[SI-LIST] Re: Frequency v/s Time Domain analysis
- From: Ron Matthews <rmatthews@xxxxxxxxxx>
- To: a.ingraham@xxxxxxxx, darshanmehta2k@xxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Thu, 04 Nov 2004 11:22:24 -0500
Another thing to consider is the ability to capture system resonances in your
simulation. If you are trying to evaluate the system over a wide range of
frequencies
in order to understand where the resonances are, then I think that a
time-domain
simulator is a much better choice. Using a frequency-domain simulation engine
is going to require that you choose your frequencies correctly. Murphy's law
says that system resonant frequencies are going to be something other than
what
you've chosen.
At 09:43 AM 11/4/2004 -0500, Andrew Ingraham wrote:
>This is an interesting question. But it's important to distinguish between
>*looking* at the data in the frequency or time domain, and *simulating* it
>in the frequency or time domain.
>
>There's little reason why you can't simulate in one domain and then look at
>the results in the other, as long as you understand the limitations.
>
>People's opinions may differ, but the way I look at it, the world is
>fundamentally a time domain universe. Things happen in time. Frequency is
>artificial. Our notion of frequency is an interesting and extremely useful
>way of looking at some things, but even in tuned circuits, those electrons
>are behaving on a moment-by-moment basis, they don't know about our man-made
>concept we call frequency. It just happens that when you look at things in
>the frequency domain, you may see some extremely useful things, things that
>you might completely miss in a time domain view, which is why we do it.
>
>Traditionally, going back some years, simulators had to simulate in the time
>domain to handle anything nonlinear. Frequency domain analysis was OK only
>for purely linear circuits. Now this distinction is blurred, because some
>programs will handle nonlinearities while doing frequency domain analysis.
>
>But not all programs. If you take a traditional SPICE based simulator and
>do a frequency domain analysis, it's going to make everything linear when
>doing the analysis, which is fine for small-signal RF analysis but not good
>for most logic circuits or digital waveforms ... even though you have
>given the simulator the full nonlinear device model parameters.
>
>I am somewhat biased because I have used mostly time domain simulations,
>unless I'm looking just at passive linear structures (transmission lines,
>connector or package models, etc.).
>
>One case where frequency domain simulations have traditionally been better,
>is when some of the input data is better (or more easily) expressed in the
>frequency domain, such as s-parameters, even skin effect loss. But there
>again, some time domain simulators can now handle this kind of frequency
>domain device data, to varying degrees.
>
>So, the answer to your question depends in part on which simulator you are
>using, and what kind of device models you have.
>
>If you are interested in loss vs. frequency, you are probably looking at
>just the analog portion of your whole circuit which is fairly linear.
>Otherwise, loss vs. frequency doesn't have much meaning, does it? For
>everything else, time domain analysis probably works best.
>
>A DDS might have special considerations. For example, to look at spectral
>purity, it might be that a time domain simulation doesn't easily provide
>enough precision to simulate a really low noise floor. But I'm only
>guessing here.
>
>Regards,
>Andy
>
>
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Ron Matthews
Principal MTS
Signal Integrity/EMI
Draper Labs
555 Technology Square
Cambridge, MA 02139
Voice: 617-258-2519
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