The difference between System Z0 and Line Z0 in a TRL calibration.
When performing a TRL or LRM calibration, you can specify in the
calibration kit whether to use "System Z0" or "Line Z0" to set the
characteristic impedance. The default is "Line Z0", but in either case,
the line or match standard still sets the reference characteristic
impedance of your measurements.
If your line standard's impedance is equal to the desired
reference impedance, then you should use "Line Z0".
After calibration, the reference characteristic impedance will equal that
of the line standard. The network analyzer's calibration software ignores the
values entered for the line standard's Offset Z0 and Offset Loss.
If you have a situation where your line standard has a characteristic
impedance different from the desired reference impedance, then you
should choose "System Z0", and enter the correct values for Offset Z0
and Offset Loss in the line standard definition of the cal kit. For
example, the physical dimensions of your test fixture may make it
desirable to have a line standard that is wider than a 50ohm line, perhaps a
33ohm line. Then you would define the line standard with Offset Z0 = 33,
leave the System Z0 = 50, and choose CAL Z0: SYSTEM Z0. This lets the
calibration software know that your line standard is 33ohm, but you want
to make measurements referenced to a 50ohm characteristic impedance.