[time-nuts] Question about frequency counter testing
k8yumdoober at gmail.com
Thu May 10 12:17:07 EDT 2018
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.
On Thu, May 10, 2018 at 9:21 AM, Bob kb8tq <kb8tq at n1k.org> wrote:
> 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 ….
> > 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
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