From: Degerstrom, Michael J. (email@example.com)
Date: Fri Nov 24 2000 - 12:30:40 PST
I think Andrew makes some very good points. Sometimes you can get
by with a signal ended measurement. But I was just doing probing
of PCML (CML run from a positive supply) and the measured
eye was lousy but the bit-error-rate was fine. Since CML
references it voltage levels from the most positive rail and
you must probe with a ground a 0 volts, then the probe picks
up power to ground noise.
I have a hard time finding good diffential probes (and good
single-ended probes as well). So I often take existing probes
and apply my own ground leads. For differential measurements
I often do not use a ground lead. Also, I've set the scope
up to measure vtrue-vfalse and have noticed a much cleaner
signal as a result.
> -----Original Message-----
> From: Ingraham, Andrew [mailto:Andrew.Ingraham@compaq.com]
> Sent: Monday, November 06, 2000 8:48 PM
> To: firstname.lastname@example.org
> Subject: RE: [SI-LIST] : Eye pattern with differencial signals.
> > According to Agilent (and using Agilent equipment), you may
> use a single
> > probe.
> > Connect
> > the probe tip to one of your +/- nets and the ground tip to
> GND. They do
> > offer
> > differential probes, but they are not be high bandwith capable.
> That's OK as long as there is no significant common-mode
> noise. If there is
> (added noise or +/- skew), then the single ended measurement
> would be in
> error. Common-mode noise would corrupt the eye pattern; the
> receiver would see bigger margins than you would measure.
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