As long as the frequency / time domain conversions cover a wide enough frequency range the FD response and the FFT of the unit impulse should match. You are not driving your transient with a step so there should not be a need to consider U(w) = H(w)/jw Steve. On 9/1/2011 5:40 PM, David Banas wrote: > Hi all, > > I've got a puzzler. I'm using this HSpice deck: > > .TITLE AC Sweep vs. Transient Correlation > > .OPTION > + POST > + PROBE > + RUNLVL = 5 > > Rin in out r=1K > Cout out 0 c=1pF > Rout out 0 10MEG > > Vin in 0 DC 1 AC 1 > > .IC V(out) = 0 > > .AC DEC 10 1 1G > .TRAN 0.05n 1u 0 0.05n UIC > > .PROBE AC V(in) V(out) > .PROBE TRAN V(in) V(out) > > .END > > to get both the transfer function, H(w), (.AC command) and the unit step > response, u(t), (.TRAN command) of a simple RC LPF. > > Then, I take the FFT of the .TRAN output and compare it to the .AC output. > And they match! > > I wasn't expecting a match, since all my textbooks say there's a `jw' factor > relating the impulse response and the step response. That is: > > U(w) = H(w)/jw > > So, when I compared the 2 spectra, I was expecting the FFT of my .TRAN output > to have an extra 6dB/octave roll-off, as compared to the .AC output, due to > the `w' in the denominator, above. > > Can anyone shed some light on this? > > Thanks, > > David Banas > Sr. Member Technical Staff > Altera > +1-408-544-7667 - desk > > Did you know Altera offers over 150 free online technical training courses? > Take one today! > > > Confidentiality Notice. > This message may contain information that is confidential or otherwise > protected from disclosure. If you are not the intended recipient, you are > hereby notified that any use, disclosure, dissemination, distribution, or > copying of this message, or any attachments, is strictly prohibited. If you > have received this message in error, please advise the sender by reply > e-mail, and delete the message and any attachments. Thank you. > > ------------------------------------------------------------------ > 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 > > > -- Steve Weir IPBLOX, LLC 150 N. Center St. #211 Reno, NV 89501 www.ipblox.com (775) 299-4236 Business (866) 675-4630 Toll-free (707) 780-1951 Fax ------------------------------------------------------------------ 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