Maximo,
A test I use to assess the capacitance added by the test jig is as
follows: Wind a high-Q air-cored coil and connect a small-value parallel
capacitor across it. Measure its resonant frequency using a
"non-contact" method. [I suspend the coil on a nylon line and place
coupling coils either side of it, several inches away. One coupling coil
goes to a signal generator, the other to a 'scope. When you sweep the
generator you quickly see the resonant frequency]. Then re-measure the
resonant frequency of the coil using S21 on the VNA. The shift in
resonant frequency enables you to assess the capacitance added by the
test set up. I typically measure values around 0.2pF.
On a related topic, its worth remembering that the "self-capacitance" of
a coil (or choke) doesn't always behave as you might expect. For
example, increasing the number of turns can sometimes _decrease_ the
self-capacitance. I've also measured examples where a small increase in
the number of turns has made an unexpectedly large _increase_ in the
self-capacitance. The problem is that models picturing capacitance
between adjacent windings are overly simplistic; the winding needs to be
treated as a transmission line in order to get results more
representative of the real world. There's a good paper on the topic here:
http://g3rbj.co.uk/wp-content/uploads/2015/08/Self-Resonance-in-Toroidal-Inductors.pdf
Steve G3TXQ
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