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10 The PCB Design Magazine • May 2015 ers/loads, layer transitions, different dielectric materials, stubs, vias, connectors and IC pack- ages. By understanding the causes of these re- flections and eliminating the source of the mis- match, a design can be engineered with reliable performance. Impedance matching slows down the rise and fall times, reduces the ringing (over/under- shoot) of clock drivers and enhances the signal quality of a high-speed design. The ringing is dramatically reduced by adding a series termi- nator as in Figure 1. From this, we can see that the impedance has to be matched, but to what value? Controlled impedance—it's all about trans- mission lines. For perfect transfer of energy, the impedance of the driver must match the trans- mission line. A good transmission line is one that has constant impedance along the entire length of the line, so that there are no mis- matches resulting in reflections. But unfortu- nately, drivers do not have the exact impedance to match the line (typically 10–35 ohms) so ter- minations are used to balance the impedance, match the line and minimize reflections. Reflections occur whenever the imped- ance of the transmission line changes along its length. This can be caused by unmatched driv- by Barry Olney In-CIrCuIT DesIGn PTy lTD BEyOND DESIGN Controlled Impedance Design feature coulmn

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