[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.
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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