Re: [SI-LIST] : 100Base-TX board SI issues

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From: Derrick Stikeleather ([email protected])
Date: Wed Nov 22 2000 - 05:10:18 PST

Would it be possible for you to cut up another board, leaving just the Jack
and the traces intact? If so, it might be beneficial to do this and look at
the crosstalk thru the jack and the majority of the traces on the PCB (maybe
up to your components or somewhere convenient?). Find a good cat5 patch
cable and cut the plug off of one end about 3 inches long from the nose.
Strip the conductors back about 0.15" and plug it into the jack, then
terminate the traces at the far end with good resistors. Look at the NEXT
for all the pair combinations through the jack and board. Load terminate
all unused pairs on the plug (I may be telling you stuff you already know.
If so, sorry)

I would think that this section of the board should have crosstalk similar
to that of a Cat5 patch panel (about 40 dB @ 100 MHz). Maybe it can be
worse; you'd probably know better.

Hope this helps
Derrick Stikeleather

> Any suggestions would be appreciated. In particular, I would like to
> know if there are recommended layout practices in the area between the
> RJ-45 connectors and the phy devices. Is there a trace width
> requirement, or trace spacing recommendations between the differential
> pairs in the region between the RJ-45 connectors and the magnetics,
> where no reference ground is used (voided to keep power and ground plane
> noise away from traces leaving the chassis)?
> Thanks in advance,
> Ellis

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