layout - What are some 'gotchas' of high-frequency board design?

I'd like to design a PCB for an analog loop controller.. something with an A/D, D/A, and processor on board. (Either DSP or FPGA, I haven't decided.) Since this should modulate analog signals at 10 kHz, it needs to be quite a fast processor.

From what I understand, designing a board for processors running above 150 MHz or so can be very challenging due to RF problems. What are some advice you can provide in designing such a board? What problems due to layout can occur? Are there any good online resources that have knowledge bases for this?

Thanks.

Answer

If you're interested in high speed digital, get a copy of High Speed Digital Design.

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.