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

[Richard Wilbur]

On Sep 28, 2017, at 03:36, Luke Kenneth Casson Leighton <lkcl at lkcl.net> wrote:
> 
> http://rhombus-tech.net/allwinner_a10/news/eoma68-a20-hdmi-275-new-keepout-taper2.jpg
> 
> ok that's the latest, a few tweaks still needed.  yes saw about the
> 15mm not 15mil... whoops :)  so all those little tweaks... gone.
> maybe shouldn't have removed the ones on the USB-OTG line but... oh
> well.

HDMI looks good without the little wiggles (which, it turns out, weren't essential) towards the connector side.  I didn't notice the wiggles on the USB OTG lines.  Were they in the pictures you released?

> anyway... moved the tracks up by 4 mil so they're now 15 mil from
> board-edge GND and 15 mil from the top line.

Very nice!  It looks cleaner and more symmetric.

>  sorted out the wiggles
> (again!!).

Courage, mate.  Be careful in the wiggles if you make a high-frequency trace turn parallel to itself that those parallel sections are at least separated by a distance of 4*trace width=20mil.  Otherwise the parallel sections look like antennae to the high-frequency signals which simply radiate straight across and bypass the intended delay path.

I notice some of your wiggles have no parallel sections and some do.

> taper... really not certain where to put it (starting point) -
> thoughts appreciated.

What type of taper?  An approximation of Klopfenstein, or just a symmetric easing of the transition?

> end... just before the 45-degree bend into the VIAs and ESD.

That's a fine place to end it.

Along the bottom margin there's a little corner in the fill that sticks out towards the trace where the keepout geometry crosses the 5mil clearance.  Not to worry, it will disappear with my recommendations for the taper.

Can I send a diagram this evening?

> a few tweaks of VIAs along the way to ensure the GND  flood-fill
> isn't broken.  the image is hi-res enough so that GND VIAs (covered by
> flood-fill) can just be identified by the word "GND".

Thanks.  Looks good.