FreeLists / si-list / [SI-LIST] Re: Frequency v/s Time Domain analysis

[Darshan Mehta <darshanmehta2k@xxxxxxxxx>]

Hi Andy,
 
I think user has to write a model in SPICE in such a way to see the Frequency 
v/s Amplitude response. Chris has sent me some example netlist of SPICE which 
serves my purpose. Thanks a lot for all your help.
 
Best Regards,
Darshan Mehta
Andrew Ingraham <a.ingraham@xxxxxxxx> wrote:
I don't think you can do what you want. I think you might be stuck with
time domain simulations and checking several output frequencies.

A simulation is usually just a shortcut of a real measurement. "If you
can't measure it, you can't simulate it." That's not entirely true; for
example, you can do simulations where you can drive or monitor internal
circuit nodes that you can't physically access; but you could have measured
it if you had ideal microprobes.

How would you measure the real DDS circuit, without making it step through
many frequencies? In other words, I don't think there is a way around that.

The concept of a Transfer Function doesn't apply to a complete DDS, from
digital input ("dialing" in the frequency you want) to analog output.

You could run a frequency analysis of just the analog portion, and it may
tell you things that you need to know, but it's probably not what you wanted
because it doesn't check the entire DDS.

Now if you were simulating just a D/A rather than a whole DDS, you could
start with the digital representation of a step or impulse function, and do
a Fourier transform on the output, which would give you data vs. frequency
(since an ideal impulse or step contains all frequencies), assuming that
linearity holds. But I don't know how you'd do this with a DDS.

Regards,
Andy



                        
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