[SI-LIST] Re: Eye diagram

Stephen -

I've seen this before also. besides trace length differences, I believe it
can also happen when the source impedance for the pull-up FET and the
pull-down FET are different. If the source-Z (aka drive-strength) is
different, then this can have an effect on the rise & fall time (less energy
initially injected onto the t-line). If the rise and fall times are not
equal, then the crossing point won't be in the middle. This should be true
for an eye diagram of a single-ended signal or an eye diagram of a
differential signal.

the short answer for both the trace length answer you gave and the one above
: no symmetry.

Jim Peterson
Honeywell

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Stephen Greenhalgh
Sent: Tuesday, January 17, 2006 4:03 AM
To: Si-List (E-mail)
Subject: [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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