VNA open circuit calibration

[Michael Katzmann]

Following the discussion yesterday ...

Professional VNAs use a model of the open circuit that contains a frequency
dependent capacitor.

There are four polynomial terms that are specified for the fringing
capacitor.
For each type of connector ( N, SAM, 3.5mm etc) this is different so you
must specify the
cal kit (or provide the model parameters) before calibration.
Below a few hundred MHz ignoring this will not give much of an error.
Likewise the short circuit is modeled as a frequency dependent inductor.

this from a Keysite note.

Open circuits radiate at high frequencies. This effectively increases the
electrical length of the device and can be modeled as a frequency dependent
capacitor, Copen, (also known as fringing capacitance). At low frequencies,
a fixed capacitance value may be sufficient; this would use only the C0
term. Most network analyzers use a third order polynomial capacitance
model. Radiation loss is assumed to be insignificant.

Copen = (C0) + (C1) f + (C2) f 2 + (C3) f 3

Michael
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