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