# Measuring resonant frequency of LRC circuit

#### dnlbunting

Joined Mar 5, 2011
1
Hi,
I have an LRC circuit and I need to measure the resonant frequency, the usual way to do this would be to hook it up to a signal gen (sine) and vary the frequency until the voltage in the circuit peaks. But I only have a pulse generator available (square wave), does it make any difference? or do I need to adjust the circuit in some way?

#### Vahe

Joined Mar 3, 2011
75
You might try to measure the step response of the LRC circuit. Make the pulse width large so that you can see the response. It is likely to have some kind of sinusoidal, decaying response. If the circuit has high Q (more reactive than resistive), measuring the ringing frequency should be a good approximation of the resonant frequency. If the circuit does not have a high Q, then the measured ringing frequency will be lower than the actual resonant frequency. Assuming this is a series LRC circuit
Q = sqrt(L/C) / R

Cheers,
Vahe

#### russ_hensel

Joined Jan 11, 2009
825
Square wave is fine. Look ( scope ) at a LC ( known C ) circuit driven thru a resistor. You will see ringing at the resonate almost right away. Adjust frequency for a max.