Resonant Frequency

In a series circuit, the resonant frequency condition occurs when the inductive reactance equals the capacitive reactance. Changing the supply frequency alters the values of X_L = 2πfL and X_C = 1/2πfC.As the frequency increases, X_L rises while X_C falls. Conversely, a decrease in frequency causes X_L to drop and X_C to rise.To achieve series resonance, the frequency is tuned to f_r (point P in the curve below), where X_L = X_C.