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Design >> High-Speed I/O Design >> Eye diagram: edges do not intersect correctly
https://designers-guide.org/forum/YaBB.pl?num=1207320051

Message started by ortis on Apr 4^th, 2008, 7:40am

Title: Eye diagram: edges do not intersect correctly
Post by ortis on Apr 4^th, 2008, 7:40am

Hello everybody. I'm testing an optical transceiver and as you can see in the attached image falling edges don't intersect the middle of rising edges. Someone could help me?
Thanks!!

Title: Re: Eye diagram: edges do not intersect correctly
Post by ywguo on Apr 6^th, 2008, 6:14am

Hi Ortis,

It looks that the output has duty cycle distortion. What was the output data stream when you recording the attached picture? I would like to observe whether it is duty cycle distortion using data stream like 0101..... , which avoids inter symbol interference. Duty cycle distortion is caused by the circuitry in the transmitter path. Usually when the driving capablitiy devates obviously from the sinking capability in the data path.

Best regards,
Yawei

Title: Re: Eye diagram: edges do not intersect correctly
Post by ortis on Apr 7^th, 2008, 2:57am

Hi Yawei, thank you for your help.
Data stream is the pseudo-casual sequence PRBS7, that means that a group of (2^7 - 1) bits is repeated. So it's not a sequence that avoids ISI.. Do you think that the problem is caused by the tracks impedance? Or by their length (and the resulting inductance..) ?

Regards
jack

Title: Re: Eye diagram: edges do not intersect correctly
Post by kanan on Apr 13^th, 2008, 9:37am

Hello,
If the cross-overs are skewed away from the centre, then it could be due to two reasons:
1. There is rise-time fall-time mismatch.
2. The "falling" starts later than "rising"

I agree with Yawei, most likely the pull-down drive is lower than the pull-up drive.

--Kanan