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

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From: e (evillaf@home.com)
Date: Wed Nov 22 2000 - 20:29:00 PST


Derrick,

I suppose I don't have to cut up the board to do a NEXT setup. I could simply
remove the phy device (or transformer) and measure crosstalk from the RJ-45
connector to the phy (or to the transformer). Suppose this is the problem, how
can I minimize it's effect? I have a frame ground between the rx and tx traces,
so interaction can only come from the two endpoints (RJ-45 connector and
transformer in one span, and transformer and phy device in another). Also, the
layout is the same for all eight ports, so I wonder why the other ports don't
exhibit the same problem. I will continue to investigate.

By the way, the unused pairs are terminated with Bob Smith termination.

Thanks for the feedback.
Ellis

Derrick Stikeleather wrote:

> 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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