[SI-LIST] Re: Eye diagram

Stephen,
    It's almost always the result of the rise times not being equal to the fall 
times.  A quick paper sketch would show this in a couple of seconds.  However, 
as we can't send pictures, solving
  Vhigh - t(falling slew rate)= Vlow + t(rising slew rate)
for cases where the falling slew rate is not equal to the rising slew rate will 
give you the result you are seeing.
It gets a little modified if the falling slew rates aren't equal and or the 
rising slew rates arn't equal


Stephen Greenhalgh wrote:
> 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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