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Impedance Controlled PCBs

What is impedance?

The impedance of a printed circuit board results from the interaction of reactance and resistance. It measures the resistance to current when an alternating current is applied to it. As an AC characteristic, impedance is frequency dependent and is expressed in ohms (Ω).
Its transmission is ideal whenever a signal passes through a conductor with the same impedance as another given impedance. The signal is affected if the impedances at the two ends are different.
Impedance control is often required in single-ended microstrip, embedded microstrip, ribbon differential pairs, and planar PCB transmission lines.
How is impedance determined?

Impedance usually ranges from 25 to 125 ohms and depends mainly on the following factors.
Width and thickness of the copper wire
Signal path
Core thickness or material thickness on both sides of the line
The dielectric constant of the core or material
Distance from the copper plane
Presence of solder resist
Circuit board designers need to realize that their choice of conductors will directly affect the target impedance value. Many advanced tools are available to help with this calculation, which we will discuss later in the article.
Impedance matching is very important in realizing high-frequency design and is closely related to the quality of the signal. The purpose of impedance matching is mainly that all high-frequency microwave signals on the transmission line can reach the load point, and there will be no signal reflecting back to the source point.
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