Definition of PCB board impedance board
Time:2023-10-27
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The definition of a PCB impedance board is that a good stacked structure can control the characteristic impedance of a printed circuit board, and its wiring can form a easily controllable and predictable transmission line structure called an impedance board.
1、 Impedance characteristics of printed circuit boards:
According to signal transmission theory, a signal is a function of time and distance variables, so every part of the signal on the line may change. Therefore, the AC impedance of the transmission line, i.e. the ratio of voltage changes to current changes, is determined as the characteristic impedance of the transmission line: the characteristic impedance of the transmission line is only related to the characteristics of the signal line itself. In practical circuits, the resistance value of the wire itself is smaller than the distributed impedance of the system, especially in high-frequency circuits. The characteristic impedance mainly depends on the distributed impedance brought by the unit distributed capacitance and unit distributed inductance of the connecting wire. The characteristic impedance of an ideal transmission line only depends on the unit distributed capacitance and unit distributed inductance of the connecting line.
2、 Calculation of characteristic impedance of printed circuit boards:
The proportional relationship between the rise time of a signal and the time required for signal transmission to the receiving end determines whether the signal connection is considered a transmission line. The specific proportional relationship can be explained by the following formula: if the length of the wire connection on the PCB board is greater than l/b, the connecting wires between signals can be regarded as transmission lines. According to the formula for calculating the equivalent impedance of a signal, the impedance of a transmission line can be represented by the following formula: wL>>R at high frequencies (from tens to hundreds of MHz) (of course, in the range of signal frequencies greater than 109Hz, considering the skin effect of the signal, this relationship needs to be carefully studied). So for a determined transmission line, its characteristic impedance is a constant. The reflection phenomenon of signals is caused by the inconsistency between the characteristic impedance of the driving end and transmission line of the signal, as well as the impedance of the receiving end. For CMOS circuits, the output impedance of the driver end of the signal is relatively small, at tens of ohms. The input impedance of the receiving end is relatively large.
3、 Characteristic impedance control of printed circuit boards:
The characteristic impedance of the wires on a printed circuit board is an important indicator in circuit design, especially in the PCB design of high-frequency circuits. It is necessary to consider whether the characteristic impedance of the wires is consistent with the required characteristic impedance of the device or signal, and whether it matches. Therefore, there are two concepts that must be noted in the reliability design of PCB design.
4、 Printed Circuit Board Impedance Control:
There are various signals transmitted in the conductors of a circuit board. When it is necessary to increase its frequency in order to improve its transmission rate, the impedance value of the circuit itself will change due to factors such as etching, stack thickness, and wire width, resulting in signal distortion. Therefore, the impedance value of conductors on high-speed circuit boards should be controlled within a certain range, which is called "impedance control". The factors that affect the impedance of PCB wiring mainly include the width and thickness of the copper wire, the dielectric constant of the medium, the thickness of the medium, the thickness of the solder pad, the path of the ground wire, and the wiring around the wiring. So when designing a PCB, it is necessary to control the impedance of the wiring on the board in order to avoid signal reflection, other electromagnetic interference, and signal integrity issues as much as possible, and ensure the stability of the actual use of the PCB board. The calculation method for the impedance of microstrip lines and strip lines on PCB boards can refer to the corresponding empirical formulas.
5、 Printed circuit board impedance matching:
In a circuit board, if there is signal transmission, it is hoped that it can be smoothly transmitted from the sending end of the power supply to the receiving end with minimal 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 emitting end in order to be called "impedance matching". Impedance matching is one of the design elements when designing high-speed PCB circuits. The impedance value has an absolute relationship with the wiring method. For example, whether to walk on the surface layer (Microstrip) or the inner layer (Stripline/Double Stripline), the distance from the reference power layer or stratum, the wiring width, PCB material, etc. all affect the characteristic impedance value of the wiring. 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 may not be able to consider some impedance discontinuous wiring situations due to the limitations of the circuit model or the mathematical algorithms used. In this case, only some terminals (such as series resistors) can be reserved on the schematic diagram to alleviate the effect of impedance discontinuity in the wiring. The real fundamental solution to the problem is to pay attention to avoiding impedance discontinuity when wiring.
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