Thanks for your reply. If there is Gnd connection from differential probe to DUT, the measured Zdiff will be still the same as that measured w/o gnd connection? My concern is: w/o gnd connection to DUT, the signal return path will be each other, (i.e., p reference to n, and n reference to p); when gnd is added, signal might partially refer to gnd, the return path might be different from w/o gnd situation. Then will the Zdiff still the same? ZG 2008/12/13 Istvan Novak <istvan.novak@xxxxxxx> > 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