I have designed many mixed-signal PCB's where the highest-frequency component is the microcontroller's crystal oscillator itself. I understand the standard best practices: short traces, ground planes, decoupling caps, guard rings, shielding traces, etc.
I've also put together a few RF circuits, at 2.4GHz and ~6.5GHz ultra-wide band. I have a working understanding of characteristic impedance, ground stitching, balanced vs unbalanced RF feed lines, and impedance matching. I've always contracted an RF engineer to analyze and fine-tune these designs.
What I don't understand is where one realm starts to cross over into the next. My current project has a 20MHz SPI bus shared between four devices, which has let me to this question. But, I'm really looking for general guidelines.
Are there guidelines as far as trace length vs frequency? I assume that ~3 inch traces are fine with 20MHz (15 meters), but what is the general case?
As frequencies increase, how to prevent long traces from radiating? Are striplines and coax the way to go?
What is the RF characteristic impedance of a typical microcontroller output stage, anyway?
Please feel free to tell me anything I'm missing :)