[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


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of steve weir
Sent: Friday, September 02, 2011 10:04 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Impulse vs. Step Response?

David. I was mistaken. Taht's what I get for not looking closely at the
deck.  However your rise time is only 1/20th of the RC time constant so
the result that you will get from the transient will have little
difference than one with a zero rise time.

Steve.
On 9/2/2011 9:48 AM, David Banas wrote:
> "You are not driving your transient with a step"
>
> Why do you say that?
> -db
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
> Behalf Of steve weir
> Sent: Thursday, September 01, 2011 6:40 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Impulse vs. Step Response?
>
> 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!
>>
>>
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>
> --
> Steve Weir
> IPBLOX, LLC
> 150 N. Center St. #211
> Reno, NV  89501
> www.ipblox.com
>
> (775) 299-4236 Business
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--
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


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