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

Luke Kenneth Casson Leighton lkcl at lkcl.net
Tue Aug 15 07:39:16 BST 2017

On Mon, Aug 14, 2017 at 10:37 PM, Richard Wilbur
<richard.wilbur at gmail.com> wrote:
> I have some time today to continue this discussion.


> So if we were to remove the ground shield traces
> from between differential pairs we could meet the
> inter-pair spacing recommendations without moving
> anything else.  This may explain the design by the
> wits-tech senior engineer you mentioned which worked
> without ground shield traces between the differential pairs.

 yehyeh.  i could then move them slightly away from the edge of the board.

> Another interesting reference on high-speed HDMI PCB layout is TI's SLLA324[2].

 nnniiiiiice.  i love it.  that's exactly the same connector being
used.  hmmm iinteresting, they bring the vias up from underneath on
all 4 diff-pairs...

> So, for the HDMI differential signals' sake, we don't necessarily need:
> 1.  Ground guard traces between neighboring differential pairs
> 2.  Ground guard traces between HDMI differential pairs and other circuits
> 3.  Multiple ground vias riveting along the side of the board to block emissions
> 4.  Perfectly matched inter-pair lengths


> On the other hand:
> 1.  Ground guard traces can be important in reducing noise radiated from single-ended circuits and coupled into other single-ended circuits on the board.
> 2.  Ground fences, traces riveted with multiple ground vias, can help even more with the goals of "reducing noise radiated from and coupled into other single-ended circuits on the board" as above.
> In other words, if we had more board space there are several things we could do differently:  increase differential pair trace width and spacing, ground shield trace spacing.
> But as it stands I believe it will likely work fine.  Without changing anything else we could drop the ground shield traces which would serve to increase our differential impedance.

 i think i will do that.

>  We would want to retain the ground vias near signal vias.


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