[time-nuts] Question about frequency counter testing

Oleg Skydan olegskydan at gmail.com
Sat May 12 13:20:48 EDT 2018


Hi!

From: "Bob kb8tq" <kb8tq at n1k.org>
> There is still the problem that the first post on the graph is different 
> depending
> on the technique.

The leftmost tau values are skipped and they "stay" inside the counter. If I 
setup counter to generate lets say 1s stamps (ADEV starts at 1s) it will 
generate internally 1/8sec averaged measurements, but export combined data 
for 1s stamps. The result will be strictly speaking different, but the 
difference should be insignificant.

> The other side of all this is that ADEV is really not a very good way to 
> test a counter.

Counter testing was not a main reason to dig into statistics details last 
days. Initially I used ADEV when tried to test the idea of making the 
counter with very simple HW and good resolution (BTW, it appeared later it 
was not ADEV in reality :). Then I saw it worked, so I decided to make a 
"normal" useful counter (I liked the HW/SW concept). The HW has enough power 
to compute various statistics onboard in real time, and while it is not 
requisite feature of the project now, I think it will be good if the counter 
will be able to do it (or at least if it will export data suitable to do it 
in post process). The rest of the story you know :)

> If you are trying specifically just to measure ADEV, then there are a lot 
> of ways to do that by it’s self.

Yes, but if it can be done with only some additional code - why not to have 
such ability? Even if it has some known limitations it is still a useful 
addition. Of cause it should be done as good as it can be with the HW 
limitations. Also it was/is a good educational moment.

Now it is period of tests/experiments to see the used technology 
features/limitations(of cause if those experiments can be done with the 
current "ugly style HW"). I have already got a lot of useful information, it 
should help me in the following HW/FW development. The next steps are analog 
front end and GPS frequency correction (I should get the GPS module next 
week). I have already tested the 6GHz prescaler and now wait for some parts 
to finish it. Hope this project will have the "happy end" :).

All the best!
Oleg 




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