70. RLC Circuit and Resonance

A simple, graphic demonstration of a series RLC circuit is to use a small light bulb for the R. Then you can tune through resonance and see the bulb's brightness reach a maximum, or you can set the frequency a little below resonance and insert an iron core into the inductor and see the bulb's brightness go through a maximum.

A parallel RLC circuit is set up so it can be driven with a signal generator and its resonance observed on an oscilloscope. The R (and thus Q), L and C are all variable. Alternatively, the circuit can be pulsed and the exponentially damped oscillations observed on a scope.

(The small input and output capacitors serve to isolate the RLC circuit.)

If the signal generator is replaced with a sweep frequency generator, the oscilloscope can be caused to actually draw the resonance curve.

The sweep generator repeatedly sweeps across a band of frequencies including the resonance. The output of the RLC circuit is then amplitude modulated by the resonance curve. Its average is still zero, but after passing through the diode which acts as a detector and being smoothed by the capacitor the final result is the response curve of the RLC circuit as a function of frequency. You can vary the position of the peak of the curve by changing C or L, or you can vary the width of the curve (Q) by varying R.