PCB impedance control refers to controlling the wiring impedance, which is also known as controlled impedance. Controlled impedance is the characteristic impedance of a transmission line formed by the PCB wiring and its related reference planes. When high-frequency signals propagate on PCB transmission lines, they are correlated. Controlled impedance is crucial for solving signal integrity problems, which involve the undistorted propagation of signals.
Why control PCB impedance
Usually, it is necessary to control the PCB impedance for high-speed digital applications. At high frequencies, the signal wiring on the PCB is like a transmission line, with impedance at every point on the signal wiring trajectory. If the impedance changes from one point to the next, there will be a signal reflection, and its amplitude will depend on the difference between the two impedances. The greater the difference, the greater the reflection. This reflection will propagate in the opposite direction of the signal, which means that the reflected signal will be superimposed on the main signal, resulting in distortion of the original signal.
To achieve undistorted signal transmission, PCB signal wiring must have a uniform controlled impedance to minimize signal distortion caused by reflection.
Factors affecting PCB controlled impedance
The factors that affect PCB impedance tolerance include the resin content percentage of the material, the Dk value of the resin, and the type of glass cloth used, as well as other physical PCB tolerances, such as the height and width of the top and bottom wiring.
