Author Topic: Viewing an LC circuit on a VNA  (Read 2159 times)

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Offline GonzoTheGreatTopic starter

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Viewing an LC circuit on a VNA
« on: January 30, 2019, 09:39:37 pm »
I got to use a 2-port Vector Network Analyzer in my school for the first time.

I can make S11, S22, S12 and S21 magnitude measurements as well as Impedance and Admittance on S11, S22, S12 and S21 at different frequencies.

I calibrated the VNA and connected an air cored coil to Port1 and tried some S11 measurements.
I am puzzled at the myriad of peaks and dips that I am seeing. 

1) Which one of them is the LC resonance ?
2) At what frequency the most MMF is generated inside the coil ( magnetomotive force, ampturns) ?    ( someone told me that if the RF current flows in the inter-turn self-capacitance (or in any other parallel capacitance) then it does not flow in the coil's windings and does NOT contribute to the MMF inside the coil )

I am well aware that only ideal coils are composed solely of inductance and that real world coils also have self-capacitance.  This capacitance forms a small LC circuit and that combined with the resistance of the wire forms an LCR circuit.
The diagram below shows that the reactance of the inductance increases with frequency and the reactance of the capacitance decreases with frequency.

3) Does that mean that as the frequency increases, less and less current flows in the inductance and more and more current flows in a capacitance ?

 

Offline timpattinson

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Re: Viewing an LC circuit on a VNA
« Reply #1 on: January 30, 2019, 10:35:22 pm »
Did you mean to attach a plot from the VNA?

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Online Bud

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Re: Viewing an LC circuit on a VNA
« Reply #2 on: January 30, 2019, 11:22:24 pm »
If you can also attach a photo of your setup showing connections of the circuit to the VNA we could provide more advice.

Edit: .....as well as what VNA you using.
« Last Edit: January 30, 2019, 11:31:18 pm by Bud »
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Offline ezalys

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Re: Viewing an LC circuit on a VNA
« Reply #3 on: January 31, 2019, 05:20:55 pm »
You're seeing the many modes of your circuit. It's true that the inductor will have some parasitic capacitance, and this paralleled with the measured inductance of the coil yields a tank circuit at some frequency, and it is at this frequency that the first resonance may present itself on the VNA as a dip. The issue is that there will be many higher modes of this circuit. The actual circuit model of a passive one-port is an infinite series of parallel LC resonators.
 


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