[time-nuts] Question about frequency counter testing

Oleg Skydan olegskydan at gmail.com
Thu Jun 21 13:05:26 UTC 2018


Hi!

From: "Magnus Danielson" <magnus at rubidium.dyndns.org>

>> I have write a note and attached it. The described modifications to the
>> original method were successfully tested on my experimental HW.
>
> You should add the basic formula
>
> x_{N_1+n} = x_{N_1} + x_n^0
>
> prior to (5) and explain that the expected phase-ramp within the block
> will have a common offset in x_{N-1} and that the x_n^0 series is the
> series of values with the offset removed from the series. This is fine,
> it should just be introduced before applied on (5).

I have corrected the document and put it here (it should be clearer now):
http://skydan.in.ua/FC/Efficient_C_and_D_sums.pdf

It should be more clear now.

> Notice that E as introduced in (8) and (9) is not needed, as you can
> directly convert it into N(N_2-1)/2.

Oh! I should notice it, thanks for the valuable comment!

>> They can be computed with small memory footprint, but it will be non
>> overlapped PDEVs, so the confidence level at large taus will be poor
>> (with the practical durations of the measurements). I have a working
>> code that realizes such algorithm. It uses only 272bytes of memory for
>> each decade (1-2-5 values).
>
> Seems very reasonable. If you are willing to use more memory, you can do
> overlapping once decimated down to suitable rate. On the other hand,
> considering the rate of samples, lots of gain already there.

I have optimized continuous PDEV calculation algorithm, and it uses only 
140bytes per decade now.

I will not probably implement overlapping PDEV calculations to keep the 
things simple (with no external memory) and will just do the continuous PDEV 
calculations only. The more sophisticated calculations can be easily done on 
the PC side.

>> ... but 2x or 4x
>> one shot resolution improvement (down to 1.25ns or 625ps) is relatively
>> simple to implement in HW and should be a good idea to try.

So, I tried it with a "quick and dirty" HW. It appeared to be not as simple 
in real life :) There was a problem (probably the crosstalk or grounding 
issue) which leaded to unstable phase measurements. So, I got no 
improvements (the results with 1.25ns resolution were worse then with the 
2.5ns resolution). I have to do more experiments with better HW 
implementation.

> Take care. Heal up properly. It's a hobby after all. :)

Thanks!

Best!
Oleg 




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