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?
Thanks,
David
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?
Thanks,
David
Dave