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When is Controlled Impedance Important?

The importance of controlled impedance hinges upon many variables, such as the PCB’s characteristics and how it is to be used. A PCB designed for digital applications will often have different impedance requirements than a circuit designed for RF applications. Within both of these categories, however, there are sub-categories of specific types of applications.

Digital applications, especially high-speed digital applications, will require consistent and controlled impedance values for signal integrity purposes. There are many different methods for checking a PCB for good signal integrity: eye diagrams, pulse distortion, bit error rate, and skew. A critical trace on a digital board with impedance variation can impact these quality measurements of the PCB.

Typically, when energy of a varying signal goes from one impedance environment to another, there will be some amount of reflection in the transition. A digital pulse going from one area of a PCB with 40 ohms impedance to another area with 50 ohms will have some reflected energy at that transition. Along with that, the pulse amplitude and shape may vary as well, depending on the significance of the reflection. The pulse distortion will impact the previously mentioned signal integrity concerns for a high-speed digital system.

Another issue with an unmatched impedance area of a PCB for digital systems is electromagnetic interference (EMI). The reflection associated with the impedance mismatch will cause electromagnetic radiation in the localized area of the transition. That radiation can couple its energy to neighboring traces or components on the board, causing electrical performance distortion of those items.

Impedance matching for RF applications sometimes share similar issues associated with high-speed digital applications. Many times in RF applications, there is a need for efficient energy transfer from one module to another. A simple example would be to consider the energy generated within the transmitter portion of a radio and how to transmit that energy to the antenna efficiently. If the feed line from the transmitter to the antenna is not well matched for impedance, some energy will be lost before it can be transmitted. The transmitting portion of the radio will not function as well as it should, possibly shrinking the distance at which the signal can be received decreasing the clarity of the signal received.

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Editor's Note: This column originally appeared in the June 2014 issue of The PCB Design Magazine.