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Group Delay Measurement Uncertainty

Question asked by jeff_philips001 on Feb 4, 2015
Latest reply on Feb 5, 2015 by Dr_joel
I just downloaded the Uncertainty Calculator and plotted Group Delay Accuracy for three different Sii.  I wanted to see how mismatched terminations affect the accuracy of a group delay measurement: they seemed to make little to no difference.

The Uncertainty Calculator plots Accuracy (ns) versus Aperture (MHz) for Group Delay.  Perhaps the vertical axis should've been labelled "Error" instead of "Accuracy."  I was surprised that the error declines with increasing frequency aperture.  Group delay is basically the derivative of phase with frequency.  The derivative only becomes exact when delta frequency approaches zero.  I understand that as delta frequency approaches zero, delta phase also becomes small and that it may no longer be large compared to the errors in the phase measurement, but the systemic phase error should fall out because the difference of adjacent phase measurements is being taken.

My intuition is apparently incorrect.  My basic question is why does Group Delay Error decline with increasing frequency aperture?  I'm looking for a brief, intuitive answer if one exists.

Please don't spend too much time on this.

Thanks, Jeff