说明：  对于包含电容和电感及电阻元件的无源一端口网络，其端口可能呈现容性、感性及电阻性，当电路端口的电压U和电流I，出现同相位，电路呈电阻性时。称之为谐振现象，这样的电路，称之为谐振电路。
(For a passive one-port network containing capacitors and inductors and resistors, its ports may be capacitive, inductive and resistive. When the voltage U and current I of the circuit ports are in phase, the circuit is resistive. It is called resonance phenomenon. Such a circuit is called resonance circuit)