Thursday, 6 March 2014

High Speed PCB Routing

I have been quite busy designing high-speed PCB's for my PCI-Express research. I found this video on YouTube from Texas Instruments which provides an excellent overview on high speed routing and I recommend it to anyone interested.

To reiterate the issues one usually faces when designing high speed circuit boards (from the video above):

  • Timing: the lengths of the tracks must be similar enough for the electrical signals to arrive at the receiver at the same time. ~0.6 times the speed of light is too slow!
  • Signal Integrity: The shape of the signal needs to be right when it arrives at the receiver.
  • Noise: There can be a lot of crosstalk and noise on a high speed PCB and this noise can adversely affect signals.
To 'solve' these concerns:
  • Maintain the correct impedance from the transmitter to the receiver. This is not always trivial and so this is usually the biggest problem!
  • Matched lengths minimise signal skew.
  • Leave space around the traces to minimise noise. More space makes an exponential difference.

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