# Resonant Frequency of a Coil (or LC tank)

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#### nix85

##### New Member
As you know there are many ways to measure resonant frequency of a coil, most often directly or by loose coupling feeding it sweep of frequencies until you see peak voltage across it on the scope. Like so..

And..

You might also measure it's inductance and capacitance by LC meter and calculate it.

Then there is the video below. Notice first, he only connected one side so circuit is open.
He is measuring voltage across the resistor in series with the coil and he finds the resonant frequency when voltage across the resistor is smallest.

Now, what exactly happens here. We know LC tank at resonant frequency is short circuit in series and open circuit in parallel (band pass filters).

But here circuit is open, only one terminal is connected, thus no current flows except tiny bit due to capacitance to ground through the air.

I assume the answer is because the coil has stray capacitance in parallel to ground and since LC tank in parallel has near infinite impedance, almost all voltage drops across it and minimum across the resistor.

Correct me if i'm wrong.

Band-stop filter Last edited:
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