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

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From: William Baldwin (bald@oscillate.eng.sun.com)
Date: Tue Nov 28 2000 - 09:46:50 PST


Ray,
We have two of the newer ones, P6330's.

This problem with the P6247's affect your measurements,
like eye diagrams, more than mine.

-WB

> Date: Tue, 28 Nov 2000 09:28:05 -0800 (PST)
> From: Ray Anderson <raymonda@ha1mpk-mail.eng.sun.com>
> Subject: FWD: Re: [SI-LIST] : Eye pattern with differencial signals.
> To: bald@oscillate.eng.sun.com
> MIME-Version: 1.0
> Content-MD5: n7ptFZa2TVa3XPq9iYpWzA==
>
>
> Bill-
>
> Some comments on Tek differential probes from si-list.
>
> -Ray
>
> ---------------------------------------------------------------
>
> Date: Mon, 27 Nov 2000 18:41:37 -0800
> From: "Bob Lewandowski" <Bob.Lewandowski@Vixel.com>
> X-Accept-Language: en
> MIME-Version: 1.0
> To: David Kaiser <david.kaiser@mcdata.com>
> CC: "'Degerstrom Michael J.'" <degerstrom.michael@mayo.edu>,
si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Eye pattern with differencial signals.
>
>
> 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.
> > >
> >
>

-WB
... --- ...


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