RLC Impedance and Resonance Calculator

The two reactances become exactly equal and cancel each other out. In a series circuit that leaves only the resistance, so the impedance drops to its minimum and the current peaks — you can see it above: set the frequency to the resonant one and the impedance equals R exactly and the phase angle goes to zero. In a parallel circuit the opposite happens and the impedance peaks instead. This is how a radio picks one station out of the air: the tuned circuit presents a very different impedance at one frequency than at any other.

How sharp the resonance is. A high Q means the circuit responds strongly over a very narrow band of frequencies and ignores everything else — good for a radio tuner, which needs to separate stations, and bad for an audio filter that should pass a whole range evenly. The bandwidth shown is the width of the band where the circuit still responds reasonably, and it is simply the resonant frequency divided by Q. Adding resistance to a series circuit lowers Q and widens the band; the two always trade off against each other.

Because they work against each other. The inductor and the capacitor push the current in opposite directions in time, so what counts is the difference between them, not the sum. Near resonance you can have hundreds of ohms of inductive reactance and hundreds of ohms of capacitive reactance and an impedance of only a few ohms, because they very nearly cancel. The voltages across the individual components can then be far larger than the supply voltage — which is real, measurable, and a genuine hazard in a high-Q circuit.