[time-nuts] Question about frequency counter testing

Magnus Danielson magnus at rubidium.dyndns.org
Fri May 11 12:45:20 EDT 2018


Oleg,

On 05/10/2018 10:46 AM, Oleg Skydan wrote:
> Hi
> 
> Now I have some questions. As you know I am experimenting with the
> counter that uses LR calculations to improve its resolution. The LR data
> for each measurement is collected during the gate time only, also
> measurements are continuous. Will the ADEV be calculated correctly from
> such measurements?

Many assume yes, actual answer is no or well, it depends.

ADEV assumes brick-wall filtering up to the Nyquist frequency as result
of the sample-rate. When you filter the data as you do a Linear
Regression / Least Square estimation, the actual bandwidth will be much
less, so the ADEV measures will be biased for lower taus, but for higher
taus less of the kernel of the ADEV will be affected by the filter and
thus the bias will reduce.

It was when investigating this that Prof Enrico Rubiola and Prof
Francois Vernotte invented the parabolic deviation PDEV.

> I understand that any averaging for the time window
> larger then single measurement time will spoil the ADEV plot.

Correct.

> Also I understand that using LR can result in incorrect frequency estimate for
> the signal with large drift (should not be a problem for the discussed
> measurements, at least for the numbers we are talking about).

That is a result of not using a LR/LS method supporting drift, at which
time its effect on frequency estimation is greatly reduced.

LR only for phase and frequency model, i.e. only linear components, is
unable to correctly handle the quadratic nature of linear drift. Thus,
by using a LR model that supports it, the quadratic terms influence on
frequency estimation can be reduced.

> Does the ADEV plots I got looks reasonable for the used "mid range"
> OCXOs (see the second plot for the long run test)?

You probably want to find the source of the wavy response as the orange
and red trace.

Cheers,
Magnus



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