Monday 18 July 2016

transmission line - How is rise time related to bandwidth of the signal?


Say, I want to limit the rise time of my digital signal edges to avoid dealing with transmission line effects.


How do I determine the maximum frequency of harmonics in my signal knowing that my rise time is, say 5ns?


How do I determine corner frequency of my low pass filter knowing that hold time on the receiver chip is, say 10ns?


In wikipedia I've found the formula


$$ BW=\frac{0.34}{t_{rise}} $$



does it apply in this case?




Edit


I failed to make myself clear, so I'll try to explain my line of thought.


Say, I have a 30HMz signal and my trace length is well below 1/10 of the wavelength. So I don't have to deal with transmission line effects in respect to that. But my edges are steep - 5ns. This adds some high frequency components into my signal that potentially will suffer from transmission line effects.


My idea is that I slow down edge transitions up to a point where I don't have to deal with transmission line phenomena. The question is twofold:



  • how do I calculate fastest rise/fall time that with the given trace length would enable me to traeat my circuit as "lumped"?

  • how do I slow down the rise/fall time?



Rise/fall time is time for voltage to change from 10% to 90% of max value. I know how to calculate the approximate speed of signal on FR4 board.




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