Re: [SI-LIST] : Eye pattern with differencial signals.

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From: Bob Lewandowski (Bob.Lewandowski@Vixel.com)
Date: Mon Nov 27 2000 - 18:41:37 PST


Tek has a newer version diff probe, the P6330, (3 GHz BW) that has significantly
lower input capacitance than the older P6247 (1 GHz BW), or the P6248 (1.7 GHz
BW). The prototype I saw about a year ago had input capacitances of ~0.3 pF
differential, and ~0.5 pF common mode. They took somewhat of a hit in making it
manufacturable, but they still claim <0.5 pF differential and no common mode
spec. The common mode cap was the killer in the older units (P6247 & P6248),
~2. pF. We had a board with through 100 ohm differential traces and SMA
connectors at both ends. We looked at the through signal (~1 GHz square wave
with Tr ~100 pS ) with and without the probes attached, and even the Tek
marketing people were surprised at the degradation of the signal when the probe
older probe was applied. The new one was quite decent, almost no degradation.

Bring money! :-) $5.4K

http://www.tek.com/Measurement/accessories/index.html

---Bob Lewandowski
    Vixel Corp.

David Kaiser wrote:

> Tektronics makes a 1.7GHz differential probe that I have had to experience
> with. I don't have the part number or probe with me right now. Extremely
> convience, but costly. About $4000.00.
>
> David Kaiser
> McDATA Corp.
> 310 Interlocken Pkwy.
> Broomfield, CO 80021
> (303) 460-4431
> davidk@mcdata.com
>
> -----Original Message-----
> From: Degerstrom, Michael J. [mailto:degerstrom.michael@mayo.edu]
> Sent: Friday, November 24, 2000 1:31 PM
> To: si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : Eye pattern with differencial signals.
>
> 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.
>
> Mike
>
> _______________________________________________________________
>
> > -----Original Message-----
> > From: Ingraham, Andrew [mailto:Andrew.Ingraham@compaq.com]
> > Sent: Monday, November 06, 2000 8:48 PM
> > To: si-list@silab.eng.sun.com
> > 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
> > differential
> > receiver would see bigger margins than you would measure.
> >
> >
> >
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