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Rules of Thumb

  • Bandwidth of a signal: BW = 350/tr , where BW is the bandwidth in MHz and tr is the risetime in nanoseconds
  • 10-90% risetime = 1.33 times the 20-80% risetime
  • Wavelength at 100 MHz = 120 inches; at 1 GHz = 12 inches; at 5 GHz = 2.4 inches
  • Speed of high-speed signals » 1 foot/nsec in air and 6 inches/nsec in FR-4
  • Therefore, a 1nsec risetime is 6 inches long in a PCB and 12 inches long in air