Rules of thumb for PCB design

Bandwidth and Risetime

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“When does digital become analog?” Is it for a 1 MHz clock, 10 MHz clock, or 100 MHz clock? In reality, clock speed is less important than rise time! Modern logic families have risetimes on the order of 1 ns. This yields a signal bandwidth of

F3db = 0.35 / 1ns = 350 MHz

Assume that a 500 MHz oscilloscope and a 500 MHz probe are available for making this measurement. The risetime observed on the display will be closer to 1.55 ns, a ~50% error. For this reason, it is very useful to compute the best-case risetime for the scope/probe combination you are using and keep this in mind as you are making measurements. If the risetime of the signal you are viewing approaches the best-case value, your scope is limiting the measurement and there may be features in the signal that are hidden from you.