[SI-LIST] Eye diagram

Recently I saw an eye diagram in which all the crossovers occurred at 70-80% 
amplitude rather than the ideal 50%. Apart from this obvious defect, the eye 
looked pretty good. Ever since, I have been wondering what would cause this 
effect. My first thought was some difference between the +ve and -ve traces of 
the differential signal path, but I cannot formulate a theory that explains 
this satisfactorily. For example, if one trace were longer than the other, I 
would expect both positive-going and negative-going edges on the longer trace 
to be delayed relative to the shorter one, which would mean that some 
crossovers were raised, and others lowered, from the 50% level.
Is this a known phenomenon that I have somehow completely missed? I would be 
glad for any explanation or suggestion that anyone can offer.

Stephen Greenhalgh

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