Broam, the BNC connectors are part of the fixture and I can not eliminate
them. Yesterday, I even questioned my own measurement of 2.7 pF. I need
to revisit that today as it was a hurried measurement. I will get back
with you after I'm satisfied I made a rigorous measurement.
Dave - W?LEV
On Tue, May 20, 2025 at 12:30?AM Brian Beezley via groups.io <k6sti=
[email protected]> wrote:
Dave, out of curiosity I created a circuit model for a common-mode choke.
It has an inductance, capacitance, and resistance in parallel. I chose the
values to mimic attenuation results for a common-mode choke someone sent me
a couple years ago. It's shown below. The choke was measured with a DG8SAQ
VNA with all four S-parameters. The software that accompanies the VNA can
implement what they call scripts. He ran the Y21 script and sent the
results. One of the neat features of the Y21 method is that it can
calculate the shunt capacitance at each port as well as the DUT response
with the capacitance nulled mathematically. Calculated shunt C was 2.8 to
7.3 pF depending on port and frequency. (I don't know why it varies with
frequency.) I used his choke to create my choke model. When I added a shunt
2.7 pF to each side of the choke in the model, I saw no difference in choke
attenuation to 30 MHz. I thought at least it would be noticeable, but the
curves overlap. I tried 27 pF to make sure the model was working, and I
could then see a small difference. 2.7 pF is about 2k ohms at 30 MHz. This
is comparable to the usual several-k ohm CMC impedance so I expected some
effect. You can notice the Y21/S21 difference below for a shunt capacitance
roughly double what you measured. The effect isn't great and is of no
consequence for choke attenuation, but it would be measurable.
One thing I often forget is that measured capacitance for a test fixture
like yours includes the connector capacitance. It is not stray capacitance.
It matches the connector inductance to form a 50 ohm impedance. You should
subtract the capacitance of a bare connector from the 2.7 pF you measured
to get the stray fixture capacitance. There is also capacitive coupling
between the choke and the VNA enclosure if it is nearby. That will
contribute to shunt capacitance.
Based on my circuit model. I think the capacitance of your fixture should
have a negligible effect at HF. It would be a good idea to verify that
expectation by measurement.
Brian
--
*Dave - W?LEV*
--
Dave - W?LEV
