According to the theory of signal transmission, the signal is a function of time and distance variables, so each part of the signal may change on the line. Therefore, the AC impedance of the connection, that is, the ratio of the change in voltage to the change in current is determined as the characteristic impedance of the transmission line: the characteristic impedance of the transmission line is only related to the characteristics of the signal connection itself. In the actual circuit, the resistance value of the wire itself is smaller than the distributed impedance of the system. Especially in the high-frequency circuit, the characteristic impedance of the PCB mainly depends on the distribution capacitance of the distribution and the distributed PCB impedance
caused by the unit distributed inductance. The characteristics of an ideal transmission line PCB impedance depends only on the unit distribution capacitance of the connection and the unit distribution inductance.
The proportional relationship between the rising edge time of the signal and the time required for the signal to be transmitted to the receiving end determines whether the signal connection is considered to be the transmission line. The specific proportional relationship can be explained by the following formula: If the length of the wire connection on the PCB is greater than l/b, the connecting wire between the signals can be regarded as a transmission line. According to the calculation formula of the equivalent impedance of the signal, the impedance of the transmission line can be expressed by the following formula: wL>>R is satisfied in the case of high frequency (tens of megahertz to several hundred megahertz) (of course, in the range where the signal frequency is greater than 109 Hz, Considering the skin effect of the signal, this relationship needs to be carefully studied). Then for a certain transmission line, its characteristic PCB impedance is a constant. The reflection phenomenon of the signal is caused by the inconsistent characteristic impedance of the driving end of the signal and the transmission line and the impedance of the receiving end. For a CMOS circuit, the output impedance of the driver end of the signal is relatively small, tens of ohms. The input impedance of the receiving end is relatively large.
The characteristic impedance of the conductor on the printed circuit board is an important indicator of the circuit design. Especially in the PCB design of the high-frequency circuit, it is necessary to consider whether the characteristic PCB impedance of the conductor and the characteristic impedance required by the device or the signal are consistent or not. Therefore, there are two concepts that must be noted in the reliability design of PCB design.