[time-nuts] Question about frequency counter testing

Bob kb8tq kb8tq at n1k.org
Thu May 10 12:23:21 EDT 2018


More or less: 

ADEV takes the *difference* between phase samples and then does a standard
deviation on them. RMS of the phase samples makes a lot of sense and it was
used back in the late 50’s / early 60’s. The gotcha turns out to be that it is an 
ill behaved measure. The more data you take, the bigger the number you get. 
( = it does not converge ). That problem is what lead NBS to dig into a better 
measure. The result was ADEV.

The point about averaging vs decimation relates to what you do to the data *before*
you ever compute the ADEV. If you have 0.1 second samples, you have to do something
to get to a tau of 1 second or 10 seconds or … The process you use to get the data
to the proper interval turns out to matter quite a bit. 


> On May 10, 2018, at 12:17 PM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
> I'm a bit fuzzy, then, on the definition of ADEV.  I was under the
> impression that one measured a series of
> "phase samples" at the desired spacing, then took the RMS value of that
> series, not just a single sample,
> as the ADEV value.
> Can anybody say which it is?   The RMS approach seems to make better sense
> as it provides some measure
> of defense against taking a sample that happens to be an outlier, yet
> avoids the flaw of tending to average
> the reported ADEV towards zero.
> Dana   (K8YUM)
> On Thu, May 10, 2018 at 9:21 AM, Bob kb8tq <kb8tq at n1k.org> wrote:
>> Hi
>> If you collect data over the entire second and average that down for a
>> single point, then no, your ADEV will not be correct.
>> There are a number of papers on this. What ADEV wants to see is a single
>> phase “sample” at one second spacing. This is
>> also at the root of how you get 10 second ADEV. You don’t average the ten
>> 1 second data points. You throw nine data points
>> away and use one of them ( = you decimate the data ).
>> What happens if you ignore this? Your curve looks “to good”. The resultant
>> curve is *below* the real curve when plotted.
>> A quick way to demonstrate this is to do ADEV with averaged vs decimated
>> data ….
>> Bob
>>> On May 10, 2018, at 4:46 AM, Oleg Skydan <olegskydan at gmail.com> wrote:
>>> Hi
>>> I have got a pair of not so bad OCXOs (Morion GK85). I did some
>> measurements, the results may be interested to others (sorry if not), so I
>> decided to post them.
>>> I ran a set of 5minutes long counter runs (two OCXOs were measured
>> against each other), each point is 1sec gate frequency measurement with
>> different number of timestamps used in LR calculation (from 10 till 5e6).
>> The counter provides continuous counting. As you can see I reach the HW
>> limitations at 5..6e-12 ADEV (1s tau) with only 1e5 timestamps. The results
>> looks reasonable, the theory predicts 27ps equivalent resolution with 1e5
>> timestamps, also the sqrt(N) law is clearly seen on the plots. I do not
>> know what is the limiting factor, if it is OCXOs or some counter HW.
>>> I know there are HW problems, some of them were identified during this
>> experiment. They were expectable, cause HW is still just an ugly
>> construction made from the boards left in the "radio junk box" from the
>> other projects/experiments. I am going to move to the well designed PCB
>> with some improvements in HW (and more or less "normal" analog frontend
>> with good comparator, ADCMP604 or something similar, for the "low
>> frequency" input). But I want to finish my initial tests, it should help
>> with the HW design.
>>> 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? I understand that any averaging for the time window
>> larger then single measurement time will spoil the ADEV plot. 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).
>>> Does the ADEV plots I got looks reasonable for the used "mid range"
>> OCXOs (see the second plot for the long run test)?
>>> BTW, I see I can interface GPS module to my counter without additional
>> HW (except the module itself, do not worry it will not be another DIY
>> GPSDO, probably :-) ). I will try to do it. The initial idea is not try to
>> lock the reference OCXO to GPS, instead I will just measure GPS against REF
>> and will make corrections using pure math in SW. I see some advantages with
>> such design - no hi resolution DAC, reference for DAC, no loop, no
>> additional hardware at all - only the GPS module and software :) (it is in
>> the spirit of this project)... Of cause I will not have reference signal
>> that can be used outside the counter, I think I can live with it. It worth
>> to do some experiments.
>>> Best!
>>> Oleg UR3IQO
>>> <Снимок экрана (1148).png><Снимок экрана (1150).png><Снимок экрана
>> (1149).png>_______________________________________________
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