印刷電路板邊緣輻射效應的量測與分析

Abstract

在微波電路中，微帶線是基本且常用的傳輸線結構，因為微帶線具有結構簡單、成本低、適用於印刷電路技術等優點。 本篇論文主要是研究微帶線在高速電路板上其邊緣效應對電磁輻射的影響，PCB線路走線越靠近板邊且為高速信號線時是需要考慮到所產生的EMI問題。實驗量測結果顯示邊緣效應會影響EMI的能力，而增加線路至板邊的間距則有抑制EMI 的效果。 當線路越接近PCB邊緣時亦會產生共模電流，我們可以使用grounded guard trace的方式作抑制。我們也驗證20H Rule的應用，其中說明了電路板上的電源面與接地面的邊緣會有邊際(Fringing)效應發生，所以需將接地面增加尺寸為厚度的20倍。

The microstrip line is the basic and popular transmission line structure in the microwave circuit, because of its simple architecture, low cost, and easy to apply in the printed-circuit technology. This thesis investigates the impact on electromagnetic radiation from high-speed circuit board by the board edge effect. PCB traces routed near board edges and carrying high-speed signals are considered to contribute to EMI problems. The measurement results show that board edge does affect the EMI performance and EMI can be suppressed by increasing the distance between trace and board edge. The common-mode current occurs due to a trace near a printed circuit board edge; it can be suppressed by a grounded guard trace. We also verify the 20H-rule. On board with power and ground planes, the fringing field at the edges of the board is contained by increasing the edge of the ground plane size by 20 times the thickness of the dielectric substrate.