From: Mike Jenkins (email@example.com)
Date: Wed Oct 11 2000 - 13:27:09 PDT
Looks like you hit a hot button on the SI list, so I'll add
my two cents, as well. On the issue of 'broadside' coupling,
while the advantages are as billed, there are two drawbacks.
One is the common mode impedance, which is probably high,
resulting in mismatched common mode. Depending on how much
common mode is in your signal, this will have EMI implications.
The second drawback is the complexity at the ends of the line.
one line (at least) of the pair must change planes. Resulting
asymmetrical parasitics on the dif'l pair layout can cause
common mode components in the signal if you're not very careful.
For short runs, it might not be worth it.
My second comment is about pre-emphasis, which was part of your
initial question. Hoping you don't mind, I will send you a PDF
showing the effect of pre-emphasis at 3.125G across 20 inches
of FR4 (microstrip). It's the same material presented at last
November's 10G Ethernet meeting, but those presentations are no
longer public. (I didn't want to spam everyone's mailbox with
435K, but if anyone else wants a copy, please let me know.)
There was another presentation on post-emphasis at the recent
Fibre Channel plenary meetings, but it's not up on the web site
yet. The performance was quite impressive.
Lastly, the required frequency content of 2.5Gb/s doesn't go very
much above 1.25G, so interpret any FR4 loss curves appropriately.
Hope that helps.
Todd Derego wrote:
> I'm working to take a present 1.25Gbps backplane to 2.5Gbps or
> faster. Any thought on transceivers with preemphasis and/or adaptive
> equalization to help get 2.5Gbps or better across a long FR4 backplane? The
> eye looks pretty collapsed at 2.5Gbps but I have seen some products the
> drive across a twisted pair and seem to recover a signal from something that
> does look much like a signal.
> Todd DeRego
> Senior Signal Integrity Engineer
> Lucent Technologies INS
> 200 Nickerson Road
> Marlborough MA 01752
> (508) 786-2168
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