[time-nuts] Question about frequency counter testing

Bob kb8tq kb8tq at n1k.org
Sat May 12 15:41:55 EDT 2018


Hi


> On May 12, 2018, at 1:20 PM, Oleg Skydan <olegskydan at gmail.com> wrote:
> 
> 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.

Except here are a *lot* of papers where they demonstrate that the difference may be *very* significant. I would
suggest that the “is significant’ group is actually larger than the “is not” group. 


> 
>> 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 :)

Again, ADEV is tricky and sensitive to various odd things. This whole debate about it being sensitive goes 
back to the original papers in the late 1960’s and 1970’s. At every paper I attended the issue of averaging
and bandwidth came up in the questions after the paper. The conversation has been going on for a *long*
time. 

> 
>> 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.

It’s only useful if it is accurate. Since you can “do code” that gives you results that are better than reality,
simply coming up with a number is not the full answer. To be useful as ADEV, it needs to be correct. 

> 
> 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" :).

I’m sure it will come out to be a very cool counter. My *only* concern here is creating inaccurate results
by stretching to far with what you are trying to do. Keep it to the stuff that is accurate.

Bob


> 
> All the best!
> Oleg 
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