[SI-LIST] Re: Ground vias and the land of ID ("It Depends")

  • From: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • To: Doug Smith <doug@xxxxxxxxxx>
  • Date: Wed, 22 Oct 2014 12:28:07 -0400

Doug
A very good point, indeed.

Scott



Scott McMorrow
Teraspeed® Consulting - A Division of Samtec
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On Wed, Oct 22, 2014 at 11:07 AM, emceed <doug@xxxxxxxxxx> wrote:

> One problem to remember, the amount of energy extracted from a signal to
> cause an EMC emissions problem is often so low that the effect on SI is
> insignificant. So EMC simulations and measurements are in order. I present
> live experiments in my seminars to illustrate this effect, very easy to set
> up an experiment to show this right on a table top, no chamber needed!
>
> Doug Smith
> Sent from my iPhone
> iPhone: 408-858-4528
> Office:   702-570-6108
> Email:    doug@xxxxxxxxxx
> Website: http://dsmith.org
>
>
> > On Oct 22, 2014, at 05:44, Loyer, Jeff <jeff.loyer@xxxxxxxxx> wrote:
> >
> > At the risk of finding myself embroiled in a furball, I thought I'd see
> if I could clear up (in my own mind anyway) some of the effects of ground
> vias.  I ran 3-D simulations on a pair of single-ended signal vias with
> various configurations of ground vias around them and compared impedance,
> NEXT, and FEXT.  The results strongly suggest that nearby ground vias are a
> good investment to reduce via crosstalk.  Of course, if you can absorb the
> extra crosstalk, it's a moot point.  And ground vias are often hard to put
> where we want them, since they impede routing on every layer, so we seldom
> have the luxury of all the vias we might like (the "no xtalk" configuration
> in the study).
> > Here's a link to the study.
> > https://www.filesanywhere.com/fs/v.aspx?v‹6e658761616dab72a2
> >
> > Note: I'm leaving the presentation a bit vague (leaving out any
> conclusions, many exact dimensions, and not stipulating simulation
> assumptions) on purpose - this should not be regarded as any formal
> "report-out".  It's only a "quick-and-dirty" peek at the issue.  Others may
> duplicate the simulation with different results, though that seems
> improbable - I don't see anything leading me to believe they are
> fundamentally flawed.  They are not overly complex topologies, so I invite
> others to replicate the simulations if the results seem questionable.
> >
> > And, of course, exact results will depend on design specifics (stackups,
> via dimensions, etc.).  Your mileage may vary...
> >
> > Cheers,
> > Jeff Loyer
> >
> >
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