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2.17 The Bottom Line

  1. The time domain is the real world and is typically where high-speed digital performance is measured.
  2. The frequency domain is a mathematical construct where very specific, specialized rules apply.
  3. The only reason to ever leave the time domain and use the frequency domain is to get to an answer faster.
  4. The rise time of a digital signal is commonly measured from 10% of the final value to 90% of the final value.
  5. Sine waves are the only waveform that can exist in the frequency domain.
  6. The Fourier Transform converts a time-domain waveform into its spectrum of sine-wave-frequency components.
  7. The spectrum of an ideal square wave has amplitudes that drop off at a rate of 1/f.
  8. If the higher-frequency components are removed in the square wave, the rise time will increase.
  9. The bandwidth of a signal is the highest sine-wave-frequency component that is significant, compared to the same harmonics in an ideal square wave with the same repeat frequency.
  10. A good rule of thumb is that the bandwidth of a signal is 0.35/rise time of the signal.
  11. Anything that decreases the bandwidth of a signal will increase its rise time.
  12. The bandwidth of a measurement is the highest sine-wave frequency where the measurement has good accuracy.
  13. The bandwidth of a model is the highest sine-wave frequency where the predictions of the model give good agreement with the actual performance of the interconnect.
  14. The bandwidth of an interconnect is the highest sine-wave frequency where the performance of the interconnect still meets specifications.
  15. The 3-dB bandwidth of an interconnect is the highest sine-wave frequency where the attenuation of a signal is less than -3 dB.
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