[SI-LIST] Re: 1 Gbps on 3.5 mil etch a hazard?
- From: "o. laney" <olaney@xxxxxxxx>
- To: paul.taddonio@xxxxxxxxxxxxxx
- Date: Tue, 19 Oct 2010 09:35:32 -0700
Check the trace under a microscope. Edge roughness might be excessive,
leading to a high distributed return loss. Also check the end to end
resistance to see if it is reasonable. The conductivity of copper is
around 5.88E7 Siemens/meter. Note that the cross section is a trapezoid
when calculating cross sectional area. This is of little consequence for
fat traces but at .0035" (88.9 microns), it matters. The microscope will
tell you what to use for top and bottom width. Height is a matter of
copper weight, about .0007" for 1/2 oz foil, but if your fab house plated
the outside foil to something thicker as part of the hole plating
process, that would increase the rms variation of the edges. Only your
microscope knows for sure. Assuming that your driving and terminations
are proper, something is wrong with those traces.
Orin
On Tue, 19 Oct 2010 12:05:07 -0400 "N. Paul Taddonio"
<paul.taddonio@xxxxxxxxxxxxxx> writes:
> Dear Signal Integrity folks
> I am running a PECL diff pair at 1 Gbps, the signal travels 2 inches
> on 3.5 mil wide microstrip traces. Un-coupled.
>
> The driver output looks OK: an isolated pulse is about 1 nS wide.
> But at the receiver via pair, I find a smeared out signal with
> insufficient bandwidth. The pulse is now 1.6 nS wide at the
> switching point.
>
> I can't get my analog simulation (microsim free eval) to duplicate
> this effect. I am using ideal transmission line models.
>
> Do you expect 3.5-mil etch to cause significant dispersion for 1
> Gbps signals at a length of only 2-4 inches?
>
> Paul
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