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VNA, S11 measurement precision vs reflection level

Question asked by jem on Sep 9, 2011
Latest reply on Sep 12, 2011 by Dr_joel
Hi there,

I'm trying to explain a behaviour I see in my experiments with actual VNA specifications and I am not too sure what to exactly look for.

I have a 8722D and measure S11. I calibrate it using an E-Cal. I measure two consecutive S11 data of the same DUT and look at the variation between the two measurement by subtracting (RE + IM) one with the other. If both S11 data are identical, I get a magnitude of 0 (in theory) and if the S11 data differs I get an higher number.... fairly simple stuff

I noticed that the variation I see are usually larger when the device I measure is not as well matched. I tested with a broadband load and a short from a 85052D cal kit and it is very obvious. After substraction I get down to -80 dB in magnitude with the load and -50 dB with the short. (exact same VNA settings).

I know the uncertainty increase with higher reflective DUT but it seems that the uncertainty value is a broader property than I would need to explain this behavior and it seems that the uncertainty is more about accuracy than precision....

Can someone help me on this ?

Thanks
Jeremie  

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