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

[Bob kb8tq]

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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