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

[Tom Van Baak]

 > 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...

Ah, welcome to the slippery slope of being a time nut. Some hints:

1) Don't expect any one oscillator to be "the best". In a pile of 
oscillators one may have best ADEV at tau 100 s, but poor ADEV at tau 
0.1 s. One may have lowest drift per day. One may have best phase noise 
at 100 Hz; one may have best phase noise at 100 kHz. So if you are 
looking for a reference oscillator be prepared to have several, each 
with its own region of proven excellent performance. Note also that some 
oscillators may have worse tempco than others, so one might be your best 
tau 1000 s reference on a cloudy day but not the one to trust on a sunny 
day.

2) A single input frequency counter is sufficient. Sure, 2 or 3 or N 
channels is nice but it's not necessary. Pick a tau, say 10 s. Then 
collect an hour of data measuring oscillator #1. Then do the same for 
#2. If their ADEV differs you have found the better one. If the ADEV is 
the same, then a) the oscillators are in fact the same, or b) they 
differ but are both better than the reference clock of the counter. It 
doesn't take much time to sort through a pile of oscillators in this 
way. At some point you will then use one of the better ones as the 
reference for your counter.

3) Let's assume that neither your reference nor your instrument is the 
limiting factor. Make ADEV plots of each oscillator and that tells you 
everything you need to know about which oscillator is best over some 
range of tau.

The 3-hat trick works with this method too. ADEV is a log-log plot and 
noise tends to add like RMS, so the gaps between different ADEV lines 
reveals their relative noise difference. You can take the pair-wise ADEV 
readings and directly compute oscillator stability using 3-hat. In other 
words, you don't need to perform 3-hat on the phase data itself, you can 
just perform it on the ADEV table. It's as easy as the Pythagorean Theorem:

http://leapsecond.com/tools/3hat1.c

/tvb


On 7/19/2022 10:25 AM, Erik Kaashoek via time-nuts 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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