The Rigexpert AA-600 has an inbuilt calibration which is convenient to use. It is capable of OSL calibration, but this article discusses only the inbuilt calibration.
The reference plane is the plane at which the instrument calibration is correct, at other locations there is a transmission line impedance transformation applied.
Last first, if you make measurements of the length of a length critical section of line, you need to know where distance zero is located.
The diagram might suggest that it is located outside the instrument, if you took the diagram literally perhaps the end of the N type collar nearest the instrument.
We should expect that the location of the reference plane under the factory calibration may vary from instrument to instrument, so I will discover its location for my AA-600.
We can use a short circuit (SC) or open circuit (OC) to help find the reference plane, a ‘good' short circuit is best as it is likely to have near zero offset from the outer face of the N socket, and it is likely to have insignificant resistance or inductance.
So, lets sweep the SC to 600MHz.
Above is the result of the sweep. Not the reactance increases pretty much proportional to frequency, indicating either a series inductance or, for a very short 50Ω transmission line, that there is some small amount of line between the reference plane and the SC. The latter is the most likely explanation.
The sweep was exported to a Touchstone file and the phase of s11 plotted.
Above is the plot of phase of s11, it is a fairly straight line up to about 480MHz, then a bit of a step and continues straight at similar slope but offset by the step. It is difficult to explain the step, and one wonders if it is some form of rude compensation in the AA-600 firmware.
Also plotted is the phase delay of a transmission line with propagation time of 92.6ps. It is a very good fit to the measured data to about 480MHz.
The offset of the SC is 18.3ps (ie the short circuit is 18.3ps further than the reference plane), so the offset from the instrument reference plane to the N connector reference plane is 74.3ps.
Since the outer face of the female connector is readily accessible, it is worth knowing that the reference plane is approximately 9.2mm or 31ps further inside the connector.
Let's calculate the equivalent length of line with VF=0.66.
The equivalent length of line with VF=0.66 is 14.7mm.
The equivalent length of other VF lines needs to be adjusted if the physical offset is calculated in terms of the other line.
Finding the reference plane offset using the AA-600
Return to the previous R,X sweep.
Let's use the subtract cable feature to compensate the reference plane offset.
Above, VF=0.66 50Ω cable is subtracted to get the best X response. In this case it is easy to get a very good compensation to about 500MHz, but difficult to get excellent compensation to 600MHz. So, choosing the offset of 18.3mm for very good results to 500MHz, we know the location of the short circuit.
Using an OC
You could use an OC termination, but typically they have a very small physical offset, the plane of the OC is very close to the end of the female pin rather than the outer plane of the socket. There is also some effect of fringing capacitance (of the order of 50fF) though quite small to 600MHz.
Above is a sweep with OC. The best cluster on the Smith chart occurs with 16.3mm offset to the open circuit, 2mm less than with the SC termination.
Velocity factor solver
The offset can use used directly in Velocity factor solver .
For example if we measured the first quarter wave resonance of a section with female SC termination (offset=0ps) of unknown vf line to be 138.2MHz, and knowing the reference plane offset (from above) to be 74.3+0ps, we can calculate VF.
If you ignored the offset, you would get a VF of 0.6343, an error of -4%.
It is possible to find the location of the reference plane using the facilities in Antscope.
It is worth knowing for many applications of the instrument.
The figures here relate to my own AA-600, yours may be different.