[SI-LIST] Re: Frequency v/s Time Domain analysis
- From: Darshan Mehta <darshanmehta2k@xxxxxxxxx>
- To: Andrew Ingraham <a.ingraham@xxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Thu, 4 Nov 2004 07:24:10 -0800 (PST)
Hi Andy,
Thanks a lot for taking time to reply my question.
My requirement is to see the response of Trace for the whole range of
Frequency. The Direct Digital Synthesizes is Digital programmed Oscillator
whose output will vary from 0 Hz to 150 MHz and I want to see the response of
Trace. If I do it in Time domain, I think I have to simulate the output for
many frequencies. So I think Frequency domain will be a good idea. I am using
ICX tool for simulation and they have facility to export the netlist for HSPICE
or Eldo. I do not have HSPICE or Eldo but I do have SPICE simulator. I exported
the .cir file from ICX and checked in SPICE but SPICE just converts the Time
Domain output in to Frequency domain by FFT. But I don't want that as this
output is valid for particular frequency only. In my case output frequency of
device is variable so I want to see Frequency response and not the FFT output.
I am expecting that the Tool should have all calculation with respect to
Frequency like x = b(f) where f is frequency. So now just substitute th
e value
of f and determine the value of 'x' and plot it. Could you please suggest me
for that?
Thanks,
-Darshan Mehta
Andrew Ingraham <a.ingraham@xxxxxxxx> 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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