[time-nuts] Re: Can ADEV of a frequency source be correctly determined using a continuous time-stamping frequency counter?

[Richard (Rick) Karlquist]

Let me just mention that when I worked at the HP Santa Clara
Division counters section, they came out with a "feature"
that they called "continuous count".  However, it was limited
to something like 3 MHz.  So a 100 MHz counter would only
continuously count signals below 3 MHz.

So you need to verify for what bandwidth your specific counter
model is truly doing continuous count.

Rick N6RK

On 11/9/2021 2:29 PM, Magnus Danielson via time-nuts wrote:
> Hi Erik,
> 
> On 2021-11-09 18:26, Erik Kaashoek wrote:
>> As far as I understood the ADEV at a Tau of 1 second is a statement 
>> about the amount of variation to be expected over a one second interval.
> Rather, the variation of readings of a frequency estimation done over a 
> span over 1 second.
>> It would be nice if we would be able to measure a frequency in an 
>> infinite short interval but any frequency measurement takes time.
> Turn out that basic white noise and systematic noise will limit our 
> frequency resolution to form a 1/tau limit slope, so infinite short 
> interval will bury it well into that noise whatever we do.
>> What if the frequency counter does a complete measurement of a 
>> frequency source every second and all the variation within that second 
>> is hidden because of the "integration" that happens over the second?
> 
> That is what happens, but that is not what the ADEV is about, it's about 
> the variations of these measures as we look for a bunch of them. So if 
> we now have say 1000 of these frequency estimates, how much variations 
> in these can be contributed to the random noise of the source, and to 
> analyse that, we need at least a tool like ADEV since standard deviation 
> will not even converge for white and flicker phase noise modulation.
> 
> What ADEV actually aims to do is to provide a low-frequency spectroscopy 
> method at a time when time-interval counters was about the only tool at 
> hand, and even those where very rare. We now have a much wider palette 
> of tools, but ADEV is relevant for how we measure frequency stability 
> and a few other applications.
> 
>> This is specially the case with continuous time-stamping counters.
>> They can provide a precise number by applying statistical methods on 
>> many measurements done during one second but they can not provide 
>> information exactly at the end of a second.
>> Is this kind of statistical measurement over a period of a second 
>> still valid for determining the ADEV at the Tau of one second of a 
>> frequency source?
> Not for ADEV, but if you use averaging counter you get the result of 
> MDEV and for linear regression / least square counter you get the 
> response of PDEV. That is the result of various statistical measures and 
> then applying the ADEV processing on these frequency estimates. The 
> upcoming IEEE Std 1139 revision, which is in approval process now 
> include language to reflect that.
>> Or should there be a correction factor depending on the method used in 
>> the frequency counter?
> 
> Yes, you then need to use the appropriate bias function for ADEV/MDEV 
> and ADEV/PDEV to convert between these scales. Knowing the response of 
> ADEV, MDEV and PDEV for a particular noise-type which is dominant at the 
> tau of interest, you can readily convert between them by forming the 
> bias functions.
> 
> You may find NIST SP-1065 a useful and handy tool, even if it does not 
> cover the more recent work such as PDEV.
> 
> https://www.nist.gov/publications/handbook-frequency-stability-analysis
> 
>> I tried to read some scientific studies on this subject but I am not 
>> smart enough to understand.
>> Hope one of you can provide some information.
> 
> It is scattered over a large number of articles, and quite a lot of 
> folks get confused. Hopefully the updated IEEE Std 1139 will be of aid 
> to you. It also has lots of useful references.
> 
> Cheers,
> Magnus
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