[time-nuts] Re: The STM32 GPSDO, a short presentation

[Bob kb8tq]

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 <andrebalsa at 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)
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