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

Luke Kenneth Casson Leighton lkcl at lkcl.net
Thu Aug 10 09:18:50 BST 2017

On Thu, Aug 10, 2017 at 9:01 AM, mike.valk at gmail.com
<mike.valk at gmail.com> wrote:
> 2017-08-09 15:23 GMT+02:00 Luke Kenneth Casson Leighton <lkcl at lkcl.net>:
>> next set...
> GND shielding parallel to the differentials is interrupted quite
> often.

 because there's simply not enough space to do otherwise.  if i could
move the entire CPU and RAM up another 0.5mm it would be doable.  but
then i would have to re-route 12 signals which go around the top area
of the board and that's (a) risky and (b) not enough space to do it.

> Those GND tracks act as shields, for emission and reception.
> I'd try to put as much parallel GND as possible.
> And trace the parallel GND around the via's, see attachment.

 ah, got it - thanks for the tip, i thought i'd done that on all
diffpairs, but i missed one.  good call.

 yes there's only one, because the layer 1 and layer 6 will be
flood-filled and that will fill the areas that "appear" to be missed.

> Make sure the'res as much solid GND on the layer above and below the
> traces, again shielding.

 these are layer 1 and layer 6, and layer 2 and 5 are solid GND.

> Also I'd personally not use curved wriggles. HF signals travel in a
> straight direction. With curves they start diffracting and start
> bouncing cross each other and might start to radiate or echo back.

 mmmmm.... *stress*!  anyone else feel the curves are "Bad"?

> But
> I see that the community is divided on that stance.
> If tight for space you can use 90% corners with a chamfered outer
> edge. I suppose the chamfer acts like a mirror.

 i'd *really* prefer not to do that :)

> https://www.maximintegrated.com/en/app-notes/index.mvp/id/5100
> Figure 6

wow that's pretty bad-ass.

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