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ecal (N4694-60003) with E8361C - why both S12 and S21?

Question asked by tom958 on Aug 8, 2010
Latest reply on Aug 8, 2010 by odanzy
Hi All

I am using the N4694-60003 Ecal module with the E8361C PNA. Before calibration, if I just want to measure S21, when I make a measurement only the Port1  light is active, and it is clear that S12 has not been measured. But after doing a 2 port calibration with the cal wizard, all further measurements seem to measure both S21 and S12. I'm not sure why this would be - I can't disable the S12 measurement. I am concerned because I intend to use this PNA with a modulator and detector for photonics test, and the system is not going to be reciprocal. I do not want to dump RF power into my detector, for one thing, and I am also concerned because I don't understand why it would perform both measurements. It only seems to do this after the ecal kit has been used.

Does anyone know why this would happen? Is it possible to rely on the S21 value even though the S12 value won't be reciprocal due to running into a photodetector?