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Measurement uncertainty and calibration power level

Question asked by mleibfritz on May 17, 2010
Latest reply on May 20, 2010 by mleibfritz

my sincere thanks to David Blackham for providing the VNA uncertainty calculator. It is a great tool that we use a lot.
In most applications we were especially interested in the uncertainty related to power level, i.e. changing the measurement
AND calibration power level to perform the uncertainty analysis of our ENA.
Now we needed to adapt only the calibration power level and I do not understand the calculation results.
Changing the calibration power from 0 to -10dBm and to -20dBm I see a global minimum in the S21 uncertainty curve
which is correlated with the calibration power - this is fine. However, I thought that lowering the calibration power level
will introduce a corresponuding amount of noise (i.e. lowering the power from 0 to -10dBm increases the noise floor or
dynamic range by 10dB). However, the results show that there is almost no influence on the measurement uncertainty.
I need to lower the power levels to something like -60dBm to become a significant additional error related to the cal-power-level.
I assume that your calculations are correct, but could you explain why this has to be like this?
I have looked through all sorts of publication, including the online manuals of the instruments/web as it has been mentioned that the formulae
are "all there". However, the formulae include the effective system data, only and do not seem to include power levels. Is there any publication
that includes this influence as well?