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Understand the definition and four elements of PCB impedance board

September 7, 2022

    The definition of a PCB impedance board is that a good laminated structure can control the characteristic impedance of the printed circuit board, and its traces can form an easily controllable and predictable transmission line structure called an impedance board. Now that we know what a PCB impedance board is, do you know some elements about a PCB impedance board?


1. Characteristic impedance control of printed circuit boards
      The characteristic impedance of the wires on the printed circuit board is an important indicator of circuit design, especially in the PCB circuit board design of high-frequency circuits, it must be considered whether the characteristic impedance of the wires and the characteristic impedance required by the device or signal are consistent. match.


2. Impedance characteristics of printed circuit boards
     According to the theory of signal transmission, the signal is a function of time and distance variables, so each part of the signal may change on the connection. Therefore, the AC impedance of the connection is determined, that is, the ratio of the voltage change and the current change is the characteristic impedance of the transmission line: the characteristic impedance of the transmission line is only related to the characteristics of the signal connection itself. In the actual circuit, the resistance value of the wire itself is smaller than the distributed impedance of the system, especially in high-frequency circuits, the impedance of the characteristic PCB impedance board mainly depends on the distributed impedance caused by the unit distributed capacitance and unit distributed inductance of the connection. The characteristic impedance of an ideal transmission line depends only on the unit distributed capacitance and unit distributed inductance of the connection.

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3.printed circuit board impedance control
   There will be various signal transmissions in the conductors in the circuit board. In order to increase the transmission rate, the frequency must be increased. If the circuit itself is different due to factors such as etching, stack thickness, wire width, etc., it will cause changes in the impedance value. Its signal is distorted. Therefore, the impedance value of the conductor on the high-speed circuit board should be controlled within a certain range, which is called "impedance control". The main factors that affect the impedance of the PCB circuit board traces 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 wire, and the trace around the trace. Therefore, when designing the PCB, it is necessary to control the impedance of the traces on the circuit board, so as to avoid signal reflection and other electromagnetic interference and signal integrity problems as much as possible, and ensure the stability of the actual use of the PCB board. The calculation method of microstrip line and strip line impedance on the PCB board can refer to the corresponding empirical formula.


4.printed circuit board impedance matching
    In the PCB circuit board, if there is a signal transmission, it is hoped that it can be smoothly transmitted to the receiving end with the minimum energy loss from the sending end of the power supply, and the receiving end will completely absorb it without any reflection. To achieve this kind of transmission, the impedance in the line must be equal to the impedance inside the sending end, which is called "impedance matching". When designing high-speed PCB circuits, impedance matching is one of the elements of the design. The impedance value has an absolute relationship with the routing method. For example, whether to walk on the surface layer or the inner layer, the distance from the reference power supply layer or ground layer, the width of the trace, the material of the PCB, etc. will all affect the characteristic impedance value of the trace. That is to say, the impedance value can only be determined after wiring, and there are also slight differences in the characteristic impedance produced by different PCB manufacturers. General simulation software cannot take into account some discontinuous impedance wiring conditions due to the limitations of the circuit model or the mathematical algorithm used. At this time, only some terminations, such as series resistors, can be reserved on the schematic diagram to ease the wiring. The effect of impedance discontinuities. The real solution to the problem is to try to avoid impedance discontinuities when wiring.