[SI-LIST] Re: RE Re: Reference planes for ethernet signals
- From: jhasson@xxxxxxxxxxxxxxxxxxx
- To: "Nijagunamurthy, Hithesh (GE Intelligent Platforms)" <hithesh@xxxxxx>
- Date: Tue, 8 Mar 2011 09:44:23 +0100
Hi,
My concern comes from the fact that other return currents could be using
the same paths as the ones used by the return current for the ethernet
signals. The idea with the moat was to prevent these alternate return
currents to use the same path.
Best regards,
JF Hasson
"Nijagunamurthy, Hithesh (GE Intelligent Platforms)" <hithesh@xxxxxx>
08/03/2011 09:40
A
<jhasson@xxxxxxxxxxxxxxxxxxx>
cc
Objet
RE: [SI-LIST] RE Re: Reference planes for ethernet signals
JF,
Why do you wanna put a moat, when most of the return currents(99.9%) are
confined to within 6x the tracewidth below the ethernet signal trace.
-Hithesh
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of jhasson@xxxxxxxxxxxxxxxxxxx
Sent: Tuesday, March 08, 2011 1:32 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] RE Re: Reference planes for ethernet signals
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
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