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Simulators >> Circuit Simulators >> AC response from transient response
https://designers-guide.org/forum/YaBB.pl?num=1213719320

Message started by manodipan on Jun 17^th, 2008, 9:15am

Title: AC response from transient response
Post by manodipan on Jun 17^th, 2008, 9:15am

Hi friends,
I want some suggestion about a problem:We know that ac response can be got from transient response at different frequencies.Now i take DFT of my transient response for a particular input signal and definitely DFT will contain the signal peak alongwith several harmonics due to nonlinearity of the system.So for a filter i want to check passband and stopband rejection.Now i pass input sinusoids at different frequencies ,the magnitude of the o/p at signal frequency for a particular amplitude input signal can give me gain.So at different frequencies the signal gain can be calculated.Suppose the system has some offset ,still can we apply the same method (i.e DFT shows some offset alongwith common mode,that means for a 0.6V common mode i/p and o/p offset shows more than 0.6V)??
What is the implication of other frequencies(harmonics) in DFT at a particular i/p frequency??

Title: Re: AC response from transient response
Post by thechopper on Jun 17^th, 2008, 7:40pm

Hi manodipan,

I would make sure that such offset in your circuit does not change the operating point. Otherwise your will get a different result from the DFT at a particular frequency with respect to the "no offset" case.
Also, if you are planning on characterizing the frequency response based on transient analysis, make sure the signal amplitudes you apply are small enough in order not to change the operating conditions. If that happens then you are not considering small signal conditions anymore and your system will not be linear.

Regards
Tosei

Title: Re: AC response from transient response
Post by manodipan on Jun 18^th, 2008, 12:15am

Hi Tosei,
my doubt is that in case of a filter upto certain frequency the signal will be transmitted to o/p with gain and passed after cutoff frequency with very low gain.Now the amplitude can be large as we want to see how much signal is passed at those frequencies.Now AC analysis does it with small signal at different frequencies..

Title: Re: AC response from transient response
Post by buddypoor on Jun 21^st, 2008, 5:03am

I don´t know exactly if I understand your problem , anyway perhaps the following clarifies something:

In ac analysis the signal amplitudes at the input are not important ; it is convinient to use always 1 volt.
The reason is as follows: in ac analysis the program linearizes the transfer curve automatically around the bias point. This is called "small signal analysis", because all amplitudes (even 100kvolts) are handled in the same way like 1mikrovolt. There is no distortion and no limiting effect.
Was this your question ?

Title: Re: AC response from transient response
Post by loose-electron on Jun 23^rd, 2008, 12:45pm

If this is a continuous time filter, why would you want to get the gain vs. frequency from a transient analysis?

It might be a good idea to clarify what you are trying to do, and then perhaps the group can help you work out an appropriate method.

Jerry

Title: Re: AC response from transient response
Post by manodipan on Jun 24^th, 2008, 12:51am

Hi all,
Definitely this is not a continuous time filter,but a switched capacitor filter.Moreover it is not opamp based,rather comparator based,and it has transients before settling to final value.Also it does not have any steady state,so i think conventional PSS simulation is not possible,thats'why i wanted to know whether from transient response we can get frequency response .

Title: Re: AC response from transient response
Post by pancho_hideboo on Jun 24^th, 2008, 1:18am

manodipan wrote on Jun 24^th, 2008, 12:51am:

You can do AC analysis at specific time point.
First run transient analysis until time you want to observe AC characteristics and save final state.
Next run AC analysis using final state of transient analysis.

Title: Re: AC response from transient response
Post by manodipan on Jun 24^th, 2008, 2:42am

Hi Paancho,
Do you mean that the time until which i want to run simulation means the minimum frequency i want to detect??I don't get how ac analysis works on transient result,how to save the final state and where this is referenced in ac simulation??please clarify..

Title: Re: AC response from transient response
Post by buddypoor on Jun 24^th, 2008, 3:24am

Hi manodipan,

I am not familiar with spectre (I suppose you are using it), however, independent on the program used, in general you can perform an ac analysis for S/C circuits when you have transformed it into a time continuous equivalent before (using delay elements). This method works exact and reveals all properties specific to S/C circuits (periodic functions, zero´s, poles,..)

Title: Re: AC response from transient response
Post by pancho_hideboo on Jun 24^th, 2008, 4:34am

manodipan wrote on Jun 24^th, 2008, 2:42am:

No. This analysis is AC analysis at specific time point.

I re-read your post.
This method is not useful for your purpose.

Title: Re: AC response from transient response
Post by pancho_hideboo on Jun 24^th, 2008, 4:45am

manodipan wrote on Jun 24^th, 2008, 12:51am:

Definitely this is not a continuous time filter,but a switched capacitor filter.Moreover it is not opamp based,rather comparator based,
and it has transients before settling to final value.Also it does not have any steady state

As far as clock and input signal are periodic and circuit is stable(this means circuit is never oscillative), circuit always results in steady state.
Settling transient is repeated periodically in steady state.

If you want to consider input signal amplitude, apply two large signal drives(one is clock, the other is signal).

There are four possibilities in simulation.

(1) PSS(two large tones, signal frequency is swept as large.
(2) PSS(two large tones/PAC, signal frequency is fixed as large but you can observe signal frequency characteristics as small.
(3) QPSS(two large tones), signal frequency is swept as large.
(4) QPSS(two large tones/QPAC, signal frequency is fixed as large but you can observe signal frequency characteristics as small.

If you want to insist on DFT from transient analysis, explain situation assuming actual instruments.
What instruments do you use ?
Have you measured actual DUT using actual instruments ?

Do you assume real time spectrum analyzer or FFT analyzer where short time interval overlapped FFT is invoked ?

Title: Re: AC response from transient response
Post by Geoffrey_Coram on Jul 1^st, 2008, 5:32am

pancho_hideboo wrote on Jun 24^th, 2008, 1:18am:

Also, do "spectre -help tran" and look at the actimes and acnames arguments ...