In circuits with resistors, inductors, and capacitors, the resistance to alternating current is called impedance. Impedance is commonly represented by Z, which is a complex number. The real part is called resistance, and the imaginary part is called reactance. The impedance caused by capacitors in a circuit is called capacitance, while the impedance caused by inductors in a circuit is called inductance. The impedance caused by capacitors and inductors in a circuit is collectively called reactance. The unit of impedance is ohms.

　　Impedance type

　　(1) Characteristic impedance

　　In electronic information products such as computers and wireless communication, the energy transmitted in the circuit of a PCB is a square wave signal (pulse) composed of voltage and time, and the resistance it encounters is called characteristic impedance.

　　(2) Differential impedance

　　Two identical signal waveforms with opposite input polarity at the driving end are transmitted by two differential lines, and these two differential signals are subtracted at the receiving end. The differential impedance is the impedance Zdiff between two lines.

　　(3) Odd mode impedance

　　The impedance of one line to ground in the two lines is consistent.

　　(4) Even mode impedance

　　The impedance Zcom of two identical signal waveforms with the same input polarity at the driving end when connecting the two wires together.

　　(5) Common mode impedance

　　The impedance Zoe of one line to ground in the two lines is consistent, usually greater than the odd mode impedance.