In high -speed design, the characteristic impedance of controllable impedance boards and lines is one of the most important and common problems. First understand the definition of the transmission line: the transmission line consists of two conductors with a certain length, one is used to send a signal, and the other is used to receive signals (remember the concept of "loop" instead of "ground"). In a multi -layer board, each line is part of the transmission line, and the adjacent reference plane can be used as the second line or loop. The key to a "good performance" transmission line is to keep its characteristic impedance constant in the entire line.

　　The key to the "controllable impedance board" is to make the characteristic impedance of all lines satisfy a prescribed value, usually between 25 ohms and 70 ohms. In the multi -layer circuit board, the key to the good transmission line performance is to keep its characteristic impedance constant in the entire line.

　　But what exactly is characteristic impedance? The easiest way to understand the characteristics of the characteristics is to see what the signal encountered in the transmission. When moving along a transmission line with the same cross -section, this is similar to the microwave transmission shown in Figure 1. Assume that the voltage ladder wave of 1 volts is added to this transmission line. It spreads, which is usually about 6 inches/nan seconds. Of course, this signal is indeed a difference between the voltage between the sending line and the circuit, which can be measured from any point of the sending line and the phase of the loop. Figure 2 is a diagram of the transmission of the voltage signal.

　　The method of Zen is to "generate signals" first, and then spread along this transmission line at a speed of 6 inches/nan seconds. The first 0.01 nano -seconds advanced 0.06 inches. At this time, the sending line has excess positive charge, and the circuit has excess negative charge. It is precise These two conductors form another capacitor.