Hello Zhangkun, Assuming that you are using a simple plate under the board and not a box (as mentioned in your subject line), then the effect you are noticing is cancellation of common-mode currents in the plate. The plate in this case acts as an image plane. The bigger the plate the more effect it has. Chas -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of zhangkun 29902 Sent: Tuesday, July 29, 2008 8:41 AM To: si-list@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] the effect of box on EMI Dear all In the recent test, I found out that the effect of metal plane on EMI is obviously. Details: There is a product with only one PCB of 4 layer. The EMI of the PCB is measured at first. Then the PCB is put on a metal plane. The EMI is lessen very much(>6db). What is the theory of this case? The second and more important question is how to apply the effct of the metal plane by means of modification of the PCB, while without additional cost. Best Regards Zhangkun 2008.7.29 ************************************************************************ ****************** This email and its attachments contain confidential information from HUAWEI, which is intended only for the person or entity whose address is listed above. Any use of the information contained herein in any way (including, but not limited to, total or partial disclosure, reproduction, or dissemination) by persons other than the intended recipient(s) is prohibited. If you receive this e-mail in error, please notify the sender by phone or em ail immediately and delete it! ************************************************************************ ***************** ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu