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56 The PCB Design Magazine • June 2014 The importance of controlled impedance hinges upon many variables, such as the PCB's characteristics and how it is to be used. A PCB de- signed for digital applications will often have dif- ferent impedance requirements than a circuit de- signed 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 dia - grams, 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 an- other, 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 an- other area with 50 ohms will have some reflected energy at that transition. Along with that, the pulse amplitude and shape may vary as well, de- pending on the significance of the reflection. The pulse distortion will impact the previously men- tioned 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 electromag- netic radiation in the localized area of the transi- tion. That radiation can couple its energy to neigh- boring 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 gener- ated within the transmitter portion of a radio and how to transmit that energy to the antenna ef- ficiently. 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 transmit- ted. 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. There is a large study within the RF/micro- wave and millimeter-wave design community de- voted solely to impedance matching. Besides the simple radio transmitting example given, there are other issues with power amplifiers, radar, low noise amplifiers (LNA), and many more. Within each of these applications there may be multiple areas where impedance matching is critical. In the case of power amplifier circuits, the PCB typically has many different areas of functional - by John Coonrod rogerS CorPoraTion, aDvanCeD CirCuiT MaTerialS DiviSion LIgHTNINg SPEED LAMINATES When is Controlled Impedance Important? column

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