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Digital Circuit Boards: Mach 1 GHz by Ralph Morrison

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2.20 Decoupling Capacitor as a Stub

Consider a decoupling capacitor placed between a voltage source, an IC, and a driven logic trace. The capacitor can be viewed as a low impedance transmission line stub. When the IC switches, the demand for energy moves on four transmission lines. These are the logic line, the power line, the capacitor as a stub, and the connections to the stub. The waveform at the receiving logic depends on the reflections and transmissions that take place for this configuration. A little thought will show that dozens of reflections are involved before the voltage at the load stabilizes. If the IC is already driving other logic lines, the wave action is even more complicated.

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