The resistance of the series circuit near resonance changes sharply according to the frequency of the generator. Accordingly, the current in the circuit also changes sharply. The circuit behaves differently at different values of power supply current frequency. These characteristics can be evaluated by the shape of the resonance curve or the Q factor of the circuit. However, since each transmitter's signal is a spectrum and occupies a certain frequency band, the concept of loop bandwidth is often more convenient for this estimation. In order to transmit and receive signals without distortion, all frequencies contained in the signal must be transmitted to the same extent by the transmitting device, received by the receiving antenna, and then equally amplified by the receiver. A series resonant circuit must pass through a frequency band corresponding to the signal spectrum. Therefore, in wireless engineering equipment, the issue of the frequency band through which the loop passes is very important.

The bandwidth of a circuit is a frequency band within which the current in the circuit decreases by no more than twice the current at resonance. Otherwise, the bandwidth of the loop is the frequency band where the loop current is equal to or greater than 0.7 of the resonant current. The diagram shows the resonance curve of a series resonant circuit. The bandwidth of this circuit is 2DW. It is generally considered that the circuit passes through the frequency range from (W0- Dw) to (W0+Dw), and does not pass through the following frequencies (W0- Dw) and the above (W0+Dw).

The bandwidth is proportional to the resonant frequency, inversely proportional to the Q factor of the circuit, or proportional to the attenuation at a constant resonant frequency.

The bandwidth of a circuit depends on its Q factor: the lower the Q factor, the more "dumb" the resonance curve, and the wider the bandwidth of the circuit.

The dependence of current modulus on frequency is usually expressed as the ratio | I |/| Ipe3 |, which is called the resonance characteristic of a series resonant circuit:

At the boundary of loop bandwidth

This means a simple formula for determining the actual Q: