Hi The gotcha of looking at one GPSDO against another is that the GPS side of things is “common mode” to all of the devices.
Bob > On Mar 31, 2022, at 4:16 PM, André Balsa <andreba...@gmail.com> wrote: > > 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 <kb...@n1k.org> 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 <andreba...@gmail.com> 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 -- time-nuts@lists.febo.com -- To unsubscribe >> send an email to time-nuts-le...@lists.febo.com >>> To unsubscribe, go to and follow the instructions there. >> _______________________________________________ >> time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send >> an email to time-nuts-le...@lists.febo.com >> To unsubscribe, go to and follow the instructions there. > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an > email to time-nuts-le...@lists.febo.com > To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.
