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

[Magnus Danielson]

Rick,

"continuous count" as in counting/time-stamping each individual cycle 
forms a sample-rate limit. However, this is not what is meant with 
continuous conting today, as that is that you have a continuous 
time-stamping for some time-base. In that some number of counted cycle 
(+/- 1) occurs between each time-stamp. Unless one attempts to use 
time-base very near the maximum sample rate per second, it cease to be a 
practical concern as one does not want to miss samples.

I have a counter that can time-stamp at 10 MSa/s and 13.333 MSa/s 
depending on mode. I extremely rarely use that even close to the 
extreme, as continuous counting I normally need is maybe up to 100 Sa/s.

Cheers,
Magnus

On 2021-11-10 01:53, Richard (Rick) Karlquist wrote:
> 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
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe 
>> send an email to time-nuts-leave at lists.febo.com
>> To unsubscribe, go to and follow the instructions there.
>>