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What is impedance control in pcb?

High Speed PCB with Impedance Control
Impedance Control PCB refers to the PCB that needs impedance control. Impedance control refers to the “resistance” generated by a certain circuit layer under its high-frequency signal to its reference layer, and its signal must be controlled within the rated range to ensure that the signal is not distorted during transmission. Impedance control is to make each part of the system have the same impedance value, that is, impedance matching.
Most high speed circuit boards currently have impedance control requirements. Therefore, it has also been said that a PCB board that does not have impedance control cannot be called a high speed PCB.

Definition & Types of Characteristic Impedance
At a certain frequency, the resistance of a high frequency signal or electromagnetic wave in the propagation of a signal line of an electronic device relative to a certain reference layer is called a characteristic impedance, which is an electrical impedance, an inductance, resistance, capacitance resistance... a vector sum.
The characteristic impedances commonly found on Impedance Control PCBs are: single-ended impedance, differential impedance, and coplanar impedance.
Single-ended impedance: refers to the impedance measured by a single signal line.
Differential Impedance: Refers to the impedance tested in two equal-width, equally spaced transmission lines for differential driving.
Coplanar impedance: The impedance measured when the signal line is transmitted between GND / VCC (signal line to its GND / VCC spacing is equal).

Factors Affecting Impedance
W --- line width / line width increase impedance becomes smaller, distance increases impedance increases;
H --- insulation thickness thickness increases impedance;
T --- copper thick copper thickness increases impedance;
H1 --- green ink thick thickness increases impedance becomes smaller;
Er --- dielectric constant reference layer DK value increases, impedance decreases;
Undercut --- W1-W undercut increases and the impedance becomes larger.

The impedance linewidth tolerance of our Impedance Control in PCB is normally controlled by ± 10%, and the minimum can be controlled to ±8%.In order to reduce the production cost, if the signal requires impedance deviation of ±10%, then we do not need to require the board factory to produce according to ±8%, resulting in unnecessary cost increase.

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