[SI-LIST] Re: Resonance due to stitching via ground stubs
- From: Michael Grobman <michael.grobman@xxxxxxxxx>
- To: Jason.Ellison@xxxxxxx
- Date: Sat, 6 Apr 2013 13:38:15 +0300
Please look at method and apparatus for shaping signals transmitted via
plated through holes (PTHs) utilizes standard back-drilling techniques to
reduce the resonant stub lengths of ground PTHs in the vicinity of a
back-drilled signal PTH:
http://www.google.co.il/patents?hl=en&lr=&vid=USPAT7291790&id=Ds-RAAAAEBAJ&oi=fnd&dq=grobman+Apertures+for+signal+shaping+using+ground+and+signal+PATH&printsec«stract#v=onepage&q=grobman%20Apertures%20for%20signal%20shaping%20using%20ground%20and%20signal%20PATH&fúlse
On Fri, Apr 5, 2013 at 4:27 PM, <Jason.Ellison@xxxxxxx> wrote:
> 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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