[SI-LIST] Re: Eye diagram
- From: David Instone <dave.instone@xxxxxxxxxx>
- To: Stephen.Greenhalgh@xxxxxxxxxxx
- Date: Tue, 17 Jan 2006 13:13:40 +0000
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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