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.
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