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

[Richard Wilbur]

On Tue, Dec 5, 2017 at 5:41 AM, 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:
>
>> 3.  Reflections are more troublesome the further you get from the
>> signal source.  Close to the source the reflection arrives at the
>>  source during the signal rise time and can be overcome by the line driver.
>
>> 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
[...]
>> 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.

That's the basic idea.

I spent a little time reflecting on the big picture (terrible pun, I know):
 differential driver ->
 connected to PCB with small clearances for the first ~100mil ->
 via from layer 1 to layer 6, sort out intra-pair skew ~100mil? ->
 differential microstrip transmission line (111 Ohm characteristic
impedance) ~500mil? ->
 ESD suppression and sort out connections to cable connector ~200mil ->
 100 Ohm cable connector ->
 100 Ohm cable ~10000mil ->
 100 Ohm cable connector ->
 100 Ohm PCB connection to ->
 100 Ohm HDMI receiver.

Many impedance improprieties can be forgiven during the signal rise
time--which is to say within the distance the signal propagates during
the first quarter wavelength of its highest characteristic frequency.
In this case we already calculated that to be 4300mil, which looks
like it will get us safely on to the cable!

In pondering the big picture I was reminded that the impedance on the
cable is again 100 Ohm, so unless we had a chance to taper back out to
that we will create a bigger impedance issue at the connector (having
tapered down to smaller clearances over ~100mil and then keeping those
clearances for another ~200mil out to the connector) than the short
problems of the incursions into our design parameters for the
differential microstrip transmission line.  The ESD and connector
lands represent pretty short distances in the direction of signal
propagation.  Their are other traces and vias that come closer but
over a relatively very short distance in the direction of signal
propagation.  (We can combat the small problems at component lands by
possibly removing the ground plane directly below--essentially moving
it lower using a different layer for ground plane.  Maybe use layer 3
or 4 instead of 2 or 5?)

So, if we had to adapt a line driver with some impedance to a line of
different impedance, the taper would be a fine solution.  We, on the
other hand, have 100 Ohm impedance on both sides of the problem area
and thus are better served leaving the conduction path at 100 Ohm
impedance and taking some small hits rather than changing it without
the room to do as good a job of changing it back.

Sorry for jumping down that rabbit hole!  The solution was worse than
the problem!  That's what I get for focusing too closely on details
without gazing at the forest again, once in a while.

Sincerely,
Richard