Excellent answer.
Many/most ethernet transformers contain two sets of magnetics:
-- an isolation transformer to separate the wire going off the PCB from the
traces to the PHY
-- a common mode choke that acts as Weston notes
Since the transformer is used for isolation, the copper is often cleared out
under the output traces from the transformer to the connector. In a traditional
EN 60950 safety examination, they will be looking for isolation between board
circuits (SELV) and Ethernet circuits (sometimes classified as TNV) in the
output area.
The result is that the output traces can pick up EMI, and that their impedance
doesn't depend on the PCB structure. Both of these are good reasons to keep the
output traces short.
Curt
Curt McNamara, P.E.
Engineering Consultant
612.305.0440 x248
www.npe-inc.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Beal, Weston
Sent: Wednesday, October 25, 2017 9:26 AM
To: edwardanto@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: 1000Base-T trace length between magnetics and connector
Edward,
I think that if you consider the purpose of the magnetics, the answer to your
question becomes obvious. The magnetics are often referred to by a more
specific term, common-mode choke. The main purpose of the coil configuration is
to suppress the common signal that might be on the differential traces. Even a
little bit of common signal out on the unshielded cable can cause a lot of EMI.
Now, once you run the differential signal through the magnetics to remove the
unwanted common signal, it makes sense to keep that signal as clean as possible
before putting it out on the cable. Keeping the traces from magnetics to
connector as short as possible reduces the risk of crosstalk and mode
conversion. This reduces the risk, but does not eliminate it. You can still
mess up the signal if you're not careful.
Notice that the optimal solution to this problem is to put the magnetics right
in the connector. That is the shortest possible path from common mode choke to
the cable.
Regards,
Weston
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Edward Anto
Sent: Wednesday, October 25, 2017 8:10 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] 1000Base-T trace length between magnetics and connector
Hi,
I was going through routing guidelines for 1000Base-T Ethernet signals,
especially constraints on trace lengths between the PHY and the magnetics and
between the magnetics and the connector.
Microchip Technology Inc has an application note on Ethernet routing guidelines
where it is mentioned to limit the trace length between the magnetics and the
RJ-45 connector to 1 inch. TI also recommends placing the magnetics close to
the RJ-45 connector, though TI doesn't put any limit on the trace length.
What could be the reason for such a constraint on the trace length? These
1000Base-T signals are supposed to run over upto 100 meters of UTP. Is this
length constraint due to to EMI concerns, or can there be any signal integrity
reason behind it?
Regards,
Edward Anto A
Senior Research Engineer
C-DOT
Bangalore
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