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Difference in TDR Plot of cable assemly using PNA vs. ENA

Question asked by dweinstein on Feb 12, 2016
Latest reply on Feb 16, 2016 by dweinstein
Recently a customer returned a cable assembly (15 ft long) claiming the impedance of the cable was 53 Ohms.  We tested using the N5225A PNA setting it up for a Low Pass TDR calibration from 10 MHz to 8.01 GHz with 801 points using an 85052D cal kit.  See the attached plot.  The customer uses a E5071C ENA and we have one also so we set it up the same way and got a significant difference in the plot.

The main difference is that on the PNA, at time less than t=0, the plot is at -10 mU and on the ENA, it is +10mU giving a 2 Ohm difference for the cable.  Without the cable attached or while measuring a short cable (6 inches), the reflection at less than t=0 is 0 (no offset).

Why is there a reference offset below t=0 and why are the 2 plots not giving the same result?  What is the actual impedance of the cable?