PCB EMC設計首要考慮的10個因素（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.復制代碼模擬電路 —— 模擬信號走線應遠離高速信號和開關信號，必有地信號保護。應該用低通濾波器來去除與周圍模擬走線耦合的高頻噪聲。此外，重要的是要使模擬地和數(shù)字地子系統(tǒng)分開。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.復制代碼去耦電容 —— 任何電源噪聲往往都會改變處于工作狀態(tài)下設備的功能。一般來說，電源耦合噪聲頻率比較高，因此需要一個旁路電容或去耦電容來濾除這種噪聲。去耦電容為電源層上的高頻電流提供了一個到地的低阻路徑。電流到地的路徑形成了一個地環(huán)路。這個路徑應盡可能短，要把去耦電容放在離IC盡可能近的地方(見圖2 b)。大的地環(huán)路會增加輻射，會成為一個潛在的EMC錯誤源。 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.復制代碼隨著頻率增加，理想電容的電抗會接近零。然而，這是不可能的，市場上沒有這樣理想的電容。此外，鉛和IC封裝也會增加電感。應該用多個低ESL (等效串聯(lián)電感)電容來改進去耦效果。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.復制代碼電纜 —— 多數(shù)EMC相關問題都是由攜帶數(shù)字信號的電纜引起的，它就像一個天線。想象一個理想情況，電流進入一個電纜把它留在另一端。事實上，寄生電容和電感會發(fā)出輻射。使用雙絞線電纜有助于保持較低耦合，去除任何感應磁場。當使用帶狀電纜時，必須提供多個地返回路徑。對于高頻信號，必須使用屏蔽電纜，在電纜的兩頭都要屏蔽到地。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.復制代碼串擾 —— 串擾可能存在于PCB上兩條走線之間，它是關于互電感和互電容的函數(shù)，和兩條走線之間的距離、邊沿變化率和走線阻抗成正比。在數(shù)字系統(tǒng)中，由互電感產(chǎn)生的串擾通常大于由互電容產(chǎn)生的串擾。可以通過增加走線之間的距離或減少與鋪地的距離來減小互電感。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.復制代碼屏蔽 —— 屏蔽并非解決方案，而是機械方式來減少EMC。可以用金屬罩(導電和/或磁性材料)來防止EMI跑出這個系統(tǒng)。可以是屏蔽整個系統(tǒng)或它的一部分，這取決于實際需求。屏蔽就像一個封閉的導電容器連接到地，通過吸收和反射輻射的一部分，有效地減小了環(huán)形天線的尺寸。用這種方法，通過衰減從一個區(qū)域到另一個地方輻射的電磁能量，屏蔽還可用來劃分兩個空間區(qū)域。通過衰減輻射波電場和磁場，可以減少電磁干擾。