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TDR excessive reactance

Question asked by Thomas_K on Nov 29, 2011
Latest reply on Mar 1, 2012 by ChengNing
Hi all,

I apologize that I post this Oscilloscope/TDR measurement question here within the NWA section, but I think this question is closer to the NWA topic.

For some weeks now, I'm glad to have an Agilent 54754A based TDR in my lab to analyze PCB structures. On one PCB I analyzed the transition from the PCB wave guide into an RF connector and found a positive bump indicating a more or less inductive behaviour for this section.
The system allows the positioning of two markers and calculates the excessive reactance in between these markers. This is really valuable to determine the correct compensation, or to find a way to reduce the excess.
For the section of interest I get round about 630nH of inductance if I place the markers left and right of it. So far so good:

Questionable to me is the behaviour that I see if I move the markers closer together to cover only a part of this inductive discontinuity. At a certain point, when the left quarter of the discontinuity is outside the markers the display jumps over and starts showing fF and further in pF values. I'd like to make clear, that all the curve in between the markers is clearly >50Ohm.

The other way around it happens in the following screenshot:
Here the impedance is clearly below 50Ohm (39.6 ... 47.2) and we see 90pH mismatch.

The system is in the market for quite some time and I checked I use the latest software from Aug2011.
I'm still searching the solution in between screen an chair fo course. 

I wonder if somebody has a similar system available, or has a good explanation for this observation.

Many Thanks, Thomas.