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Characteristic impedance of PCB technology in high speed des

  Time:2017-09-29 17:10
In high-speed design, the problem of characteristic impedance of controlled impedance plates and lines has plagued many Chinese engineers. In this paper, the basic properties, calculation and measurement methods of characteristic impedance are introduced by simple and intuitionistic methods.
In high-speed design, the characteristic impedance of controllable impedance plates and transmission lines is one of the most important and universal problems. First look at the definition of the transmission line, transmission line consisting of two conductors with a certain length, a conductor used to send signals, another for receiving signals (remember the "loop" to replace "concept). In a multilayer plate, each line is a component of the transmission line, and the adjacent reference plane serves as the second circuit or loop. The key of a line to be a "good" transmission line is to keep its characteristic impedance constant throughout the line.
The key of a circuit board to become a "controllable 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 a good linear transmission in a multilayer circuit board is to keep its characteristic impedance constant throughout the circuit.
But what exactly is a characteristic impedance? The easiest way to understand the characteristic impedance is to see what happens to the signal during transmission. When moving along a transmission line with the same cross section, this is similar to microwave transmission shown in figure 1. The voltage to 1 volts this step wave transmission lines, such as the 1 volt battery is connected to the front end of the transmission line (which is located in the transmission line and the loop between), once connected, the voltage wave signal along the line travels at the speed of light, its speed is usually about 6 inches / ns. Of course, this signal is indeed the voltage difference between the transmission line and the loop, which can be measured at any point in the transmission line and at the point of the loop. Fig. 2 is a schematic diagram of the transmission of the voltage signal.
Zen's approach is to "generate signals" and then propagate along the transmission line at speeds of 6 inches / nanoseconds. The first 0.01 ns forward about 0.06 inches, then the transmission line has a positive charge extra, and have a negative charge loop redundant, it is two kinds of charge difference maintained 1 volts between the two conductor, the two conductor is composed of a capacitor.
In the next 0.01 nanoseconds, the voltage of a 0.06 inch transmission line will be adjusted from 0 to 1 volts, which must add some positive charge to the transmission line and add some negative charge to the receiving line. Each 0.06 inch move must add more positive charge to the transmission line and add more negative charge to the circuit. Every 0.01 nanoseconds, the other segment 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 of the transmission line. When moving along the line, it charges the continuous part of the transmission line, thus creating a voltage difference of 1 volts between the transmission line and the loop. Each forward 0.01 ns gets some charge (+ Q) from the cell, and a constant current (+ Q) that flows out of the battery at constant time intervals (+ T) is a constant current. The negative current in the inflow circuit is actually equal to the positive current flowing out, and just at the front of the signal wave, the AC current passes through the capacitor consisting of the upper and lower lines to end the whole cycle.