AC Impedance & Resonance

At resonance the two reactances cancel exactly, which makes a series RLC a short circuit and a parallel one an open circuit. Q behaves oppositely too: series Q rises as resistance falls, parallel Q rises as resistance rises. Getting that backwards is the usual source of a filter that does the opposite of what was intended.

Positive phase means the impedance is inductive and current LAGS voltage; negative means capacitive and current leads. Leave L or C at zero to remove that branch. This is the ideal-component picture: real inductors have winding resistance and self-capacitance, and real capacitors have ESR and ESL, all of which matter near and above self-resonance. For a filter’s frequency response use Filter Response.