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

[Richard Wilbur]

On Sun, Dec 17, 2017 at 9:05 PM, Luke Kenneth Casson Leighton
<lkcl at lkcl.net> wrote:
>  so... ah.... key question here... is the taper needed or not? :)

The taper is a nice idea for changing the context smoothly but it
requires enough space that we can't return to our original context
before the cable connector (which is specified to be 100 Ohms).  So I
think we're better off living with the small, brief discontinuities
due to incursions into our design geometrical constraints, than
introducing a hulking change in our design geometrical constraints to
cover up the incursions (with the likely effect of changing our
impedance) and having no space left to taper the new impedance to 100
Ohm at the connector.

> should i instead be just setting 15mil clearance all round?  (and put
> a GND keepout underneath the ESD)?

Are there signals beneath the ESD components on layer 3 or 4?  If not,
we could put our ground reference planes on those layers under the ESD
components which would move them both one layer deeper.  (We already
have several conveniently placed ground vias.)  Otherwise, I would
just copy the lands for the ESD pads connected to the high-speed
signals and put them as ground keepouts on the normal ground reference
planes.  (In other words, only keep out the copper on the reference
plane just under the signal path where it goes through a wide pad for
the ESD component.)

Likewise with the connector, I would put a ground keep out under the
lands on layer 2 (probably best to just draw a keepout under the whole
connector on layer 2) but allow layer 5 to provide a full ground
shield.  (Provided my assumption is correct that the connector is
soldered on layer 1.)