Why controlled impedance is important.
Every day, PCB designs and components become smaller, faster – in other words, more complicated. It is now crucial to slow certain circuits down in order to allow specific functions of components to perform before others. The time of simple interconnecting traces and conductors is over.
Typically, you will need controlled impedance for PCBs used in high-speed digital applications, such as RF communication, telecommunications, computing using signal frequencies above 100 MHz, high-speed signal processing, high-quality analog video, etc.
At high-frequency signals, the signal traces on a PCB act like transmission lines, which have an impedance at each point on the signal trace trajectory. If this impedance varies from one point to the next one, there will be a signal reflection whose magnitude will depend on the difference between the two impedances. The larger the difference is, the greater the reflection will be. This reflection will travel in the opposite direction of the signal, which means that the reflected signal will superimpose on the main signal.
As a result, the original signal will be distorted: the signal intended to be sent from the transmitter side would have changed once it gets to the receiver side. The distortion may be so much that the signal may not be able to perform the desired function. Therefore, to have an undistorted signal travel, it is essential that the PCB signal traces have a uniform controlled impedance to minimize signal distortions caused by reflections. This is the first step to improve the integrity of the signals on the PCB traces.