High Speed Transmission - Fiber Weave Effect

隨著傳輸速度持續上升（5Gbps and beyond!），慢慢開始看到各大SoC廠商警告＂Fiber Weave Effect＂將會影響到較長的高速差分訊號。而究竟什麼是Fiber Weave Effect？

這要從ＰＣＢ介電層結構看起，各層介電層在製造上其實是將一條條的玻璃材質編織線＂上下、垂直地＂交織而成，再依據所需的厚度和介電常數有不同的材質和密度可選擇（如圖）。

當一組高速差分訊號走在ＰＣＢ上時，如上圖紅色藍色線條組成一對差分走線。若仔細觀察就會發現當其中一條長距離走在編織空隙上時，其所對應到的Er值會將大於另一條訊號線，而Er值不同代表的結果是Time skew or phase skew（眼圖閉合、CMRR下降），進而失去差分訊號提高Bit rate的功用。這種因介電層玻璃編織對高速差分走線造成的現象即為Fiber weave effect，且其編織密度愈疏, 其造成的影響愈嚴重！

借用網路上找到的資料：

Fiber weave effect is a statistical problem. It is not uncommon for PCB designs to have long parallel lengths of track routing without any bends or jogs. This is particularly true in large passive backplane designs. Because the fiber weave pattern tends to run parallel to the x-y axis, any traces running the same way will eventually encounter a situation of worst case timing skew if you build enough boards. This was demonstrated by Intel after compiling more than 58,000 TDR and TDT measurements over two years. In 2007, Jeff Loyer et al presented a DesignCon paper “Fiber Weave Effect: Practical Impact Analysis and Mitigation Strategies” where they published the data and proposed techniques to mitigate the effect of fiber weave skew. They showed statistically it is possible to have a worst case timing skew of approximately 16ps per inch representing a delta Dk of approximately 0.8.

Altera公佈了一份Application note建議如下：

1. 刻意使玻璃編織與長距離走線有夾角，需要PCB製造商配合

2. 使用特殊均勻性介電材質製作，可能造成Cost rise

3. 使用編織密度較高材質（eg. 2112...），但可能影響成本及疊購尺寸

4. Zig-zag routing

5. Jogged routing

主要的概念是在長距離走線時, 固定每段距離(eg. 500-800 mil)做一次trace turn/offset, 強迫pair中P/N的路徑所對應到的介電層Er值接近彼此。