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 <[email protected]> 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>_______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
