Generating a stable, high-frequency signal from a crystal is simple, but you can’t always get just the frequency you want. Of course, you can always order custom-made crystals, but not everyone can afford this and it’s not necessarily terribly practical. A more flexible solution is direct digital synthesis (DDS). Ready-made DDS devices do exist, but are complicated to use. Here’s a compromise between the flexibility of a DDS device and the inflexibility of a crystal: programmable logic.