High Speed Digital Design

What is the difference between high speed digital circuits and the others? The high speed ones aren't digital!

The higher the clock rates and the shorter the rise and fall times in digital circuits become the more these circuits escape the rules of the digital domain and enter the analog domain. Not just the analog world, but that one of microwave technology. Here, there are no wires any more, but transmission lines; no shields, but damping factors and antennas and of course, a completely different time scale. In other words: Physics at it's best.

This article wants to introduce the newcomer to the very basics of high speed designs (not necessarily only digital ones, but mostly them). It is definitely not intended to substitute any good textbook or training.

When designing with clock rates above app. 100MHz or with slew rates less than 5ns there are some pitfalls to avoid. Because at these high frequencies a normal wire or trace on a board behaves differently as usual. First, high speed devices need a closer specified timing. The relevant data can be found in the data books and is not subject of this article. Neither is the calculation of the loads of a driver and the associated slowdown of the signal it sources. These are all easy to understand and should pose no problem to an experienced designer. But there are some other aspects one can easily overlook: