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Impedance Characteristics Of PCB

According to the signal transmission theory, the 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, that is, the ratio of voltage change to current change, 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 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 characteristic impedance mainly depends on the distributed impedance caused by the unit distributed capacitance and unit distributed inductance of the wire. The characteristic impedance of an ideal transmission line only depends on the unit distributed capacitance and unit distributed inductance.

Common Impedance Types

Characteristic impedance In computer, wireless communication and other electronic information products, the energy transmitted in PCB circuit is a square wave signal (called pulse) composed of voltage and time, and the resistance it encounters is called characteristic impedance. Differential impedance Two identical signal waveforms with opposite polarity are input at the driving end and transmitted by two differential lines respectively, and the two differential signals are subtracted at the receiving end. The differential impedance is the impedance zdiff between two lines. Odd mode impedance The impedance Zo of one of the two lines to the ground is the same. Even mode impedance The impedance zcom of two same signal waveforms with the same polarity input at the drive end when the two wires are connected together. Common mode impedance The impedance Zoe of one of the two lines to the ground is the same, which is usually larger than the odd mode impedance.

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