Measuring the resonant frequency of an RLC circuit

I have a simple RLC circuit (100 ohm resistor, 10mH inductor and 1uf capacitor in series) and I need to measure the circuit's resonant/natural frequency and damping coefficient. The input to the circuit is a 2% duty cycle, 1khz 1V square wave (like the unit impulse).

My understanding is that when the circuit resonates, the combined impedance of the inductor and capacitor is zero, so the circuit's impedance is at a minimum. If I measure the voltage across the resistor, this should be at a maximum (since the current is at a maximum).

I calculate the resonant frequency for this circuit to be ~1.6KHz however I don't measure any maximum or minimum voltage around this point. The voltage just keeps decreasing as the frequency increases.

This seems to have something to do with using an impulse input because if I use a sine wave instead it is easy to see this effect. Why does it not resonate in this case?

I also calculated the damping coefficient to be 0.5. This should mean the system is underdamped and should oscillate. I haven't been able to work out how to measure this yet. Any suggestions?

• Did you look at the resulting waveform on a scope? Commented Aug 30, 2014 at 8:27
• I don't see your problem, the results are what I would expect. All the resonant-gain-bodeplot thing works for pure tone inputs, aka sinusoids. If you sen in a SUM of sinusoids, e.g. a square wave, the output will be a sum of sinusoids, weighted by the RLC series. And that's not something naked eye can easily see. Try to make the FFT of the output, then sweep your square wave from 160Hz to 16kHz, looking at the first spike (fundamental harmonic). Do you see it now? Commented Aug 30, 2014 at 8:34
• @jippie yes the signal on the scope is right (the impulse response of the system). Commented Aug 30, 2014 at 9:36
• I think you improve your question with a circuit diagram, including AC source and your meter/scope. Commented Aug 30, 2014 at 9:51
• @Vladimir can you explain why it doesn't oscillate /resonate with the square wave input? The fourier transform spike would make sense because the unit impulse includes all frequencies Commented Aug 30, 2014 at 10:49