Re: [SI-LIST] : May 9th Presentation: "Radiation from Edge Effects in Printe...

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From: rachild.chen (rachel.chen@huawei.com.cn)
Date: Thu May 11 2000 - 18:53:20 PDT


Peter,

I am very interesting in the fencing by via on the board side.Many high speed backplanes have this kind of fencing vias.All vias are connected with ground plane.It can reduce the emissions.The via size and locations are very important.How much distance should have a via to ground?We don't have any datas to help us design.Can you email your simulation and test results to me?I wish you can give me some advice on designing the fencing vias size and locations.

Regards,

Rachild
----- Original Message -----
发件人: ARNOLD,PETER (HP-Cupertino,ex3) <peter_arnold@hp.com>
收件人: <si-list@silab.eng.sun.com>
发送时间: 2000年5月11日 0:38
主题: RE: [SI-LIST] : May 9th Presentation: "Radiation from Edge Effects in Printe...


> Doug,
>
> Here is what I think I heard at the EMC society presentation. The
> simulations presented showed that 20H structures they examined actually
> resulted in more emission at the board edge. However, the presenters were
> neutral on whether this was detrimental to compliance for the system as a
> whole, pointing out that EMC solutions are rarely universally applicable. I
> hope they post this on the web, I found it most instructive.
>
> The main points I remember:
>
> 1) Time-varying currents on vias can inject radial TEM-mode waves into the
> space between planes.
>
> 2) The energy thus injected bounces around the cavity volume between the
> planes. The board edge is a discontinuity in the medium and so results in
> partial reflection of the propagating wave and partial transmission, i.e.
> radiation from the board edge.
>
> 3) Fencing the board edge with grounded vias is equivalent to changing the
> PCB-edge discontinuity to a short to ground, so the reflection coefficient
> becomes -1 and all energy is kept inside of the fenced area.
>
> 4) By contrast, a 20H-rule example showed that that structure, which looks a
> little like a patch antenna, allows for more efficient radiation from the
> edge. The exposed area allows a propagation mode where energy can travel
> around the outside edges of the board also. Thus less energy is trapped
> within the board area and more gets radiated.
>
> 5) Is this good? Energy bouncing around between planes can be picked up by
> structures like the one that initially injected it, e.g. vias, and then
> travel along conductors to outside surface components where it can be
> emitted. This is not especially desirable. On the other hand, the more
> efficient radiatiing edge (20H) puts more energy into the system chassis,
> which moves the problem one level higher.
>
> 6) Closely spaced ground vias all across the board had the effect of fencing
> in the injected energy to a small area. This seems to cut radiation from the
> edges drastically. I would like to know more about this particular case.
>
> The data was obtained from an FDTD simulation of a small board with a single
> off-center via as the point of injection. Excitation was with both a
> continuous sine-wave at 1GHz and also a Gaussian-derivative pulse.
> Dielectric losses were included in the FR4 model. The results of lengthy
> simulations were presented as captivating animations, with color variation
> showing the magnitude of the Poynting vector across the whole board. The
> test setup was of necessity rather artificial but it did help to give a feel
> for the physical behavior underlying radiation from board edges.
>
> peter.
>
> -----Original Message-----
> From: Doug McKean [mailto:dmckean@corp.auspex.com]
> Sent: Tuesday, May 09, 2000 3:27 PM
> To: si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : May 9th Presentation: "Radiation from Edge
> Effects in Printe...
>
>
> I won't be able to make the meeting.
> Any chances of having what's presented
> put up on a website? - Doug
>
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