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Dynamic accuracy questions

Question asked by b.d.hall on Jun 19, 2011
Latest reply on Jun 21, 2011 by b.d.hall
I would like to better understand what dynamic uncertainty is and how it should be used in uncertainty calculations.

For example, lets say that a measurement of the substitution loss of a step attenuator is required. Two VNA transmission measurements will be made: one with the step attenuator in the datum position and one in the step position. Then the SL could be calculated by taking the ratio of the results, etc.

Dynamic accuracy is, if I understand correctly, predominantly associated with linearity errors in this type of measurement. Is that correct?

Agilent presents DA as associated with a sytematic effect, which suggests that the errors do not change from one measurement to the next (unless the measurement conditions change). So, is there some potential for these errors to combine and cancel in compound measurements?

In a single transmission measurement (S21 say) two amplitudes are measured on different channels, and ratioed to get the transmission (reference and testport). There will be an error in each measured amplitude, but it is the net effect of these errors in different signal channels (reference and testport) that is presumably reported in specifications of the DA. Since the receivers are different, I suppose that it is fair to say that the errors are independent? In that case, the DA specification will be larger than the uncertainties associated with the two individual errors.

So, what should one think in the case of the SL measurement, which combines two transmission measurements?

One might expect that the (two) errors associated with the linearity of the (two) reference measurements will cancel (to some extent), because the reference level is unchanged and the receiver is the same. That would leave the (two) measurement errors at the test port, which have different amplitudes (one is attenuated). But how would the uncertainty of the combined error be related to the DA specifications? Since these errors now arise on the same receiver channel, I would (naively) expect there to be some cancellation (correlation) of the resultant error here too.

Does this makes any sense?

Comments would be appreciated.  

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