[SI-LIST] Re: Length matching on Gb lines
- From: "richard moffat" <richard.moffat@xxxxxxxxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Thu, 19 Jan 2006 17:24:59 +1300
Actually, I haven't tried it. I expect the wires inside the connector to not
be long enough. Any time I've used a TDR, it needs a some time to settle
after a major impedance mismatch, eg through a connector, PCB side.
If I've got my maths correct, lets say there's a 50ps rise time on a
single-ended diff pair signal. If I want the crossover point to be no more
than +/-5% of that, that implies I need to match them to +/- 15 mils (1.7ps =~
10mils). I don't think a TDR can do that. I can easily see a far greater
variance in the coil windings.
Back to the CMR, in which I've had further thoughts: I don't see that it's a
CM rejection issue unless there is a major mismatch and there is parasitic
coupling onto one side of the diff pair. The fact that it's CM implies that it
needs tightly matched for the crossover point to control the EMC.
Any one got any comments on any of this?
Regards,
Richard
>>> "Kai Keskinen" <kalevi@xxxxxxxxxx> 19/01/2006 12:54 p.m. >>>
Richard:
I can't answer about the matching but why is it impossible for a tdr? You
can build an adapter to coax with a RJ45 male which you build with known
matching.
Cheers,
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of richard moffat
Sent: Tuesday, January 17, 2006 7:04 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Length matching on Gb lines
As an aside, does anyone know how well the wires are matched inside a RJ45
connector (between the PHY and magnetic)? It's difficult to tell; they're
usually encapsulated in epoxy, and it's impossible for a TDR measurement :(
Still banging my head about not recognising CMR,
Richard
>>> "richard moffat" <richard.moffat@xxxxxxxxxxxxxxxxxxx> 18/01/2006
12:46:42 p.m. >>>
Steve,
"common mode rejection" is the key. I don't know why I didn't realise that.
All is clearer now.
Thanks,
Richard
>>> steve weir <weirsi@xxxxxxxxxx> 18/01/2006 11:50:46 a.m. >>>
Richard, common mode rejection, and EMI compliance are the big
issues. Mismatch between the PHY and the CM magnetics will result in mode
conversion which is very bad. that will erode all: EMI, EFT, ESD and
ground noise rejection. From an EMI standpoint, we want the CM choke as
close to the box exit as possible. On the inside, we want the stray
coupling to be as uniform as possible.
The better you match the members of a GIVEN diff pair, the better off you
will be vis a vis the mode conversion issues. Match between multiple pairs
isn't anything to be concerned about. So, what you want to do is keep
these traces away from mounting hardware.
Steve.
At 10:25 AM 1/18/2006 +1300, you wrote:
>Hi all
>
>This is a question about matching Gb signal lengths in the primary-side
>and secondary-side of magnetics.
>
>Spec sheets for PHYs usually have very tightly-matched lengths within a Gb
>port between the PHY and the magnetic. A ethernet cable on the other side
>of the magnetic (in the outside world) can have huge length mismatching
>between the wires.
>
>Why is the primary PHY side of a magnetic more sensitive to length
>matching on the secondary ethernet side? Is it due to the center-tap on
>the PHY side?
>
>My next question would be: how close do lines have to be matched within
>the diff pair and between diff pairs within the same port?
>
>Any help or links would be great.
>
>Regards,
>Richard
>
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