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</span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'> 串扰的实质：</span><span
lang=EN-US style='font-size:9.0pt;font-family:Arial'><br>
</span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'> 串扰是现代数字系统的必然产物。我们不能消除它，但我们要找到控制它的办法，从而更好的管理，或者是能够使其降低到可容许的范围内。</span><span
lang=EN-US style='font-size:9.0pt;font-family:Arial'><br>
</span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'> 观察以下的电路，用串扰的概念分析，</span><span
lang=EN-US style='font-size:9.0pt;font-family:Arial'>A</span><span
style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:Arial;
mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>点的门电路是干扰源，</span><span
lang=EN-US style='font-size:9.0pt;font-family:Arial'>D</span><span
style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:Arial;
mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>点的门电路是受害源。只要干扰源一改变状</span><span
lang=EN-US style='font-size:9.0pt;font-family:Arial'><br>
</span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>态，我们就可以观察到受害源处的脉冲串扰。</span><span
style='font-size:9.0pt;font-family:Arial'> <span lang=EN-US><o:p></o:p></span></span></p>
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<p align=center style='text-align:center'><span style='mso-bidi-font-family:
Arial'>图１Ａ<span lang=EN-US><o:p></o:p></span></span></p>
<p><span style='mso-bidi-font-family:Arial'> 串扰中的一个有趣的现象是其方向性。串扰信号的波形是电流方向的函数。例如，上图中，如果我们将干扰源导线中的电流方向颠倒，则在受害源端感应到的串扰也将改变其极性。<span
lang=EN-US><br>
方向性是一个极其重要的概念，因此要一步一步地详细讨论。首先，构建如图1所示的电路，当A处的门电路由低电平跳到高电平时，在D处测量串扰，会发现当干扰信号到达B的同时，在D处便会出现一个负极性的串扰尖峰。<br>
第二步，将干扰信号源中的电流方向颠倒，也即是说，将干扰源放在右端B处，原来右端的三负载放在左端A处，再次进行串扰测量，同时观察D点的信号有何变化，你会发现一个正极性的串扰尖峰。（极性改变）<br>
极性发生改变说明串扰不是由电容耦合引起的。许多数字电路工程师都假设串扰是由寄生电容引起的，但其实并非如此。两个单端电路间的互电容耦合仅会产生正极性的串扰。<br>
极性反转表明，干扰是（或部分）由于互感耦合造成的。这与变压器中的耦合机理一样。大家都知道，将变压器初级绕组的接线端子对调，就可以使次级电压的极性反向。计算机主板电路印制线中的耦合与之非常相似。如果你将每块电路板中的印制线视为极小的电流环路，你就会明白“串扰”变压器的工作原理。<br>
我们可以首先假设A处门电路中的电流通过导线流到B端的负载，然后沿着电源和地线系统回流到门电路A。干扰电流形成一个环路，可以将此环路视为变压器的初级绕组。<br>
同一变压器的次级绕组离初级不远，它是门电路C开始处就形成的环路，并且沿受害源导线逐渐移动到负载端，然后又顺着电源及地线系统返回到门电路C。<br>
这两个环路在许多方面极象一个弱耦合的单匝变压器。<br>
这种类似变压器的互感耦合对于数字电路设计具有深刻的含意。首先，这暗示了串扰是可以改变的，这取决于电路中所使用的负载。<br>
例如，在上面电路图中，我们假设传输干扰电流的轨线很短，这时干扰电流是负载的函数。负载越重，干扰电流也越大，产生的串扰也越大。图中的三个负载的网络产生的串扰信号大约是同样电路、同样布局，但仅有一只负载的网络产生的串扰的三倍左右。<br>
在驱动SIMM内存插槽的情况下，这种负载效应尤其突出。这种走线一般极短，但负载很大，因此驱动电流总体上差不多是由SIMM输入端的负载电容所控制的。当插入较多的SIMM内存条时，串扰便会增加。<br>
当你在元件密集的多层电路板上解决串扰问题时，电路板印制线负载影响串扰的有关原理可帮助你发现和解决问题。现在有许多应用IBIS I/O模型的串扰预测工具，许多这样的新工具都能自动、高效地计算出串扰，包括负载效应。<o:p></o:p></span></span></p>
<p><span style='mso-bidi-font-family:Arial'> <span lang=EN-US><o:p></o:p></span></span></p>
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