SV: Re: [sdiy] Simulating SW?

[René Schmitz]

Magnus Danielson wrote:
> Well, not quite... you are correct in the sense that one really
> should attempt to match the window length to an integer number of
> periods, however this will always be difficult to do in real life,
> but if you can come close to it and use propper windowing you are
> probably going to get the best result.

You're right of course, in a general context it isn't always possible.
In the context of circuit simulation however, you should be able to
choose the number of points.

> The FFT algorithm is not limited to powers of 2, I agree, but it is
> the easiest to implement. You can acheive practically any length, but
> if you have prime- number lengths you have problems. Powers of 2 is
> quite nice...

Yes, factoring primes is a bit difficult. :-P You would need to do a DFT...

> The choice of window function greatly affect the broadening, but it
> improves relative to non-windowed data.

Windowing IMO is a (sometimes neccessary) kludge to make the long time
transform into a short time transform. (I.e. generating some sense of
time-resolution, which the FT actually doesn't have.)
My point is that in this special case one can use the properties of the
FFT so that one gets the same result as with a true FT of an infinitely 
long signal (which can't be technically realised due to it starting at 
t=-inf.).

> The longer window, the less will the artifacts at the ends of the
> window apply. There is no absolute truth in Fourier analysis, since
> the artifacts by its missuse cripples the data. For the FFT to make
> good data, you must have a true integer multiples of waveform in the
> window and when the window is extended to represent the signal from
> -inf to +inf it shall be the same signal, i.e. transient signals or
> non-covered amplitude changes may not apply.

True, but in the context of THD measurements, you can assume (or rather
enforce) a steady input sinewave without any transients or amplitude
changes. So if you can perform a FFT that is essentially simulating a
long term analysis you get the results you would expect.

> The curse with FFTs is that people don't understand the basic
> limitations of the tool and learns to trim their setups accordingly.

That goes for circuit simulations as well...

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159