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LC resonance "sonde"

Wojtek SP9WPN 2021/11/24 12:09


Just a small concept I've built today and would like to share. I needed to measure a resonance of a LC tank inside a circuit.
It all took one RG-316 pigtail and 5 minutes of work. Results are satisfactory (apply you preferred disclaimers here).

Wojtek SP9WPN

OK1VAW 2021/11/25 22:45

Hello Wojtek,
I have one practical remark - as far as the NanoVNA has 50Ohm low impedance  output, measurement of parallel LC circuit should be not done in series, but the best way is to couple to the parallel LC circuit inductively with a small loop or similar, such as when you measure with the GDO.  By connecting to the parallel LC in this way the LC  circuit is significantly loaded. The graph is unclear, what was measured- was it S11 or S21?
The measurement in this way is more suitable for series LC circuit. This is the theory.
Vojtech, OK1VAW.

Wojtek SP9WPN 2021/11/26 02:50

Hi Voytech,

Yes, I have used GDO-like loop (connected in series as S11) for other measurements, but inside a tight circuit it is physically impossible to get a reading this way. Thus I have crafted this tool. I calibrate these "tweezers" for trough and isolation, and in theory any pure-resistive loading (50R by nanoVNA) should not affect dip frequency.

Tweezers are just connected to both SMA ports, so I measure S21 loss - it is in the top left of the chart.


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