source:Industry News release time:2022-07-16 Article author:yu Popular:pcb
On a circuit board, if there is a signal transmission, it is hoped that it can be smoothly transmitted from the power source to the receiving end with minimal energy loss, and that the receiving end absorbs it completely without any reflections. In order to achieve this transmission, the impedance in the line must be equal to the impedance in the transmitter before it is called "impedance matching". Impedance matching is one of the key elements in designing high-speed PCB circuits. There is an absolute relationship between impedance and routing mode.
For example, walking on the surface layer (Microstrip) or inner layer (Stripline/Double Stripline), distance from the reference power layer or layer, line width, PCB material, etc. will affect the characteristic impedance value of the line. That is, the impedance value can only be determined after wiring. At the same time, the characteristic impedance produced by different PCB circuit board manufacturers is also slightly different. Conventional simulation software cannot account for some discontinuous impedance routing due to limitations of the circuit model or the mathematical algorithms used. At this point, only a few resistors, such as series resistors, can be left on the schematic to mitigate the effects of discontinuous impedance. The real solution to this problem is to avoid impedance discontinuities when routing.
The proportional relationship between the rise time of a signal and the time it takes to send the signal to the receiving end determines whether a signal connection is considered a transmission line. The specific proportional relationship can be explained by the following formula: If the length of the wires on the PCB impedance board is greater than l/b, the wires between the signals can be regarded as transmission lines. From the formula for calculating the equivalent impedance of the signal, the impedance of the transmission line can be expressed by the following formula: wL>R is satisfied at high frequencies (tens to hundreds of megahertz). (Of course, in the signal frequency range greater than 109 Hz, taking into account the skin effect of the signal, this relationship needs to be carefully studied. For a given transmission line, the characteristic impedance is a constant. The signal reflection phenomenon is caused by the driving end and the signal transmission line It is caused by the inconsistency between the characteristic impedance of the signal and the impedance of the receiving end. For CMOS circuits, the output impedance of the signal driving end is relatively small, which is tens of euros. The input impedance of the receiver is relatively large.
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