[Arm-netbook] HDMI High-Frequency Layout: Recommendations

Luke Kenneth Casson Leighton lkcl at lkcl.net
Thu Aug 10 09:23:00 BST 2017

On Thu, Aug 10, 2017 at 9:14 AM, Hrvoje Lasic <lasich at 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.


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