[SI-LIST] AW: Resonance due to stitching via ground stubs
- From: "Havermann, Gert" <Gert.Havermann@xxxxxxxxxxx>
- To: "Jason.Ellison@xxxxxxx" <Jason.Ellison@xxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 5 Apr 2013 15:11:42 +0000
Hi Jason,
I think the explanation is in the fact that Return currents follow the signal's
field. If the field surrounding the signal trace happens to couple into the
"GND-Stub"
, then the return current will be forced to take a "detour" from the GND plane
into the via stub and back again. This will result in reflections due to timing
differences between signal and return current wich show up as a IL resonance.
You should activate a current monitor in the 3D solver with fine phase
resolution, then you should see the field effects.
BR
Gert
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-----Ursprüngliche Nachricht-----
Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im
Auftrag von Jason.Ellison@xxxxxxx
Gesendet: Freitag, 5. April 2013 15:28
An: si-list@xxxxxxxxxxxxx
Betreff: [SI-LIST] Resonance due to stitching via ground stubs
Hello,
We recently found a somewhat interesting outcome of using stitching vias
without having a ground plane on the top and bottom of the PCB . In our case,
we voided ground in a footprint for impedance reasons and did not put ground on
the top layer.
We backdrilled the signal vias, but left all the grounds alone. An unexpected
resonance in IL that appeared to be from a stub was observed.
We found the quarter wave length related to the resonance frequency in our
board material. We found that the distance was 4 mils from the stitching via
stub length. We modeled this geometry in a 3D full wave solver and sure enough
we recreated the problem.
The question I have for the group is, why does this happen with a ground stub?
We all know that quarter wave stubs on signals lines cause this kind of
response, but it is not intuitive that a ground would do the same thing.
Thanks!
Jason Ellison
FCI - Signal Integrity Engineer
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