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仕傑 發問時間: 社會與文化語言 · 2 0 年前



Designing the USB to meet EMI emission

requirements needs a combination of electrical and

mechanical design involving component placement,

trace routing, connector placement and circuit design.

Some of the critical issues are discussed here. As

shown in Figure 8, when routing the D+ and D- outputs

of the USB controller chip avoid running the traces near

the high-speed clock lines or other such signals.

Induced common mode noise can be reduced by

running pairs of USB signals parallel to each other and

running grounded guard traces on each side of the

signal pair. Locate the USB controller chip near to the

USB connector.

(Fig.8 EMI layout consideration of USB)

The V b u s and ground in a PC are subject to noise

both from onboard sources and also from the switching

power supply. Hence, both VBUS and ground should be

decoupled. Separate ferrite bead rated for 500mA he

used on each line. Further a small capacitor in the order

of l000pF can be connected on the connector side of

Vbus to the connector shell.

In order to prevent noise from coupling to the

connector and subsequently to the cables shield, it is

important that a good high frequency connection to be

made between the connector and the chassis. This can

be accomplished by a spring metal EMI gasket clip that

makes multiple contacts between the USB connector

and the back of the chassis panel.

Ineffectiveness of shielding can be traced to improper

termination of the cable shield to the connector or

connector shell to the receptacle. Full speed USB

cables should have a woven or spirally wound copper

shield. The shield should be crimped to the connector

shell yielding 360" shield coverage at the connector.

For the low speed applications, care should be taken

so that the clock does not couple to the USB cable. The

amount of high frequency energy generated by the

clock circuit should be limited and kept as low as


Ferrite beads may also be used on the data lines for

low speed application. However, the frequency

response through the ferrite beads on D+ or D- must not

roll off by more than 2.0dB at 6.0MHz.

Full speed devices must be able to send and receive

12Mbps signal. Oscillator is the single largest

contributor of EMI in this case. A ground plane does

not behave like an equipotential surface at high

frequencies. The location of the connector shell's

termination to the ground plane is critical. The

connection needs to be made to the quietest area of the

ground plane from coupling to the shield. The quietest

location on the ground plane is on the opposite edge of

the board far away from the crystal and other high

frequency signals.


Conducted emission noise was observed in the USB

wires during the evaluation of a satellite receiver, which

drives power from the USB. Similar emissions are

possible when the USB is used with any type of

peripheral. Due to the high-speed data requirement, the

emissions cannot be totally eliminated from the USB by

using external filter and other noise suppression

techniques. It will be wiser to take care of the emission

in the PCB design and by following proper layout

considerations so that the USB can be used for any type

of peripheral devices.


[1] Universal Serial Bus Specification Revision 1 . 1

[2] High Speed USB Platform Design Guidelines Revision 1.0

3 個解答

  • 匿名使用者
    2 0 年前

    5. 設計USB滿足EMI 發射要求的PCB 設計REQUREMENTS需要與組成部分安排,尋跡路由有關的電和機械設計的結合, 連接器安排和電路設計。 一些關鍵的問題在這裡被討論。 如圖8中所示, 在高速時鐘線或者其他這樣的信號附近當鋪設USB 控制單元晶片的D+ 和D產量時,避免運轉蹤跡。 引起 共同模式噪音可能透過運轉對與彼此平行的USB信號並且在顯著對的每邊運轉接地的警衛蹤跡被降低。 找到靠近USB 連接器的USB 控制單元晶片。 (圖8 EMI 佈局考慮 USB的)那些s u V b 和接地PC 以噪音為準既從艙中來源又從那些開關電源那裡。 因此,VBUS和地應該被分離。 分開為他在每條線上使用的500 mA 評價的ferrite小珠。 促進一小電容器按照l000pF次序可能在連接器Vbus 給連接器殼的邊上連結。 為了防止噪音對連接器和后來對電報盾連合, 它重要被在連接器和底盤之間製造的一個好高的頻率連接。 這可能因為做在USB 連接器和底盤板的背面之間的多接觸的春天金屬EMI 墊片痛擊被完成。 保護的無效可能被跟蹤到電報盾的不恰當的終端設備到對容器的連接或者連接器砲彈。 電報USB全速有應該一編造或者螺旋盾銅幣傷口。 盾應該給產生360"盾新聞報導在連接器的連接器殼卷邊。 對低速應用來說,小心應該被帶,因此鐘不對USB電報連接。 被這條時鐘電路產生的高的頻率能量的數量應該是有限的並且保持儘可能低。 Ferrite小珠可能也被在低速應用的基準線上使用。 不過,頻率附應透過ferrite小珠在D+或者D上不可以2.0分貝超過滾下在6.0MHz。 全速設備必須能送並且收到12Mbps信號。 振蕩器是在這種情況裡的EMI 的單個的最大的捐助者。 一張透視地平面不象在高的頻率的一個equipotential 表面一樣表現。 對透視地平面的連接器殼的終止的位置是評論性的。 連接需要被從連合到盾做到透視地平面的最安靜的領域。 關於透視地平面的最安靜的位置遠離水晶和其他高的頻率信號在這塊板的對面的邊緣上。 結議處理髮射噪音在對衛星接收器的評估期間被在USB 電線內觀察,這從USB驅動動力。 相似散發物可能USB被用于任何外圍的類什麼時候。 由於高速數據要求,散發物不能透過使用外部過濾器和其他噪聲抑制技術被全部從USB 消除。 管理將是聰明的發射在PCB裡設計, 在遵循適當佈局考慮時,USB可能用于任何類外部設備。 參考 [1]普遍的連續的公共汽車說明修正1。 1 [2]高速USB 平台設計指南修正1.0

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  • 1 0 年前

    你們好,ferrite可單獨一個字,或者為ferrite core,業界稱呼叫做"鐵粉心",它的作用是降低訊號傳輸時產品的雜訊(EMI)干擾。

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  • 1 0 年前


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