Dear List members, I have done some experiments and have gotten what seems to be reasonable results. I seek the list's wisdom to comment on whether the technique is valid or it was just a coincident that the results looked right but can't be trusted. My application is very limited. We are making board assemblies with HDMI receivers. One of the HDMI compliance requirements is that the differential impedance must be between 85 and 115 ohm. We do not own a differential TDR scope, but we do have a high performance DSA70804 with a 300 psec risetime FAST EDGE output from the scope and ~100 psec risetime probe+front end. I thought I could press it into giving a reasonable estimate if the line length is more than a couple of inches. Starting from the 2x2 impedance matrix and the relationship between the two voltages and the two currents as shown on page 14 of this very nice article by Daniel Wu: http://www.ansoft.com/hfworkshop02/Danile_Wu.pdf It seems that even if the -phase is not driven, we should be able to observe the induced voltage and thus infer the differential impedance. Assuming symmetry and rewriting the equations: V1 = i1 * Z11 + i2 * Z12 V2 = i2 * Z11 + i1 * Z12 Rearranging and combining: Z11 = (V1 - i2/i1 * V2) * (i1) / (i1^2 - i2^2) Z12 = (V1 - i1/i2 * V2) * (i2) / (i2^2 - i1^2) V1 and V2 are the instantaneous voltages and i1 = (V1open - V1) / 50ohm i2 = (V2open - V2) / 50ohm Since all unknown variables can be measured, we can compute Z11 and Z12. Then: Zdiff = 2 * (Z11 - Z12); Zcm = (Z11 + Z12) / 2; I have made some measurements and plotted some results here: http://alfred1520.wiki.zoho.com/HomebrewTdrMeasurements.html The first graph is the V1, V2, and the computed Zdiff and Zcm of stuffed board. The second graph is the Zdiff of 12 measurements. They all seem to give believable results. I'll be more than happy to share more details about the procedure here or else where if this is really a valid technique. Best Regards, Alfred Lee ------------------------------------------------------------------ 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 Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu