Hi Bob, "You need an external / stable reference that is (hopefully) much more accurate than the GPSDO to compare it to."
I fully agree, and of course I don't have an atomic clock at hand. But I was thinking that there is an alternative: the Three-Cornered Hat method. In any case, we are not there yet, I still have to write the code for tab delimited reporting, and then collect the data from the 4 different GPSDOs I have here (3 x STM32 GPSDO + 1 x Lars' DIY GPSDO), and then setup the Three-Cornered Hat measurement apparatus, and then collect more data, etc... Oh the joys of time-nuttery! On Thu, Mar 31, 2022 at 9:01 PM Bob kb8tq <[email protected]> wrote: > Hi > > One word of caution: > > You really can’t compute things like ADEV by observing the device against > itself. You need an external / stable reference that is (hopefully) much > more > accurate than the GPSDO to compare it to. > > Bob > > > On Mar 31, 2022, at 2:18 PM, André Balsa <[email protected]> wrote: > > > > Hello time-nuts, > > This is a quick follow-up to my short presentation of the STM32 GPSDO > > project, because as some of you noticed, the EEVblog thread is > > unfortunately 27 pages long and growing... :( > > A few pointers: > > 1. If you are interested in the technical details or further > understanding > > of the project, I suggest reading **only the first post** in the EEVblog > > thread and then jumping straight to the schematics in post #378, page 16 > in > > the thread, and then reading the generously commented source code on > GitHub > > (download the zip file for the latest release and check the GPSDO.ino > > sketch). > > 2. The core of this project is made of just three components: > > a. The STM32F411CEU6 "Black Pill" > > b. The 10 MHz OCXO > > c. The ublox M8N GPS receiver module > > These three components plus a few resistors, capacitors and an LED and > > voila!, you have a working GPSDO. Everything else is optional. But of > > course the optional parts are where the fun is, really. > > 3. Likewise the core of the firmware is just 15 lines of C code, which > are > > used to setup the 64-bit counter (10 lines) and the interrupt service > > routine that reads the counter (5 lines). Everything else in the firmware > > is built around these 15 lines of C code. Also to be honest, while these > 15 > > lines of C code are well written, the rest... not so much. ;) > > 4. So how does it work? Essentially we measure the frequency of the OCXO > > every second (the 64-bit counter), average it over the last 10/100/1000 > > seconds (data stored in circular buffers), and adjust the OCXO control > > voltage Vctl accordingly: if the frequency is too high, we decrease Vctl, > > and if the frequency is too low, we increase Vctl. In other words, it's a > > simple frequency locked loop, which all of you are familiar with. > > 5. The performance? Hmmm... honestly I started working on this project > with > > the modest aim of 1ppb stability and that was achieved almost > immediately, > > also from the anecdotal data I and others have collected, the STM32 GPSDO > > achieves 0.1ppb stability after a couple of hours and 0.01ppb is also > > achievable, without any correction being applied from the various > > environmental sensors. I think ultimately 0.001ppb should be achievable > but > > that will take some work on the firmware. > > 6. Those nice ADEV plots? Ouch: I don't have any. In the coming days I > will > > push to GitHub a new version of the firmware with reporting with proper > > tab-delimited fields that will allow people to plot various charts and > > study the correlation between supply voltages, OCXO power consumption, > OCXO > > frequency, atmospheric pressure, humidity, etc... A true smorgasboard for > > statisticians! > > > > Greetings from France, > > Andre' (Andrew) > > _______________________________________________ > > time-nuts mailing list -- [email protected] -- To unsubscribe > send an email to [email protected] > > To unsubscribe, go to and follow the instructions there. > _______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send > an email to [email protected] > To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
