In high-speed design, the characteristic impedance of controllable impedance plate and line is one of the most important and common problems. First of all, understand the definition of transmission line: transmission line consists of two conductors with a certain length, one conductor is used to send signals and the other is used to receive signals (remember that "loop" replaces the concept of "ground"). In a multilayer board, each line is an integral part of a transmission line, and an adjacent reference plane can be used as a second line or a second loop. The key for a line to become a "good performance" transmission line is to keep its characteristic impedance constant throughout the line. [ 1]
The key for the circuit board to become a "controllable impedance board" is to make the characteristic impedance of all lines reach a specified value, usually between 25 ohms and 70 ohms. In multilayer circuit board, the key to good performance of transmission line is to keep its characteristic impedance constant throughout the line.
But what exactly is characteristic impedance? The easiest way to understand the characteristic impedance is to look at what the signal encounters during transmission. When moving along the transmission line with the same cross section, it is similar to the microwave transmission shown in figure 1. Suppose a voltage step wave of 1 volt is added to this transmission line, for example, a battery of 1 volt is connected to the front end of the transmission line (between the transmission line and the loop). Once connected, this voltage wave signal travels along this line at the speed of light, which is usually about 6 inches/nanosecond. Of course, this signal is indeed the voltage difference between the transmission line and the loop, which can be measured from any point of the transmission line and the adjacent points of the loop. Fig. 2 is a schematic diagram of voltage signal transmission.
Zen's method is to "generate a signal" and then propagate along this transmission line at a speed of 6 inches/nanosecond. The first 0.0 1 nanosecond advanced by 0.06 inch. At this time, the transmission line has excess positive charge and the loop has excess negative charge. It is these two charge differences that maintain the voltage difference of 1 volt between two conductors, and these two conductors form a capacitor.
In the next 0.0 1 nanosecond, to adjust the voltage of a 0.06-inch transmission line from 0 to 1 volt, it is necessary to add some positive charges to the transmitting line and some negative charges to the receiving line. For every 0.06 inch of movement, more positive charges must be added to the transmission line and more negative charges must be added to the loop. Every 0.0 1 nanosecond, another section of the transmission line must be charged, and then the signal begins to propagate along this section. The charge comes from the battery at the front end of the transmission line. When it moves along this line, it charges the continuous part of the transmission line, thus forming a voltage difference of 1 volt between the transmission line and the loop. A part of charge (q) is obtained from the battery every 0.0 1 nanosecond, and the constant amount of electricity (q) flowing out of the battery in a constant time interval (t) is a constant current. The negative current flowing into the loop is actually equal to the positive current flowing out, and at the front end of the signal wave, the alternating current passes through the capacitor composed of the upper and lower wires, ending the whole cycle.