Brian: All depends on your accuracy requirement , how you build your calibration standards … for special case of serial resonance you can use 2 ports but you can only measure this frequency. The self resonance frequency is not very useful on RF applications, this is a problem. Is good to know it to have it far from working frequencies.For this reason my approach are more focused to measure other parameters more useful in my point of view. We measure a parameter for a reason, my reason is to design an inductor ro my application or validate a calculus. In general this is one more of parameters under analysis to measure. I posted an example in other subject I will repeat it :

Finally I got some time to perform measurements. I used an E5061A VNA (a nanovna can be used as direct replacement, this is nothing special). I started calibrating my port 1 on SMA male. I build a simple coil over a plastic form and I used a 160pF mica plate capacitor in series. I got a series resonance frequency of fs= 2.718 MHz and a series resonance due pasitic capacitance (Cp) of fp=15.757 Mhz. I used segmented sweep to have more frequency resolution at fs.

If I short the coil and I measure the capacitance this is @fs C= 162.14 pF performing some calculations* Cp = 4.97 pF , then we can compute L = 20.518 microH at fs the series resistance was Rs= 2.314 ohm and this is Q= 151. There are more information on attached pictures.
If we not consider the Cp(=0) … the results are : L= 21.147 microH Q= 156.

*The series capacitance is close to the ratio of fp and fs squared minus one times Cp : C= ((fp/fs)^2 -1) Cp

In my opinion this is a complete set of measurements for example to design an inductor of a resonant LC for a trap or part or any major design. Is interesting the idea to shunt the inductor to measure the capacitor C and then compute the rest of values. Measure only the self resonance would be interesting for a quartz resonator but not for an inductor where you need to know his self inductance value and the Q factor … again : with error on results would be acceptable in your project ? it defines how to measure. A Q factor with an error of 10% is very good for the most applications, this is a reference value , in actual designs it don´t defines the bandwidth of an amplifier for example , modern filters are used now.


Ing. Patricio A. Greco
Laboratorio de Calibración ISO 17025 AREA: RF/MW
Gral. Martín Rodríguez 2159
San Miguel (1663)
Buenos Aires
T: +5411-4455-2557
F: +5411-4032-0072
www.servicios-electronicos.com.ar

On 13 Dec 2024, at 11:44, Brian Beezley via groups.io <k6sti@...> wrote:

Patricio, thanks for correcting me about calibration nullifying port capacitance. However, I don't think it can nullify stray capacitance to ground due to a fixture or large inductor size. When using a single VNA port, this capacitance appears in parallel with the inductor and alters its SRF. But if the inductor is placed between ports, the capacitance appears from the ports to ground. This does not affect SRF if measured as the frequency of maximum transmission loss. To verify this, I modeled the circuit shown below with stray shunt capacitance and stray lead inductance. C2, C3, L2, and L3 had no effect on the transmission null frequency.

I didn't make the measurements myself, but SRF values near 1 MHz for 2.2 mH inductors differed for single-port and two-port VNA measurements.

Though not needed to measure SRF, this writeup describes a method to avoid the effects of stray capacitance when measuring inductor resistance and reactance:

https://k6jca.blogspot.com/2020/07/the-y21-method-of-measuring-common-mode.html

I've implemented the Y21 method here:

http://ham-radio.com/k6sti/stray.zip

Brian





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