That's why lots of RJ45 connectors come with integrated magnetics!
- binayak
On Wed, Oct 25, 2017 at 8:08 PM, Curt McNamara <CurtM@xxxxxxxxxxx> wrote:
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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