> Tim Wescott wrote:
> >> >
> >>> Hi 2 all,
>
> >>> is there any program to derive transfer function from circuit?
> >>> Something like PSpice, but instead of numerical to do analytical
> >>> "computation".
>
> >> What are you trying to do? =A0You can't choke transfer functions out o=
f
>> Spice, but if you look at it from the right angle you can often get
> >> loads of useful information.
>
> >> Most circuits are either simple enough that someone has already
> >> cataloged their transfer functions (or you can derive them easily
> >> enough yourself), or complicated enough that if you ignore all of the
> >> parasitics and nonlinearities you're doing yourself no favors -- yet
> >> factoring in the parasitics makes the transfer functions exceedingly
> >> opaque, while factoring in the nonlinearities renders the whole concep=
t
>> of transfer function analysis invalid.
>
> > A symbolic algebra program is probably your best bet, or else using
> > something like Octave to fit some suitable curve to the output of a
> > SPICE AC analysis.
>
> Depends on whether you want to plug in numbers or keep the component
> values as symbolic (which gets _really_ messy _really_ fast).
>
> Something like Scilab (and I think Octave) will let you fairly easily
> reduce a circuit to a transfer function with numeric coefficients (or a
> state-space representation as polynomials are insanely sensitive to
> numerical accuracy issues). =A0I've used this when I want to include a
> filter or something in a larger system model.
>
> --www.wescottdesign.com
The HP3562 DSA has a mode to derive the transfer function from what it measures. Sometimes it worked, sometimes it didn't. Of course, I've never fed the box something that I didn't know the transfer function of already.
The RF network analyzers can fit a response to a model, but that's not quite the same thing.