On Thu, Aug 10, 2017 at 9:14 AM, Hrvoje Lasic lasich@gmail.com wrote:
the case for GND around differential pairs cant hurt, maybe even can help. But is it better to have GND in plane below that actually is doing same things? If there is no clear path for signal to go back then I guess put GND in parallel is good but if you have clean GND below than make it somehow redundant. Or am I wrong? I am discussing these because most probably there is tight space even without GND lines...
the most amazing borad i saw was a 2-layer 5-port GbE router. man you should have seen the diff-pairs on that. it was... beautiful. every ethernet diffpair - bear in mind this is GbE with 5 ports - so that's TWENTY pairs - had GND vias equally spaced an absolute specific distance from them, absolutely regularly like clockwork every couple of mm.
what that does is make *absolutely* certain that there's no cross-talk between the diff-pairs. with only a 5 mil GND trace between pairs i am really pushing it, but there really isn't any choice here.
the first design (done by a superb senior engineer at wits-tech) didn't even have the GND separation between diff-pairs, and yet amazingly it worked. i don't feel comfortable leaving them out, but i can't get vias in at both ends on all pairs.
l.