Signal analysis system and calibration method for measuring the impedance of a device under test

A method of calibrating a signal path of a signal analysis system having a signal source coupled to a passive circuit device (PCD) in the signal path for measuring the impedance of passive circuit device comprising the steps of:

• acquiring sets of samples in the time domain of a signal under test from the signal source 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 from the signal source in the time domain to a spectral domain representations;
• characterizing transfer parameters of the signal source within a spectral domain from the spectral domain representations of the sets of samples by computing for each selected impedance load, combined transfer parameters associated with two-port network representations of the signal path;
• retrieving a stored value representing a reference impedance Zref associated with the transfer parameters of the signal analysis system;
• processing the combined transfer parameters of the signal source with the reference impedance (Zref) in the spectral domain to effect thereby a representation of the signal source impedance (Zeqsource) as a function of frequency;
• coupling the signal source to the passive circuit device;
• acquiring sets of samples in the time domain of a signal under test from the passive circuit device combined with the signal source 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 from the passive circuit device combined with the signal source in the time domain to a spectral domain representations;
• characterizing transfer parameters of the passive circuit device combined with the signal source within a spectral domain from the spectral domain representations of the sets of samples by computing for each selected impedance load, combined transfer parameters associated with two-port network representations of the signal path;
• processing the combined transfer parameters of the combined signal source and the passive circuit device with the reference impedance (Zref) in the spectral domain to effect thereby a representation of the combined signal source and passive circuit device impedance (Zeqcomb) as a function of frequency,
• processing the representations of the signal source impedance (Zeqsource) and the combined signal source impedance and passive circuit device impedance (Zeqcomb) in the spectral domain to effect thereby a representation of the passive circuit device impedance (ZeqPCD) as a function of frequency.

A signal analysis system for processing a signal under test from a device under test to represent an impedance of the device under test comprising:

• a signal analysis device having a memory and acquiring time domain digital samples of the signal under test from the device under test via a signal path with the memory storing the time domain digital samples of the signal under test and transfer parameters associated with the signal analysis device;
• a probe disposed in the signal path providing the signal under test to the signal analysis device with the test probe having associated with it transfer parameters stored in a memory, and a controllable impedance device disposed in the signal path coupled between the device under test and the probe with the controllable impedance device having selectable impedance loads with each selectable impedance load having associated with it transfer parameters stored in a memory;
• and
• a controller in the signal analysis device communicating with the controlled impedance device for selectively coupling a selected impedance load in the controllable impedance device to the device under test with the signal analysis device receiving the acquired time domain digital samples of the signal under test for each selected impedance load and converting the time domain digital samples to a spectral representation for each selected impedance load, and characterizing the transfer parameters of the device under test within a spectral domain from the spectral domain representation for each selected impedance load by computing for each selected impedance load, combined transfer parameters associated with two-port network representations of the of the signal path, the controller retrieving a stored value representing a reference impedance Zref associated with the transfer parameters and computing at least a first impedance (Zeq) of the device under test as a function of frequency using the characterized combined transfer parameters of the device under test and the reference impedance (Zref).