The "set Zo" function does not appear to affect the displayed S-parameters of my network analyzer. How does this function work?
If the characteristic impedance of the device under test (DUT) is not equivalent to the impedance of the analyzer in use, then the impedance value can be modified using the "set Zo" function.
An example would be performing 75-ohm DUT measurements with 50-ohm network analyzer. The effects of changing the value of the system impedance, "Zo", will be realized within the Smith chart format. Specifically the R+jX marker values in the Smith chart mode. Modifications within the analyzer, due to changing "set Zo" are mathematical. There is no change in the inherent physical characteristic impedance of the analyzer. The measured impedance is mathematically normalized to the user entry value of Zo. The center of the Smith chart is now adjusted to the new characteristic impedance value of Zo. Other S-parameter formats are not affected by the change of the characteristic impedance.
Note that when working with waveguide (W/G) based DUTs there exist a special requirement to "set Zo" to "1". This value is a flag to the analyzer to account for W/G dispersion effects when performing certain internal calculations. This is only true for analyzers that support waveguide calibrations. The "set Zo = 1 ohm" should be entered prior to performing a calibration.