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Weird Eye jitter. Can anybody explain why?

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gilbertomaldito

gilbertomaldito

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Hi,

Ok, I simulated my LVDS receiver using input K28.5 pattern at 1Gbps. The eye diagram shown below is the eye pattern at the output. My question is, why is it that the waveforms that constitutes the eye are grouped into separate strands?

The data pattern i used is K28.5 or 0011111010110000010...

--andrew
 

That seems to me, the classic form of intersymbol interference.
Is your driven line, longer than 1/2 electrical time-of-flight?
 

Hi dick,

What do you mean by electrical time-of-flight?
 

Wire lines propagate signal at about 1nS/foot. To null a transmission
line after a signal step requires a wavefront round-trip (with near-
perfect termination). If you have "one in the pipe" and launch a
second transition, the "symbols" will collide and add / oppose at
the receiver, modulating the "eye" edges.

I'd say at 1GBPS you would be likely to see this at any length
over 6" or so. Most line interface parts for high speed and long
wire have some sort of pre-emphasis and try to adjust it
based on signal history, or so I gather, to correct that effect.
 

Yes, case of data dependent jitter. I can't figure out why low to high has the more classic form whereas the high to low has 3 strands.

btw, there can be other things causing DDJ other than jsut board trace. It'll be good if the OP Can specify exactly what his setup looks like
 

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