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

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

A short is 1 /_180, a 0.2 degree shift causes a 0.035 residua, (tan(0.2)),l which is about -50 dB. The same shift for a 30 dB return loss load (mag 0.03) is about -80 dBc. When you subtact one from another, the phase shift will generate a residual for the same standard being measured.