Cable movement in measurement journal

[Claes Wingqvist]

Hi,

When you enter a cable movement in the measurement journal, an uncertainty is added that is specified in the database. In our case, we have measured the electrical specifications of the cable movement according to the VNA guide.
If you repeat several measurements, for example on a thru measurement and one cable is fixed and the other movable. The moving cable will always have the same position when re-connected during the repeated measurements.
What does VNA Tools expect, how to enter cable movement in the measurement journal to get a good estimate of the uncertainty due to the cable movement.

Regards

Claes

RISE, Research Institutes of Sweden

[Juerg Ruefenacht]

Dear Claes,

cable influences are not an easy topic and how to minimise them are discussed in detail during our VNA Tools training course.

How to characterise and define a test port cable is described here: https://groups.google.com/g/vnatools/c/bMacNaEPJfY/m/gtMEKotcAgAJ

The predefined cable specification stored in the database of VNA Tools are based on the one-way manufacturer specifications for a 90 deg bend effect. This is typically a rather conservative assumption and may be much smaller if the cables are moved less and if they are not twisted during the mating process by using a clamp system.
To keep one side fixed with a clamp and moving the cable only on the other port side is always a good recommendation to minimize cable effects with two port measurements.

Each cable movement entry in Measurement Journal has the following information:
Time stamp, Cable definition, Port number, Position number, Movement number

Position: is incremented from 0 by default (could be influenced by the user). Equal positions get the same uncertainty ID in VNA Tools and are therefore treated as correlated. The original idea behind this: if the cable is moved again to the same physical position (same index number), it has exactly the same behavior as before.
However, as we have learned, this is not the case. The problem is that any movement of the cable to make a new connection can have effects on the cable that vary from movement to movement, even if the cable returns to the same physical position.
Therefore, each cable movement requires an increase in the Position number. Each time the Position number is changed, the cable uncertainties specified in the database are added to the measurement data.

Movement: is incremented from 1 (cannot be influenced by the user).

Hope this helps,
Juerg