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?v6e658761616dab72a2 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 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu