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

[Bob kb8tq]

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 <andrebalsa at 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 <kb8tq at 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 <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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