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Can I trust my AFR results?

Question asked by philarama on Sep 13, 2018
Latest reply on Sep 18, 2018 by philarama

Hi there,


I wonder if anyone can give me a bit of advice regarding the AFR process (speaking as a complete VNA novice).


I am using a PNA-L to characterize a transmission line which is on a flexible circuit.  My eCal kit is N4431B.  

The connectors to get down onto the board are Samtec bulls eye RF test connectors (pictured).  These, as well as the thin blue coax cables, are the fixtures I am trying to de-embed.  I am making differential measurements so using the 2X Thru AFR method.   


Samtec don't make a barrel adaptor for these connectors so I have made my own!  I glued the plastic housings together with an epoxy, ensuring that the conductive ground rings are aligned.  I plugged in the thin blue coax cables and tested the central conductor for continuity.  Then I performed the measurements.


Attached are my SDD21 results before and after AFR is applied.  


If you look at the corrected version you can see that at some frequencies I end up with a negative insertion loss.  An insertion gain in fact at higher frequencies.  My question is: can I trust the results I have obtained at my frequencies of interest (640 MHz and 1.28 GHz)?  Or does this prove that the maths has collapsed across all frequencies?


Any advice is much appreciated!