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  • Some knowledge of PCB impedance
    Date :2017-2-25


    PCB Impedance Features:
    According to the signal transmission theory, the signal is a function of time, distance variable, so the signal on each part of the connection is likely to change. Therefore, determine the AC impedance of the connection, that is, the voltage change and the current change ratio of the transmission line characteristic impedance (Characteristic Impedance): transmission line characteristic impedance only with the characteristics of the signal connection itself. In the actual circuit, the resistance of the wire itself is less than the distributed impedance of the system. In the high frequency circuit, the characteristic impedance depends mainly on the distributed impedance of the unit distributed capacitance and the unit distributed inductance. The characteristic impedance of the ideal transmission line depends only on the unit of distributed capacitance and the unit distributed inductance.
    PCB characteristic impedance calculation:
    The rising edge of the signal and the ratio of the time it takes for the signal to be transmitted to the receiver determines whether the signal connection is considered as a transmission line. The specific relationship between the following formula can be explained: If the PCB board wire length greater than l / b can be between the signal connection wire as a transmission line. From the signal equivalent impedance calculation formula, the impedance of the transmission line can be expressed by the following formula: in the case of high frequency (tens of megahertz to hundreds of megahertz) to meet wL >> R (of course, the signal frequency is greater than 109Hz range, then Taking into account the skin effect of the signal, the need to carefully study this relationship). Then for a certain transmission line, the characteristic impedance is a constant. The reflection of the signal is caused by the characteristic impedance of the drive and the transmission line of the signal and the impedance of the receiver. For the CMOS circuit, the signal output side of the output impedance is relatively small, for dozens of Europe. While the input impedance of the receiver is relatively large.
    PCB Characteristics Impedance Control:
    The characteristic impedance of the wire on the PCB 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 impedance of the wire and the characteristic impedance required by the device or signal are consistent and matched. Therefore, there are two concepts in the reliability design of the PCB design that must be noted.
    PCB Impedance Control:
    There are various signals in the conductor in the circuit board. When the transmission rate must be increased, the circuit itself must be changed due to the different factors such as etching, stack thickness, wire width and so on, which will cause the impedance to change. Signal distortion. Therefore, in the high-speed circuit board conductor, the impedance value should be controlled within a certain range, known as the "impedance control." The factors that affect the impedance of the PCB trace are the width of the copper wire, the thickness of the copper wire, the dielectric constant of the medium, the thickness of the medium, the thickness of the pad, the path of the ground, and the alignment of the traces. So in the design of the PCB must be on the board to control the impedance of the line in order to avoid the signal as much as possible to reflect and other electromagnetic interference and signal integrity issues to ensure the practical use of PCB board stability. PCB board microstrip line and stripline impedance calculation method can refer to the corresponding empirical formula.
    PCB impedance matching:
    In the circuit board, if the signal is transmitted, it is hoped that it will be transmitted to the receiving end in the case of the minimum energy loss, and the receiving end will completely absorb it without any reflection. To achieve this transmission, the impedance in the line must be equal to the impedance inside the transmitter, known as "impedance matching." In the design of high-speed PCB circuit, the impedance matching is one of the elements of the design. And the impedance value and the alignment way has the absolute relation.
    For example, whether it is in the surface layer (Microstrip) or the inner layer (Stripline / Double Stripline), with the reference power plane or formation distance, alignment width, PCB material, etc. will affect the characteristics of the line impedance. In other words, to determine the impedance after wiring, while the different PCB manufacturers to produce the characteristic impedance is also a slight difference. General simulation software will be limited by the line model or the use of mathematical algorithms and can not take into account some of the impedance of the wiring is not continuous, this time in the schematic can only reserve some termination (Temninators), such as series resistance, etc. The effect of mitigating trace impedance discontinuities. The real way to solve the problem or wiring as much as possible to avoid the occurrence of impedance discontinuity.



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