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

[mike.valk at gmail.com]

2017-08-16 11:33 GMT+02:00 Luke Kenneth Casson Leighton <lkcl at lkcl.net>:
> On Wed, Aug 16, 2017 at 10:31 AM, mike.valk at gmail.com
> <mike.valk at gmail.com> wrote:
>
>> Looks pretty. Seeing that does raise a question too me. Is it
>> necessary to match length between the different pairs? I didn't think
>> that was a requirement. Because I see pairs wriggling and wasting a
>> lot of space.
>>
>> I thought that only matching was required on a single pair. Impedance matching.
>
>  that's what we've been discussing.  read richard's message and my response.

I've read it again. But did not digest that from Richard's responses.

Inter-pair skew: Length (un)matching between two traces making op one
differential pair?

Intra-pair skew: Length (un)matching between differential pairs? Not mentioned.

What else I read so far:

Possibly remove the GND traces between pairs. Differential pairs are
designed to cancel each other out thus limit radiation. The pair
coupling creates force to repel incoming radiation noise. Correct me
if I'm wrong

The same construction as in twisted pair cables. But there you have
differential pair twisting creates an even bigger effect. But there we
also have types with shielding. Shielding around the whole set and
even with shielding per pair. The HMDI cables I've  butchered had per
pair shielding and the other lines, clock, cec, etc, unshielded
bundled in one extra shield.

Removing the, intra pair, GND traces improves impedance, but decreases
shielding from external incoming radiation. But I suspect that effect
is limited due to the GND layer below, far bigger and nearer than
those traces.

Differential pairs should have a bigger, dielectric, space surrounding
them than they have to each other. Because the nearer you get to a
pair the less the differential cancelling effect. With the exception
for GND, which should act as a sink for EM emissions.

Removing the, intra pair, GND traces won't give you more space because
the pairs should keep the extra distance from each other.

Via's should occur only when, inter pair, length is matched.
Differential via's should have a rounded, oval,  common, dielectric,
space surrounding them so the Z-axis radiation can cancel out
uninterrupted.

Digital differential signals might be skewed to begin with. Limiting
the differential EM canceling effect to begin with.

I'd say keep the intra pair GND traces. Maybe loose the intra pair
length mathing.

There should be no electric/magnetic coupling between intra pairs. But
if their length differs the parallel digital signals might become time
skewed. But I doubt that on this length that would be a problem.

Richards math should help with that along with max allowed digital
signal skew. Don't have the time to convert the math in a spreadsheet
calculator to confirm.