[SI-LIST] Re: Differential TDR
- From: olaney@xxxxxxxx
- To: istvan.novak@xxxxxxx
- Date: Sun, 14 Dec 2008 01:10:47 -0800
Istvan:
Given that Mssr. Chen specified a diff probeand measurement of Zdiff in
his query , I responded to that. Perhaps you are referring to the lack
of mention of a step source, which obviously cannot be sent through a
high-Z probe. My impression is that the TDR source is a separate part of
the setup, and what is really wanted is the ability to probe anywhere
along a differential line without disturbing the signal. That would
require a high-Z probe (in this context a 10:1 probe with 500 ohms at the
tip, 1000 differential is high-Z compared to a 50 ohm trace). The
measurement mode would be TDR and TDT mixed together, except at the ends
of the line where it would be pure TDR at the step source, and pure TDT
at the termination. Sorting through terminology and phraseology, I don't
think you and I disagree. If ZG wishes to share more complete
information about his setup and what he wants to do, we could get more
specific about how to do it.
Orin
On Sat, 13 Dec 2008 23:30:56 -0500 Istvan Novak <istvan.novak@xxxxxxx>
writes:
> ZG, Orin,
>
> If the probe is really part of a TDR setup, I dont think
> high-impedance
> probe is an option.
>
> If the DUT is passive and well balanced, the differential probe
> without
> ground connection
> measured the differential response. It can not, however, measure
> the
> common-mode response
> without a ground connection to the DUT. So to answer the original
> question, without a ground
> connection to the DUT, only the differential impedance can be
> measured.
> And since without
> ground connection the single-ended impedance can not be measured,
> both
> Z_single and
> Z_mutual will remain unknown.
>
> Regards,
>
> Istvan Novak
> SUN Microsystems
>
> olaney@xxxxxxxx wrote:
> > If you mean a hi-z diff probe, then the line needs to be
> terminated with
> > matched resistors to ref ground. If dual 50 ohm probes into 50
> ohm
> > inputs, the instrument itself provides termination. The
> instrument
> > doesn't need to "know" what the single ended impedance is to
> measure
> > Zdiff. That's not to say that Zse or Zcm aren't also available
> from the
> > typical setup. The most important consideration for Zdiff
> accuracy is
> > that measurement path delays be identical for both probes
> (automatic for
> > a unitary diff probe), and that the probe grounds be tied to each
> other
> > (not necessarily to other grounds) at the measurement end (also
> automatic
> > for a unitary diff probe). Also note that Zdiff can be well
> defined even
> > if Zcm is not, hence the successful application of unshielded
> twisted
> > pairs dangling in air and over heating ducts in the form of CAT5
> LAN
> > cabling. The large swings in Zcm barely affect Zdiff.
> >
> > Orin Laney
> >
> > On Sat, 13 Dec 2008 12:22:54 -0800 "ZHENGGANG CHENG"
> > <zhenggang.cheng@xxxxxxxxx> writes:
> >
> >> Hi all,
> >> When using differential probe to do the TDR for differential
> pair,
> >> if the
> >> probe only has P and N tip no Gnd tip, does the TDR gives accurate
>
> >> values?
> >>
> >> Here is the concern:
> >>
> >> Zdiff =2*(Z_single-Z_mutual), if no Gnd tip is used, how does the
>
> >> TDR know
> >> what's the single-ended impedance, and consequently how does TDR
> >> know the
> >> Zdiff?
> >>
> >>
> >> Thanks,
> >>
> >> ZG
> >>
> >>
> >>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject
> field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
>
____________________________________________________________
Win the battle of the bulge with great liposuction solutions. Click now!
http://thirdpartyoffers.juno.com/TGL2141/fc/PnY6rw2auzNy7nU5byZTDhFOUP64kVOJ7SwnIeYboontwJ49motxN/
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
