The definition of PCB impedance board is that a good laminated structure can control the characteristic impedance of the printed circuit board, and its wiring can form an easily controlled and predictable transmission line structure called the impedance board. Now that we know what a PCB impedance board is, do you know some of the elements of a PCB impedance board?
First, the printed circuit board characteristic impedance control
The characteristic impedance of the wire on the printed circuit board is an important indicator of circuit design, especially in the design of the PCB circuit board 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.
Second, printed circuit board impedance characteristics
According to the transmission theory of the signal, the signal is a function of time and distance variables, so the signal is likely to change in each part of the line. Therefore, the AC impedance of the line is determined, that is, the ratio of the change in voltage and the change in current 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 less than the distributed impedance of the system, especially in the high-frequency circuit, the impedance of the characteristic PCB impedance board mainly depends on the distributed impedance brought by the unit distributed capacitance and unit distributed inductance of the wire. The characteristic impedance of an ideal transmission line depends only on the unit distributed capacitance and unit distributed inductance of the line.
Three, printed circuit board impedance control
The conductor in the circuit board will have a variety of signal transmission, when to improve its transmission rate and must increase its frequency, the line itself if the etching, laminated thickness, wire width and other factors are different, will cause the impedance value changes, so that the signal distortion. Therefore, the conductor on the high-speed circuit board, its impedance value should be controlled within a certain range, called "impedance control". The main factors that affect the impedance of the PCB circuit board 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 routing around the wire. Therefore, when designing the PCB, it is necessary to control the impedance of 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 PCB board can refer to the corresponding empirical formula.
Four, printed circuit board impedance matching
In the PCB, if there is a signal transmission, it is hoped that from the source of the power supply, in the case of minimum energy loss, it can be smoothly transmitted to the receiving end, 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 originating end, which is called "impedance matching". When designing high-speed PCB circuits, impedance matching is one of the design elements. The impedance value has an absolute relationship with the routing mode. For example, whether to walk on the surface layer, or the inner layer, the distance from the reference power layer or formation, the width of the line, and the PCB material will affect the characteristic impedance value of the line. That is to say, the impedance value can be determined after wiring, and the characteristic impedance produced by different PCB manufacturers also has a small difference. General simulation software can not take into account some impedance discontinuity wiring due to the limitations of the line model or the mathematical algorithm used, at this time, only some terminations can be reserved on the schematic diagram, such as series resistance, to mitigate the effect of linear impedance discontinuity. The real fundamental solution to the problem is to try to avoid the occurrence of impedance discontinuity when wiring.
These are the four elements of the impedance board explained by the Mosaic technology for you (printed circuit board impedance control, printed circuit board characteristic impedance control, printed circuit board impedance characteristics, printed circuit board impedance matching, etc.), I hope it will be helpful to you.
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