If the series RLC circuit is driven by a variable frequency at a constant voltage, the magnitude of the current I is proportional to the impedance Z. Therefore, during resonance, the power absorbed by the circuit must be at its maximum value, as P=I 2 R

If the frequency is reduced or increased now until the average power absorbed by the resistance in the series resonant circuit is half of its maximum value at resonance. We will generate two frequency points, called half power points. They have decreased by -3dB compared to the maximum value, with 0dB as the maximum current reference.

These -3dB points give us a current value that is 70.7% of its maximum resonance value, defined as: 0.5 (I 2 R)=(0.707 × 1) 2R, and then the point corresponding to the lower frequency with half the power is called the "lower cutoff frequency", marked as ƒ large and the point corresponding to the highest frequency, while the half power is called the "upper cutoff frequency", marked as ƒ ^ h. The distance between these two points, i.e. (ƒ - ƒ large), is called the bandwidth, （BW）， And it is within the frequency range set at at least half of its maximum power and current as shown in the figure.

The frequency response of the circuit current amplitude is related to the "sharpness" of the resonance in the variable frequency series resonant circuit. The sharpness of the peak value is quantitatively measured and is called the quality factor Q of the circuit. The quality factor relates the maximum or peak energy (reactance) stored in the circuit to the dissipated energy (resistance) during each oscillation cycle, which means it is the ratio of the resonance frequency to the bandwidth, and the higher the circuit Q, the smaller the bandwidth Q, Q=ƒ - R/BW.

Since bandwidth is obtained between two -3dB points, the selectivity of a circuit is a measure of its ability to suppress any frequency on either side of these points. Circuits with higher selectivity will have narrower bandwidths, while circuits with lower selectivity will have wider bandwidths. Due to Q=(X L or X C)/R, the selectivity of a series resonant circuit can be controlled simply by adjusting the resistance value while keeping all other components constant.