# Finding resonance frequency

## Main Question or Discussion Point

how does one perform a frequency sweep to obtain resonant frequency?

for instance, if the inductance is 50mH and the capacitance is 20nF, how do I achieve a resonant frequency of 200khz? what parameters are involved?

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Redbelly98
Staff Emeritus
Homework Helper
You may need to describe your application a little better. Do you have access to a function generator? Run it at 200 kHz and connect it to your circuit. Look at the voltage across either the cap or inductor on an oscilloscope. Adjust the generator frequency for maximum voltage amplitude, and then you are at resonance.

edit added: I'm assuming the cap and inductor are connected in series. If your setup is otherwise, let us know. Again, a more detailed description generally helps to get a better answer.

Andy Resnick
Mechanically, a resonant frequency is found by tapping with a hammer- a standard hammer, if you can believe it. Accelerometers record the response and the data is processed to obtain the resonant behavior.

You could probably do the same thing with an electrical circuit- excite with a spark, record the ring-down.

Redbelly98
Staff Emeritus
Homework Helper
Good point, Andy. Or perhaps an impulse or step from a function generator would also work.
If one doesn't have a function generator, just flipping on a switch from a DC source might even suffice.

Theoretically resonant frequency is obtained by using the following formula:
f=1 / sqr rt(L* C).
// assuming the value of r is very very small.
When the values are substituted, a frequency of nearly 32 KHz is obtained.
Varying the values of C will yield a different bandwidth. To obtain a frequency of 20 KHz; the frequency desired, a different capacitance value could be used. Using 50 millihenrys and 20 nanofarads, I calculate w = 31,623 radians per sec, and f = 5033 Hz. The sqrt(L/C) impedance is 1581 ohms. Put the L and C in parallel, put a 100k resistor in series, and take a signal generator (sine wave ppreferred) and put it across the LC circuit plus resistor, put a high impedance voltmeter across the LC circuit, and run the frequency from manually from 100 Hz to 100 kilohertz. The volts across the LC circuit will rise when you near resonance. See attached pdf for frequency sweep

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