On Jun 29, 2017, at 20:45, Luke Kenneth Casson Leighton lkcl@lkcl.net wrote:
On Fri, Jun 30, 2017 at 3:32 AM, Richard Wilbur richard.wilbur@gmail.com wrote:
Have you identified which signals are affected?
no - i do not have access to equipment which will allow me to make such a determination.
I'll check to see whether I can get some lab time up at the university (where I took the lion's share of my electrical engineering coursework).
Have you been able to determine which signals are leaking into the affected signals?
ditto... basically this is all guess-work and experimentation.
Well congratulations on many good guesses and what sounds like a relatively successful experiment!
The fact that I don't know the layer stack for the board throws in some more uncertainty. I don't know whether those differential pairs were spending a lot of time over a ground plane. When the differential pairs crossed paths did they do so over separate ground planes, et cetera?
ok, so what i'm planning to do, richard, is a redesign of this entire area, starting by widening the PCB by 1mm.
Does that still meet your design goal for EOMA68 form factor? I wouldn't sacrifice an important design goal, yet. I wouldn't be surprised if we can pack a working layout into your original outline. On the other hand, more space can make the layout easier.
this should allow me to put several diff-pairs on the same layer (i'll start by trying to put them all on layer 3, see how that goes).
You said the board has 6 layers. Does that mean layer 3 is in the midst of the stack or on the outside? If it's inside, you could use striplines for the differential pairs[1]. It'll require three layers (two of them ground) but it is about as close to a TEM waveguide as you can get in a PCB.
would you be happy to advise before it goes to pre-production?
I would be overjoyed to put this somewhat arcane knowledge to some good use. In other words, "Yes!"
Reading an application note on HDMI from Texas Instruments[2] I noticed they mentioned a clock rate of 340 MHz and with a data rate of 3.4 Gb/s. In the interest of characterizing the problem at hand I would like to find a reference that describes the signals of this standard (HDMI v1.4).
Sorry for the noise. My last two posts were prematurely sent when I was trying to type (on my phone keyboard) and accidentally hit a/an conveniently/inconveniently placed "Send" button. I am trying to make peace with this user interface.
My wife gave me a tip: remove the "To:" address until you are ready to send the message as without a destination address the "Send" button is disabled.