[SI-LIST] [Fwd: Re: RE Re: Reference planes for ethernet signals]
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list <si-list@xxxxxxxxxxxxx>
- Date: Tue, 08 Mar 2011 00:55:49 -0800
-------- Original Message --------
Subject: Re: RE [SI-LIST] Re: Reference planes for ethernet signals
Date: Tue, 08 Mar 2011 00:09:50 -0800
From: steve weir <weirsi@xxxxxxxxxx>
To: jhasson@xxxxxxxxxxxxxxxxxxx
References:
<OF399D7AF1.FE4994FD-ONC125784D.0027EB4E-C125784D.002A8BF3@xxxxxxxxxxxxxxxxxxx>
JF, cutting up planes will create HF loops and resonances that are as
likely to bite you as solve/avoid any problems. Moating is a very
powerful tool, but it has consequences and must be undertaken only with
great care and attention to the side effects.
You only said you had RJ jacks. I took that to mean you were direct
coupling without either isolation transformers or CM magnetics.
You have homework to do. You need to assess the extent of CM EMFs vs.
frequency that you have, and then determine how efficiently those EMFs
can couple through your I/O. If your geometry has a high degree of
symmetry, you can estimate your EMFs with a hand calculator or Excel.
If your geometry has complex shapes, then you can use a field solver to
find response.
Steve
jhasson@xxxxxxxxxxxxxxxxxxx wrote:
>
> Hi,
>
> To cope with this potential issue we were more thinking of trying to
> segragate the reference planes of the gigabit ethernet signals from
> other return currents (for instance creating a moat to prevent the
> return currents from interfering with the return current for the
> ethernet signals). Putting in CM filtering seems a bit tricky (to us
> anyway) as there is already the CM choke of the ethernet magnetics.
> Moreover, in the setup we are considering the distance between the PHY
> and the magnetics is quite long (about 4'') and it is over this
> distance that we could have some common impedance coupling. What tools
> could be used to evaluate the level of the disturbance we could get ?
>
> Best regards,
>
> JF Hasson
>
>
>
> *steve weir <weirsi@xxxxxxxxxx>*
> Envoyé par : si-list-bounce@xxxxxxxxxxxxx
>
> 04/03/2011 21:31
>
>
> A
> jhasson@xxxxxxxxxxxxxxxxxxx
> cc
> si-list@xxxxxxxxxxxxx
> Objet
> [SI-LIST] Re: Reference planes for ethernet signals
>
>
>
>
>
>
>
>
>
> JF, your concerns are with respect to common impedance. The conditions
> under which common impedance will matter are those where you have
> currents across that impedance that alter the difference voltage seen by
> a receiver of a particular signal(s). For any I/O, the reference
> couples directly to that I/O and induces CM noise. You always need to
> pay attention to common mode noise common impedance causes. In order to
> meet EMC regulations you may need to add CM filtering close to your RJ
> connectors.
>
> Steve.
>
> jhasson@xxxxxxxxxxxxxxxxxxx wrote:
> > Hi,
> > We are working on a setup where gigabit ethernet links are managed by
> > daughtercards plugged in a backplane. The gigabit ethernet links
> would be
> > routed through the backplane and to the outside world via cat 6
> cable. The
> > RJ45 on the backplane would be either with or without magnetics as
> we have
> > both configurations. The backplane could be managing quite a lot of
> time
> > varying current over the ground planes due to daughter card
> activity. We
> > are concerned that the reference plane for the ethernet links would be
> > also carrying these return currents. Could someone elaborate on the
> > coupling mechanism that could trouble the link ?
> >
> > Best regards,
> >
> > JF Hasson
> >
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>
> --
> Steve Weir
> IPBLOX, LLC
> 150 N. Center St. #211
> Reno, NV 89501
> www.ipblox.com
>
> (775) 299-4236 Business
> (866) 675-4630 Toll-free
> (707) 780-1951 Fax
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--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
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- » [SI-LIST] [Fwd: Re: RE Re: Reference planes for ethernet signals] - steve weir
