10-29-2014 12:48 AM
I'm setting up a statistical eye pattern scanner for use on a custom board, but I'm testing it on KC705. It's based upon XAPP743, but several adaptations were made to work in the target environment. In particular:
The tested channel ran at 10 Gbps through the SMA ports of the KC705 board (40 bits with 8b/10b enabled, data randomized with a scrambler, DFE enabled). Two RG-316 cables of 6” each were used to loop back TXN to RXN and TXP to RXP.
The channel ran practically error free (no errors detected for several minutes).
After running an eye scan with max_prescale=9, both horizontal and vertical increments at unity (took 30 minutes or so), this is what I got:
The turquoise segment in the middle is the no-error zone. Purple marks the high-error region, and intermediate colors show a logarithmic transition between the two regions.
I would be happy with this eye scan, had it not been for it's vertical asymmetry: I can't see any reason why the no-error zone is pushed towards a positive voltage when sampling earlier in time.
The cables are not to blame. I've also run a similar test with RG-316 cables of 36” (!) and got very similar results. The equalizer does a good job.
This is not a matter of differences in cable lengths either: When disconnecting either of the cables (running only through an P or N channel), the turquoise region gets smaller and rounder, but with the same downwards slope.
So I said OK, maybe it's because I'm running at 10 Gbps through lousy cables. I reduced the rate to 1 Gbps, with the same parameters, and changed max_prescale=3 to speed up the scan.
This is what I got:
Looks somewhat better: There's still a slight vertical asymmetry, but the really weird thing is around horizontal shift zero: There are errors. And it gets weirder: These errors appear only at horizontal positions from 1 to 6 with an error rate of 2*10^(-5) at position 1 and going towards zero at 6. What makes this even more weird, is that no errors were measured at the respective negative horizontal shifts or at zero. Let's keep in mind that the scan starts at horizontal shift 0, and then jumps to -1, 1, -2, 2 etc. So the errors are consistent with the horizontal positions. Needless to say, I ran this twice and got very similar results.
So -- has anyone an idea on what's going on here? Have I made a silly mistake?
Thanks in advance,
10-29-2014 01:16 AM
1G eye must be much better..
however even your 1G looks too much off from ideal, please look the 1G eye with TE0712 there is almost full rectangle of open area!
At 10G the eye opening can disapper with lousy cables.
You should run Xilinx IBERT on your boards to compare the eye scans.. then you know what they would need to look ,if your octave stuff is working properly.
10-29-2014 09:29 AM
Thanks for trying, but --
I don't think the cables can be blamed, because of the variations I tried with them. Besides, lousy cables tend to cause a narrow eye opening, not an assymetric pattern.
As for the 1Gb/s pattern -- it seems OK to me, except for the errors near the sampling point. This is not a problem with the graphical representation -- I've seen the numbers in the text file generated by the scanning program.
And of course, running Xilinx reference design as is could shed some light on this. But it would be even nicer, if someone spotted an obvious problem which could lead to the patterns I got.
11-23-2018 03:20 AM
I know this was a long time ago, but did you ever get to the bottom of this, as I have exactly the same problem now. Getting a very similar eye to yours running Aurora at 6.25GHz on a KC705 using xapp743 derived software, yet an IBERT at 6.25GHz shows a good clean eye ?
11-23-2018 03:28 AM
It was a long time ago indeed. Frankly speaking, I don't remember how this particular issue was resolved, if at all. I usually write follow-ups when there's a solution, so the answer is most likely that I stopped being fussy with this eye scan thing and went on with the project.
But I can confirm that the link remained error free and it all ended up with a working, functional project.