David, Let's neglect with second (large) resistance, so that R=1K and C=1pF The transfer function of your circuit and also the spectrum of the impulse response is: H(p) = 1/(pRC+1), where p = 2*Pi*j*freq The spectrum of the step response is: S(p) = 1/[p*(pRC+1)] You can represent S(p) as a sum: S(p) = 1/p - (RC)/(pRC+1) The second summand is similar to H(p) but has additional scaling (-RC) that is about 1e-9. You should be able to see large difference in magnitude compared to the AC response. If all computations are correct, then you are right, we should see the difference between the AC response and FFT from step response. However, there could be a possibility that FFT method used eliminates the constant from the step response, if it requires zero at infinity: S(t) = 1 - exp(-t/RC) Thus taking the exponential term only, that differs from impulse response by sign and magnitude: H(t) = (1/RC)*exp(-t/RC) Plus, if we compare the AC response to FFT from step response, we may not see the small magnitude in S(p) because FFT did not multiply the result on the time step (should be done because of integrating in time domain). Typically, such normalization should be done outside FFT, but I'm not sure what FFT method FFT was used. Vladimir -----Original Message----- From: David Banas <DBanas@xxxxxxxxxx> Date: Fri, 2 Sep 2011 10:20:00 -0700 Subject: [SI-LIST] Re: Impulse vs. Step Response? Okay, so if we're agreed that, for all intents and purposes, I am driving this circuit with a unit step, as opposed to a unit impulse, why does the spectral content of my transient simulation output exactly match the output of the ac sweep, rather than having an additional 6dB/octave roll-off, as one would expect? Thanks, -db ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu