On Wed, Aug 16, 2017 at 6:11 AM, Richard Wilbur richard.wilbur@gmail.com wrote:
yehyeh. i could then move them slightly away from the edge of the board.
I'm curious, what would you move? The goal of this was to get >= 15mil between any differential signal trace and any trace not from the same differential pair.
ahhhh ok. i'm glad you're paying attention :)
The ground shield traces with 5mil spacing, 5mil trace width, and another 5mil spacing enforce this spacing on the differential signal traces. So if we remove the ground shield traces, and don't move anything closer, we get that spacing for previous effort.
ha!
ok.
so.
if i just take *out* the ground intermediary traces that would do the trick of bringing the impedance back up, is that right?
what would you suggest, here - leave the intermediary GND traces in or take them out.
also, i think i "Get It" about the intermediary wiggles. when the transmit end does automatic compensation that results in the signals coming out in such a way that, really, the inter-pair length-matching should be done from the *OPPOSITE* end i.e. from the CONNECTOR.
why?
because the automatic compensation will result in the signals coming out with a small delay, which by the time they go round that big set of bends they *WILL BE IN SYNC*.
ok they'll be in sync as long as all pairs are exactly the same length from that point up until they meet the connector.
so the only bit that would be out-of-sync would be that huge set of bends just after the transition from CPU-layer-1 onto layer 6, where i've had to put in huge amounts of bend-compensation.
by adding in the down-stream inter-pair compensation just before the rclamp0524p's) that *entire straight section* is out of sync... and the set of bends is also out-of-sync so it's no improvement.
Are you talking about moving the differential pairs further from the edge of the board?
yes. but from what you're saying it's not possible anyway.
Another interesting reference on high-speed HDMI PCB layout is TI's SLLA324[2].
nnniiiiiice. i love it. that's exactly the same connector being used. hmmm iinteresting, they bring the vias up from underneath on all 4 diff-pairs...
I think that is to keep the path as similar for all 4 pairs as possible.
yehyeh.
Vias add delay and (if not properly tuned) reduce the impedance. So it seems they are working with the stratagem that it is better to treat each component of the signal the same.
indeed. however i don't want to change the BOM, apart from anything that's a TI part not a "Well Known Easily Sourceable Part In The Shenzhen Huaqiang Road Eco-System".
dual rclamp0524's, one each side, it is.
l.