PCB EMC设计首要考虑的10个因素（2）

6.; Analog Circuits – Traces carrying analog signals should be kept away from high-speed or switching signals and must always be guarded with a ground signal. A low pass filter should always be used to get rid of high-frequency noise coupled from surrounding analog traces. In addition， it is important that the ground plane of analog and digital subsystems not be shared.复制代码模拟电路 —— 模拟信号走线应远离高速信号和开关信号，必有地信号保护。应该用低通滤波器来去除与周围模拟走线耦合的高频噪声。此外，重要的是要使模拟地和数字地子系统分开。7. Decoupling Capacitor – Any noise on the power supply tends to alter the functionality of a device under operation. Generally， noise coupled on the power supply is of a high frequency， thus a bypass capacitor or decoupling capacitor is required to filter out this noise. A decoupling capacitor provides a low impedance path for high-frequency current on the power plane to ground. The path followed by the current as it travels toward ground forms a ground loop. This path should be kept to a minimum possible level by placing a decoupling capacitor very close to the IC (see Figure 2b).A large ground loop increases the radiation and can act as a potential source of EMC failure.复制代码去耦电容 —— 任何电源噪声往往都会改变处于工作状态下设备的功能。一般来说，电源耦合噪声频率比较高，因此需要一个旁路电容或去耦电容来滤除这种噪声。去耦电容为电源层上的高频电流提供了一个到地的低阻路径。电流到地的路径形成了一个地环路。这个路径应尽可能短，要把去耦电容放在离IC尽可能近的地方(见图2 b)。大的地环路会增加辐射，会成为一个潜在的EMC错误源。 The reactance of an ideal capacitor approaches to zero with increasing frequency. However， there is no such thing as an ideal capacitor available on the market.3In addition， the lead and the IC package add inductance as well. Multiple capacitors with low ESL (Equivalent Series Inductance) should be used to improve the decoupling effect.复制代码随着频率增加，理想电容的电抗会接近零。然而，这是不可能的，市场上没有这样理想的电容。此外，铅和IC封装也会增加电感。应该用多个低ESL (等效串联电感)电容来改进去耦效果。8. Cables – Most EMC-related problems are caused by cables carrying digital signals that effectively act as an efficient antenna. Idealy， the current entering a cable leaves it at the other end. In reality， parasitic capacitance and inductance emit radiation. Using a twisted pair cable helps keep coupling to a low level by cancelling any induced magnetic fields. When a ribbon cable is used， multiple ground return paths must be provided. For high-frequency signals， shielded cable must be used where the shielding is connected to ground both at the beginning and at the end of the cable.复制代码电缆 —— 多数EMC相关问题都是由携带数字信号的电缆引起的，它就像一个天线。想象一个理想情况，电流进入一个电缆把它留在另一端。事实上，寄生电容和电感会发出辐射。使用双绞线电缆有助于保持较低耦合，去除任何感应磁场。当使用带状电缆时，必须提供多个地返回路径。对于高频信号，必须使用屏蔽电缆，在电缆的两头都要屏蔽到地。9. Crosstalk – Crosstalk can exist between any two traces on a PCB and is a function of mutual inductance and mutual capacitance proportional to the distance between the two traces， the edge rate， and the impedance of the traces. In digital systems， crosstalk caused by mutual inductance is typically larger than the crosstalk caused by mutual capacitance. Mutual inductance can be reduced by increasing the spacing between the two traces or by reducing the distance from the ground plane.复制代码串扰 —— 串扰可能存在于PCB上两条走线之间，它是关于互电感和互电容的函数，和两条走线之间的距离、边沿变化率和走线阻抗成正比。在数字系统中，由互电感产生的串扰通常大于由互电容产生的串扰。可以通过增加走线之间的距离或减少与铺地的距离来减小互电感。10. Shielding – Shielding is not an electrical solution but a mechanical approach to reducing EMC. Metallic packages (conductive and/or magnetic materials) are used to prevent EMI from escaping the system. A shield may be used either to cover the whole system or a part of it， depending upon the requirements. A shield is like a closed conductive container connected to ground which effectively reduces the size of loop antennas by absorbing and reflecting a part of their radiation. In this way， a shield also acts as a partition between two regions of space by attenuating the radiated EM energy from one region to another. A shield reduces the EMI by attenuating both the E-Field and H-field component of radiating wave.复制代码屏蔽 —— 屏蔽并非解决方案，而是机械方式来减少EMC。可以用金属罩(导电和/或磁性材料)来防止EMI跑出这个系统。可以是屏蔽整个系统或它的一部分，这取决于实际需求。屏蔽就像一个封闭的导电容器连接到地，通过吸收和反射辐射的一部分，有效地减小了环形天线的尺寸。用这种方法，通过衰减从一个区域到另一个地方辐射的电磁能量，屏蔽还可用来划分两个空间区域。通过衰减辐射波电场和磁场，可以减少电磁干扰。