Do you remember 20H Rules? 是否还记得20H原则

As the operating frequency of electronic circuits continues to increase, today’s package and printed circuit board designers face more radiation problem than ever before. The control of radiated emissions to make the package comply with radiation constraints is one of the most important aspects of the EMC study. Some “rules-of-thumb” are employed to help the designers to reduce the radiation problems. This paper presents an investigation on the effects of the 20-H rule and the shielding vias on radiation from printed circuit boards.

 

The 20-H rule states that the ground planes are extended beyond the power planes by about 20 times the distance between the planes. Let us consider the simple structure consisting of one power plane and one ground plane.

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It can be seen that the larger the separation between planes, the smaller the amplitude of the reflections coefficient and the larger the radiation. The reflection coefficient of the 20-H rule structure can be estimated by that of the power plane.

 

Because the separation between the power plane and its image plane is twicethe reflection coefficient of the 20-H rule structure is smaller in the

amplitude. Therefore, more radiation is expected to come out of the edges of the board implemented with 20-H rule.

 

随着电子电路工作频率的不断增加,今天的封装和印刷电路板设计人员面临的辐射问题比以往任何时候都多。辐射排放的控制,使包符合辐射约束是最重要的方面的电磁兼容研究。一些拇指规则是用来帮助设计人员减少辐射问题。本文提出了对20-H规则影响的调查和对辐射屏蔽通孔的印制电路板。

 

20-H规则规定,接地面超出约20倍的电源平面之间的距离。让我们考虑一个电源平面和一个接地平面的简单结构。

 

可以看出,平面之间的间隔越大,越小反射系数的幅度和辐射较大。20-H规则的结构的反射系数可以通过对电源平面估计。

 

因为在电源平面和图像平面的两倍20-H规则结构反射系数之间的分离是小幅度。因此,更多的辐射有望走出与20-H规则实施的板边。