[SI-LIST] Re: the effect of box on EMI

  • From: "Long Yang" <long.0.yang@xxxxxxxxx>
  • To: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
  • Date: Wed, 30 Jul 2008 18:01:15 +0800

Hi, Zhangkun:
       Charles told a true story related to image plane. In general, two
factors, the distance between the object and the plate, and the size of the
plate, play important role in image plane theory.
       But you should notice that you will take into account the difference
of EMI radiation below and above the image plane.
Regards
Long Yang
Dan Quayle  - "For NASA, space is still a high priority."

On Wed, Jul 30, 2008 at 2:44 AM, Grasso, Charles <
Charles.Grasso@xxxxxxxxxxxx> wrote:

> 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
>
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