[time-nuts] Re: Comparing 3 oscillators using a 2 channel frequency counter?

[Eric S]

Also using one as the external refrence of the counter will super impose
its errors on to channel 1 and 2. So the errors of channel 1 will be
channel 1 error + ref errors.

Eric

On Tue, Jul 19, 2022, 4:06 PM Bob kb8tq via time-nuts <
time-nuts at lists.febo.com> wrote:

> Hi
>
> This process is typically referred to as a “three corner hat” approach.
> There are
> a lot of papers about doing it and the often surprising results. These
> days it is
> commonly done with simultaneous sampling ADC based setups.
>
> Back when they tried it with “analog” sorts of approaches ( take three
> data sets
> and then subtract this from that) the results often came out a bit crazy.
> That improved
> with the ADC setups. You still can easily get nutty results. This is often
> true if the
> noise levels of this or that DUT are much better / much worse than the
> others.
>
> Sometimes “nutty” is very obvious. You can’t have noise that low ( or
> negative
> results). The more pesky case is when the result *might* be correct.
> Fortunately
> the really crazy outcome is the more likely failure mode.
>
> By far the most simple approach ( and one with very few gotchas) is to
> simply
> compare A to B, B to C, and A to C. One of those pairs will be better than
> the
> others. Put the oscillator that is not part of that pair back on the shelf
> and start
> with another. It’s not fast, but neither are the sorts of runs involved
> with the ADC
> setups.
>
> Bob
>
> > On Jul 19, 2022, at 9:25 AM, Erik Kaashoek via time-nuts <
> time-nuts at lists.febo.com> wrote:
> >
> > As a good engineer I want to check the short and long term stability of
> all my oscillators but to do that a need a better oscillator...
> > But I've read that it should be possible to measure 3 independent
> oscillators together and use some kind of statistical tools or voting to
> get some better insight in the performance of each of the 3 oscillators.
> > Now the problem is: where to find a 3 input frequency counter? I don't
> have one, but I do have a 2 input frequency counter that can use an
> external reference. So I connected 2 oscillators to the two inputs and the
> 3rd to the counter reference and for each measurement of the two
> frequencies I also calculated also the ratio of the two measured
> frequencies scaled back to the oscillator frequencies (all 10MHz) and
> imported in Timelab. [1]
> > Looking at frequency difference chart two oscillators (DOCXO-RB) seem to
> be long term more stable with respect to each other compared to the two
> other combinations (OCXO-Rb and DOCXO-OCXO), these two act as if they are
> each others opposite. Of course this is just looking at the plot so my
> questions are:
> > - Is this the correct way to use a 2 channel frequency counter to get
> info on 3 oscillators?
> > - What (mathematical) tools can be used to get insight in the
> performance of the 3 oscillators individually?
> >
> > I've read about 3-cornered hat and using the three frequency
> measurements I know how to calculate the taus of each pair and import into
> stable 32 to do the 3-cornered hat calculation but what would that tell?
> >
> > [1] http://athome.kaashoek.com/time-nuts/3-freq.png
> > Erik
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