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

[Richard Wilbur]

On Dec 22, 2017, at 07:51, Luke Kenneth Casson Leighton <lkcl at lkcl.net> wrote:
> On Fri, Dec 22, 2017 at 8:54 AM, Luke Kenneth Casson Leighton
> <lkcl at lkcl.net> wrote:
[…]
>> this is a pain!  i might see if i can set a clearance to GND on
>> individual tracks / connections as opposed to NETs.
> 
> ok that worked.  just uploading a video here:
> 
> https://youtu.be/LfQc89CS4m0
> 
> turns out that there's a feature i'd not used before, called
> "conditional rules".  you can specify that *if* GND meets HDMI Group,
> clearance rules shall be different.  ordinarily you have to forcibly
> set the *entire* GND plane to specific clearances (to ALL objects), or
> the *entire* HDMI group to specific clearances (to ALL objects)...
> this "conditional" rule does the trick.

Nice work!  That certainly simplifies things!

> richard i go over it in the video but i believe the layer... 5
> keepout needs to also be extended under the layer 6 (blue, bottom)
> tracks leading to the VIAs that jump up to the DC3 connector pads.
> also i believe that i should be adding some tracks (pink) which,
> particularly if there is to be a hole in layer 5 underneath, should be
> around a 5 mil clearance, to match the fact that it's swapping
> vertical distance for horizontal distance, what do you think?

I believe that the right thing is to not extend the layer 5 ground keepout under the differential nets on their way out to the connector because it is the ground plane for layer 6.  The reason for dropping the ground plane under the connector pins (layer 1) from layer 2 to layer 5 is that the pins are so close to each other.  But we are still interested in the shielding effect of ground plane below the high-frequency signals (on the differential pairs) on layer 1 and between the signals on layer 1 and those sneaking under on layer 6.  That's my reason for keeping layer 5 ground fill everywhere except under the layer 6 high-frequency pads of the ESD (where he drop to ground on layer 4).