Zhenggang, In case of symmetrical differential excitation (or probes), only the differential mode will be excited and propagating along the transmission line, regardless of how the excitation structure is implemented, with or without connection to the ground or reference planes. The differential mode impedance does not depend on the excitation - it is property of the transmission line geometry with the reference planes (does not matter ground or power). However the transition from the probes to the differential mode will be different in two cases and may cause the impedance transformation and differences in the observed impedances. If probes are not connected to the reference plane, the transition into the differential mode may take longer line segment - simply because of zero currents in the plane at the location of the probes must transition into the differential mode with the currents in the reference plane (reflection and radiation are side-effects of such transition). The observed impedances may also be different due to the impedance transformation effect in the transition area. In general, the closer the current distribution in excitation structure to the current of the mode to be excited, the shorter the transition area and the closer to the unit the impedance transformation coefficient. Note, that all those parameters can be effectively estimated with a 3D full-wave field solver for a particular stackup and measurement setup. Best regards, Yuriy Shlepnev www.simberian.com -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of ZHENGGANG CHENG Sent: Saturday, December 13, 2008 11:03 PM To: istvan.novak@xxxxxxx Cc: olaney@xxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Differential TDR 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 ------------------------------------------------------------------ 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