1. A method of calibrating a signal path of a signal analysis system for measuring the impedance of a device under test in the signal path comprising the steps of: acquiring sets of samples in the time domain of a signal under test from the device under test via the signal path including a plurality of selectable impedance loads with the signal path for each set of samples having a selected impedance load of the plurality of selectable impedance loads; converting the sets of samples of the signal under test in the time domain to a spectral domain representations; characterizing transfer parameters of the device under test within a spectral domain from the spectral domain representations of the sets of samples; retrieving a stored value representing a reference impedance Z ref associated with the transfer parameters of the signal analysis system; processing the transfer parameters of the device under test with the reference impedance (Z ref ) in the spectral domain to effect thereby a representation of the device under test impedance (Z eq ) as a function of frequency.