[SI-LIST] Re: Reference planes for ethernet signals
- From: steve weir <weirsi@xxxxxxxxxx>
- To: jhasson@xxxxxxxxxxxxxxxxxxx
- Date: Fri, 04 Mar 2011 12:31:10 -0800
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
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www.ipblox.com
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