[time-nuts] Question about frequency counter testing

Magnus Danielson magnus at rubidium.dyndns.org
Fri May 11 16:15:07 EDT 2018


Hi,

On 05/11/2018 05:35 PM, Bob kb8tq wrote:
> Hi
> 
> If you do the weighted average as indicated in the paper *and* compare it to a “single sample” computation, 
> the results are different for that time interval. To me that’s a problem. To the authors, the fact that the rest of
> the curve is the same is proof that it works. I certainly agree that once you get to longer tau, the process 
> has no detrimental impact. There is still the problem that the first post on the graph is different depending 
> on the technique. 

Check what I did in my paper. I made sure to check that my estimator of
phase and frequency is bias-free, that is, when exposed to stable phase
or stable frequency, that comes out of the phase and frequency unbiased,
but 0 as you switch them, as a good estimator should do.

> The other side of all this is that ADEV is really not a very good way to test a counter. It has it’s quirks and it’s
> issues. They are impacted by what is in a counter,  but that’s a side effect. If one is after a general test of 
> counter hardware, one probably should look at other approaches.

Well, you can tell a few things from the ADEV, to give you a hint about
what you can expect from that counter when you do ADEV... and measure of
frequency. The 1/tau limit is that of the counter. It's... a complex
issue of single-shot resolution and noise, but a hint.

> If you are trying specifically just to measure ADEV, then there are a lot of ways to do that by it’s self. It’s not
> clear that re-invinting the hardware is required to do this. Going with an “average down” approach ultimately
> *will* have problems for certain signals and noise profiles. 

The filtering needs to be understood and handled correctly, for sure,
and it's not doing anything good for lower true ADEV measures. Filtering
helps for improving the frequency reading, as the measures deviation
shifts from ADEV to MDEV or PDEV, but let's not confuse that with
improving the ADEV, it's a completely different thing. Improving the
ADEV takes single-shot resolution, stable hardware and stable reference
source.

Cheers,
Magnus



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