[time-nuts] Question about frequency counter testing
kb8tq at n1k.org
Thu May 10 10:21:41 EDT 2018
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:
> 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.
> Oleg UR3IQO
> <Снимок экрана (1148).png><Снимок экрана (1150).png><Снимок экрана (1149).png>_______________________________________________
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