[SI-LIST] Impulse vs. Step Response?

  • From: David Banas <DBanas@xxxxxxxxxx>
  • To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
  • Date: Thu, 1 Sep 2011 17:40:18 -0700

Hi all,

I've got a puzzler. I'm using this HSpice deck:

.TITLE AC Sweep vs. Transient Correlation

+ 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)


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?


David Banas
Sr. Member Technical Staff
+1-408-544-7667 - desk

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