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I'm very new to LC circuits and signal generators but I would like to learn more about them. I have a capacitor which I do not know the exact value. But it is in the range of pF. Therefore I decided to build an LC tank circuit and use resonance to find my capacitance value. Before I put an unknown capacitance into the circuit, I bought some known capacitors and inductors to test my circuit. I used 470pF film capacitor and 680µH inductor and 10kohm resistance for providing damping and a diode for the test circuit. According to the formula, I should see the resonance around 281 kHz. You can see my circuit in the attachment. I feed the circuit with square wave and I used Agilent 33220 function/arbitrary waveform generator (up to 20MHz).

LC circuit

I saw something that I believe was resonance behavior but the oscilloscope reading was unstable. I checked the cables, signal generator, and oscilloscope (I connected directly signal generator to oscilloscope and applied sin and square wave with a certain frequency and saw the value that I applied on the oscilloscope screen). They are working.

What could be the reason for unstable reading? What do you think?

P.S. I read couple of things in the forum and saw suggestions such as using 0.01 µF capacitor. I have already ordered it but I think the problem is not the capacitor.

Link for the unstable reading: https://www.dropbox.com/scl/fi/35f8w2o0eyvagf29a1cn4/20230809_164401.mp4?rlkey=7ajpvt5ub804clisixep9kdbw&dl=0

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  • \$\begingroup\$ You need to adjust your trigger. Perhaps do a one-shot. \$\endgroup\$
    – winny
    Aug 10 at 8:58

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Hello Edgy Current (love your name!)

Thanks for the video, it shows a lot of possible issue!

The first I see is probably a triggering issue. Your oscilloscope trigger doesn't seem to be properly set and what you are seeing might be an harmonic of your fundamental signal. I think your signal might be at a much lower frequency that you are expecting and those older Rigol scope don't have the best reputation in that matter.

The second thing I'm seeing is that you are trying to measure with very long ad loose leads. Which will give you a lot of extra capacitance and inductance. With the order of magnitude of your circuit, you might even have some resonance between your cable!

Third, you are using a breadboard. These are notorious to add passive capacitance and provide very little high frequency isolation.

Finally, what you are measuring with that function generator isn't comparable. The function generator has a 50 ohm output impedance (yes I know it can be set to Hi-Z, but it's still a 50 ohm source). Your LC tank has a much weaker impedance. The impedance of the scope will be much more significant.

So, what should you do,

First, I would sync the FGEN to the scope using something else. I would probably use the sync port of the FGen and connect it to a second port of your scope and trigger of that (or split the input signal, in that case, it shouldn't make any difference). Then, I would use an oscilloscope probe, in a 10:1 ratio. If that doesn't work, I'd remove the breadboard. Don't forget, you can use a spice software to simulate your setup. If the Spice doesn't match up, try to find the parasitic capacitance and inductance.

Good luck!

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As pointed out by @julien, it would be a "triggering" issue.

enter image description here

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