What are some 'gotchas' of high-frequency board design?
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Key points:


The main determining factor of you circuit is the rise time of the logic. Even if you operate at a slow clock rate, fast edges can create problems.
The maximum rise time of your system then gives you the critical length of your circuit. Essentially if the propagation delay of your signal over the length of the circuit is longer than the rise time of the signal, you need to worry about the high frequency aspect of the design.
If it turns out the critical length is shorter than the circuit layout, then you need to use controlled impedance layout. This includes:

Track geometry (track width and hight above a ground plane) to give the circuit a defined characteristic impedance.
Terminating the drivers and/or recievers with the line's characteristic impedance.