On Tue, Nov 23, 2004 at 04:51:58PM -0800, Samuel A. Falvo II wrote: > > It's a handy formula to have -- however, I'm curious though: where does > the factor of 0.35 come from? >
from a 1st order system. BW (Hz) = 1/(2*pi*tau), step response = 1 - exp(-t/tau). Time to 10% is t10 = -tau*log(0.9), time to 90% is t90 = -tau*log(0.1), 10-90% risetime is tr = t90 - t10 = tau*(log(0.9) - log(0.1)) = tau*log(9) = 1/(2*pi*BW) * log(9) finally, BW * tr = log(9) / (2*pi) = 0.3497 = 0.35 And for other order systems, its an approximation, but a useful one at that. -Dan --
