slew threshold limits - need physical explanation please

[mikkie_r]

Hey all

When simulating electrical circuits, I calculate the transition time with 20%-80% threshold limits. That means I sample the time in which the voltage arrives to 20% of it's highest value and at 80% of highest value.

In that are of 20-80% the transition curve is linear. In the edges of that curve there is non linearity caused by a physical effect.

What is this effect?

Thanks
Mikkie

 

[Fahmy]

I doubt that the rise/fall time is linear in most of the electrical circuits even between the 20% and 80% thresholds. They may look linear but in fact they are not.
If you analyzed the problem of applying a square wave into a simple RC low pass filter you will find the output contains exponential term , this term may looks linear in small region but in fact it is not.