Separate magnetics allow for more flexibility to reduce emissions when
needed by adjusting the Bob Smith termination parameters. This can make a
several dB improvement in emissions in some cases.
Doug Smith Sent from my iPhone IPhone: 408-858-4528 Office: 702-570-6108
Email: doug@xxxxxxxxxx Website: http://dsmith.org
On Wed, Oct 25, 2017 at 23:49, Edward Anto <edwardanto@xxxxxxxxx> wrote:
Thank you Weston. That was excellently explained. And Thank you Curt and
Binayak for adding more info.
Regards,
Edward.
On Wed, Oct 25, 2017 at 8:18 PM, Binayak Shrestha <binayaks@xxxxxxxxx>
wrote:
That's why lots of RJ45 connectors come with integrated magnetics!wrote:
- binayak
On Wed, Oct 25, 2017 at 8:08 PM, Curt McNamara <CurtM@xxxxxxxxxxx>
cleared
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
isolationout under the output traces from the transformer to the connector. In a
traditional EN 60950 safety examination, they will be looking for
classifiedbetween board circuits (SELV) and Ethernet circuits (sometimes
[mailto:si-list-bounce@xxxxxxxxxxxxx]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 <0612%20305%200440>
www.npe-inc.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
toOn 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
removeyour 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
asthe unwanted common signal, it makes sense to keep that signal as clean
crosstalkpossible before putting it out on the cable. Keeping the traces from
magnetics to connector as short as possible reduces the risk of
Youand mode conversion. This reduces the risk, but does not eliminate it.
magneticscan still mess up the signal if you're not careful.
Notice that the optimal solution to this problem is to put the
moderight in the connector. That is the shortest possible path from common
[mailto:si-list-bounce@xxxxxxxxxxxxx]choke to the cable.
Regards,
Weston
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
connectorOn 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
magnetics
Hi,
I was going through routing guidelines for 1000Base-T Ethernet signals,
especially constraints on trace lengths between the PHY and the
theand 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
onmagnetics close to the RJ-45 connector, though TI doesn't put any limit
Thesethe trace length.
What could be the reason for such a constraint on the trace length?
this1000Base-T signals are supposed to run over upto 100 meters of UTP. Is
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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Regards.
Binayak Shrestha
Research Engineer,
C-DOT Centre for Development of Telematics