Resonance
An LC circuit resonates when inductive reactance equals capacitive reactance. At this point, the inductor and capacitor exchange energy. The resonant frequency depends on L and C.
fr = 1/(2π√LC)
Series resonance
At series resonance, the net reactance is zero. The circuit impedance is at its minimum and current is at its maximum. The voltage across L or C can be much larger than the supply voltage.
Parallel resonance
At parallel resonance, the ideal circuit presents a high impedance to the source. Source current is low. Real circuits have losses, so the impedance is finite.
Selectivity and bandwidth
A tuned circuit selects a range of frequencies. A narrow bandwidth rejects nearby signals more strongly. The quality factor Q is the ratio of resonant frequency to bandwidth for the usual resonator definition.
Q = fr / BW
Coupled circuits
Coupled circuits transfer energy between circuits. Inductive coupling uses a shared magnetic field. The amount of coupling affects bandwidth and response. Loose coupling often gives a narrower response; tighter coupling can widen it.
Applications
Tuned circuits are used in receiver input stages, local oscillators, IF filters, antenna matching networks and RF amplifiers.