat的Drawbridge。本文即利用時域有限差分法(FDTD)來探討單一線路的信號傳輸特性受一簡單Moat/Drawbridge的影響，其佈置方式可能會對跨越兩個隔離區的信號完整性(signal integrity)產生變化。 在本文的研究裡分成兩大類的問題，第一類問題是Trace由Drawbridge正上方通過的狀況，當Drawbridge的寬度愈小時，類似一個slot antenna的Moat的長度就愈長，因而將會輻射出更多的能量。就一特定Moat的長度而言，當Moat的寬度(即Drawbridge的長度)在約 以下的頻率範圍內時，跨越Moat的切線電場在寬度方向的分佈約為均勻，因此輻射隨頻率上升而增強，但在較高的頻率時，由於電場不再均勻分佈，輻射即不再隨之增加了，最大輻射發生在Moat的長度約等於 的頻率。而基板較薄時輻射亦較小。 第二類的問題是Trace未經Drawbridge而直接跨越Moat的狀況。 我們發現當Drawbridge愈往外移時，電流回流被迫繞道的狀況將愈惡劣，電波傳播所受到的擾動就愈嚴重，此外這也將造成一個更大的電流回路區域，因此輻射損耗便會增加，而能夠傳播的信號也就隨之減少了。 結論是如果要有較好的信號完整性和更低的輻射損耗，signal trace必須直接由Drawbridge的正上方通過，而在我們討論的例子裡，Drawbridge的半寬度必須是約三倍的基板厚度(或四倍的trace半寬度)。

To prevent RF coupling in printed circuit board design and layout, engineers often use the moat to provide the isolation, and so the products may achieve better signal quality and functional integrity. If it is desired to connect some components or devices between different functional subsections or to maintain DC continuity, one may apply one or more bridges across the moat. In this paper, the signal propagation of a single signal trace affected by a simple moat/drawbridge is analyzed with Finite-Difference Time-Domain (FDTD) method. The signal integrity across two isolated sections may be changed by the layout. We divide our research problems into two cases. The first one deals with the circumstance of signal trace crossing the moat through an underlying, centered drawbridge. When the width of drawbridge becomes smaller, the length of the moat, which behaves as a slot antenna, becomes larger so that it could radiate more energy. For a given moat length, tangential electric field across moat would distribute uniformly in the direction of width if the moat width (i.e. the drawbridge length) were within frequency range below . Radiation enhances as frequency increases. However, since electric field would not uniformly distribute at higher frequency, radiation could not enhance more. Maximum radiation happens at the frequency that moat length is about . Radiation is generally smaller for thinner substrate. The second case is the circumstance when signal trace doesn’t pass though drawbridge but span moat directly. It is found that as drawbridge shifts from trace outward toward the substrate edge, return current is forced to change the direction of flow, which causes more signal to be reflected back. In addition, it also results in a larger current loop area and, thus, increases radiation. Together, the transmitted power is lowered. In summary, for better signal integrity and lower radiated emission, signal trace should pass through drawbridge of appropriate width which, for the case examined here, amounts to about three-times the thickness of the substrate.