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

[Richard Wilbur]

On Dec 5, 2017, at 05:41, Luke Kenneth Casson Leighton <lkcl at lkcl.net> wrote:
> 
> On Tue, Dec 5, 2017 at 12:30 AM, Richard Wilbur
> <richard.wilbur at gmail.com> wrote:
>> 
>> Deviations from path in due NorthEast direction (+ signifies change in the
>> NorthWest direction, - signifies change in the SouthEast direction, units in mil)
>> <step>  <Northern keepout> <TX1> <TX0> <TXC> <Southern keepout>
>> 0     0  0  0  0  0
>> 1     -1  1  2  3  4
>> 2     -1  1  2  3  4
>> 3     -1  1  2  3  4
>> 4     -2  2  4  6  8
>> 5     -2  2  4  6  8
>> 6     -1  1  2  3  4
>> 7     -1  1  2  3  4
>> 8     -1  1  2  3  4
> 
>  okaay so the idea is, just after the long straight you make a series
> of very tiny corrections by bringing each of the tracks inwards -
> closer together - so that when you get to the point where you *have*
> to be 5-7mil apart you're already neeearrrrly that far apart *anyway*
> so it's not so bad.
> 
> ok :)  that's perfectly doable.

Sweet!

The reason for all the detail is to try and make the changes gradual enough to avoid causing big reflections from the taper itself but also make the geometry symmetric so we avoid turning differential signal into common mode.

Notice that all the tracks and keepouts start a move "in" towards TX2 once every 15mil and then stay at that distance for the remainder of that step.  Notice also that all the tracks and keepouts move in except TX2 (since our goal is that it be the shortest--or moreso that TXC be the longest).  At the 5mil-inter-pair-distance end of the taper the manual keepouts become superfluous in light of your 5mil minimum Cu-Cu spacing design rule which then simplifies the connector end of the layout.

I threw "step 0" in there to say let's allow things to at least settle a little after making the turn before we start into the taper.

Once we get into that tight bundle then we have to carefully pull the pairs off in order to avoid undoing all our work.  I'll send another drawing later this afternoon.  (I'm waiting for the dental hygienist, right now.)

The idea is to have the bundle running NE, then simultaneously turn the bottom (southernmost) pair due E while the rest of the bundle turns due N for at least 15mil  before turning back NE.  This makes a 90 degree corner between the bundle and the pair which is leaving and gives enough space to allow ground fill between immediately.