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Problem with de-embedded return loss measurements

Question asked by alhum on Nov 16, 2015
I am doing some measurements on a frequency converters with a N5242A. Connected to the VNA, I have 20 inches of cables that connect to a switch matrix. At the output of the switch matrix I have approximately 10 feets of cable up to the frequency converters. The total insertion loss between the VNA and the frequency converter is approximately 4 dB at the RF frequency (1.6 GHz).

My calibration plane is at the input of the switch matrix just after the 20 inches of cables, I use an e-cal to do the calibration. I also measure the s-parameters of the switch matrix + 10 feet cables and I use the s2p files I get to de-embed the measurements of my UUT. I chose to do this because I have a high number of path between the analyzer and the frequency converters and it was not practical to calibrate the analyzer directly at the converter ports. I get some great results for all of my measurements (conversion gain, noise figure, IMD3, group delay ...), matching nearly exactly the results I get at the calibration plane. However, for the return loss measurement, I don't get good results.

I have made some tests with a 50 Ohm load. When I do the VNA calibration at the frequency converter ports, and I measure the 50 Ohm load there, I get a 50 dB return loss. If I de-embed the measurement like described above, I get something close to 35 dB. I don't understand why I get this result. The network I de-embed starts exactly at the VNA calibration plane and ends exactly at the measurement plane.

Any idea of what could be wrong with my setup?