PCB Design Introduction

The characteristic impedance of controlled impedance boards and lines is one of the most important and common issues in high-speed design. First, let's understand the definition of a transmission line: a transmission line consists of two conductors of a certain length, one for sending signals and the other for receiving signals (remember that the concept of "return" replaces the concept of "ground"). In a multilayer board, each line is part of a transmission line, and the adjacent reference plane can be used as a second line or return. The key to a line being a "good performance" transmission line is to keep its characteristic impedance constant throughout the line. [1]
The key to a circuit board being a "controlled impedance board" is to make the characteristic impedance of all lines meet a specified value, usually between 25 ohms and 70 ohms. The key to good performance of a transmission line in a multilayer circuit board is to keep its characteristic impedance constant throughout the line.
But what exactly is characteristic impedance? The easiest way to understand characteristic impedance is to look at what a signal encounters during transmission. When moving along a transmission line with a constant cross-section, this is similar to the microwave transmission shown in Figure 1. Suppose a 1-volt voltage step wave is added to this transmission line, such as connecting a 1-volt battery to the front end of the transmission line (it is located between the transmission line and the return line). Once connected, this voltage wave signal propagates along the line at the speed of light, and its speed is usually about 6 inches/nanosecond. Of course, this signal is indeed the voltage difference between the transmission line and the return line, which can be measured from any point on the transmission line and the adjacent point on the return line. Figure 2 is a schematic diagram of the transmission of this voltage signal.
Zen's method is to "generate a signal" first, and then propagate it along this transmission line at a speed of 6 inches/nanosecond. The first 0.01 nanosecond advances 0.06 inches. At this time, the transmission line has excess positive charge, and the return line has excess negative charge. It is these two charge differences that maintain the 1-volt voltage difference between the two conductors, and the two conductors form a capacitor.
In the next 0.01 nanosecond, the voltage of a 0.06-inch transmission line must be adjusted from 0 to 1 volt, which requires adding some positive charge to the transmission line and some negative charge to the receiving line. For every 0.06 inches of movement, more positive charge must be added to the sending line and more negative charge must be added to the loop. Every 0.01 nanoseconds, another section of the transmission line must be charged before the signal begins to propagate along this section. The charge comes from the battery at the front end of the transmission line, and as it moves along this line, it charges the continuous part of the transmission line, thus forming a voltage difference of 1 volt between the sending line and the loop. Every 0.01 nanosecond forward, some charge (±Q) is obtained from the battery, and the constant amount of electricity (±Q) flowing out of the battery within 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 just at the front end of the signal wave, the AC current passes through the capacitor composed of the upper and lower lines, ending the entire cycle.