From: Jinhua Chen (firstname.lastname@example.org)
Date: Thu Oct 12 2000 - 06:32:59 PDT
Could you send a copy of pre-emphasis at 3.125G across 20 inches
of FR4 (microstrip) to me? I am very interested in the pre-emphasis
data. In general, the loss is linear to the length of PCB. The pre-emphasis
is trying to compensate for the high frequency PCB loss (dielectric loss +
skin effect loss). Assume the pre-emphasis is designed to compensate for L
of FR4 material. When the real PCB length is shorter than L, the pre-emphasis
may over compensate. When the real PCB length is longer than L, the
may under compensate. Different dielectric material might have different
L. It is hard to optimize the signals with pre-emphasis.
This is just my 2 cents!
At 01:27 PM 10/11/00 -0700, you wrote:
>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
> > does look much like a signal.
> > Thanks!
> > Todd
> > Todd DeRego
> > Senior Signal Integrity Engineer
> > Lucent Technologies INS
> > 200 Nickerson Road
> > Marlborough MA 01752
> > (508) 786-2168
> > email@example.com
> > >
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