[time-nuts] Re: Simple simulation model for an OCXO?

[Attila Kinali]

On Tue, 10 May 2022 08:20:35 +0200
Carsten Andrich <carsten.andrich at tu-ilmenau.de> wrote:

> If you happen to find the paper, please share a reference. I'm curious 
> about implementation details and side-effects, e.g., whether 
> implementing the filter via circular convolution (straightforward 
> multiplication in frequency-domain) carries any penalties regarding 
> stochastic properties due to periodicity of the generated noise.

Yes, for one, noise is not periodic, neither is the filter you need.
You can't build a filter with a 1/sqrt(f) slope over all the
frequency range. That would require a fractional integrator, which
is a non-trivial task. Unless you actually do fractional integration,
all time domain filters will be approximations of the required filter.

> Also, any reason to do this via forward and inverse FFT? AFAIK the 
> Fourier transform of white noise is white noise, because the underlying 
> Gaussian is an Eigenfuntion of the transform. Generating in time-domain 
> as follows (Python code using NumPy)

I had the same question when I first saw this. Unfortunately I don't have a good
answer, besides that forward + inverse ensures that the noise looks like it is
supposed to do, while I'm not 100% whether there is an easy way to generate
time-domain Gauss i.i.d. noise in the frequency domain.

If you know how, please let me know.

But be aware, while the Gauss bell curve is an eigenfunction of the Fourier
transform, the noise we feed into it is not the Gauss bell curve. It's the
probability distribution of the noise that has this bell curve, not the noise
itself. Hence the probability distribution of a random variable is not necessarily
invariant over the Fourier transfom.

> On 05.05.22 18:34, Magnus Danielson via time-nuts wrote:
> > Both is being revisioned and 1139 just went out for re-balloting 
> > process after receiving ballotting comments and 1193 is just to get 
> > approved to be sent to balloting.
> Any details you can share regarding the changes over the current 
> version? Are drafts publicly available?

Both 1139 and 1193 saw major rework. Large portions have been replaced and rewritten
and the rest re-arranged to make more sense. I would look at the upcoming revisions
more as complete rewrites instead instead of just mere updates. 

I am not aware of any public drafts. As the groups working on them are well networked
with those who would use the standards, having a public discussion never came up
and thus no public draft policy was formed. But the revision process is mostly
complete, both should be published at by the end of the year, probably earlier.


				Attila Kinali
-- 
In science if you know what you are doing you should not be doing it.
In engineering if you do not know what you are doing you should not be doing it.
        -- Richard W. Hamming, The Art of Doing Science and Engineering