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

[Neal Pollack]

So then, I think what I hear you saying, is that if a <insert
person/newbie/time-nut> wants to
see if a GPSDO that they built, or purchased on ebay, is working OK, then
they really need to
purchase a non-GPS rubidium oscillator on ebay, and use that as a ref to
compare their GPSDO under
test?

And, I assume that since we have no idea if the used rubidium oscillator
from ebay is working properly
anymore (aside from output seen on a counter), then we should take that
rubidium oscillator to a
calibration vendor and pay them to test it, correct?

Then, we can measure?

Neal


On Thu, Mar 31, 2022 at 4:43 PM Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> Consider the case:
>
> GPS shifts ( possibly due to any of a number of issues). All three modules
> “move” forward in time by 15 or 20 ns over some time period.
>
> The GPSDO’s all do their thing and shift frequency by the appropriate
> amount.
>
> Since all three devices in your three corner hat did the same thing at the
> same time, all of the delta this / delta that numbers just sit there. They
> do
> not show the frequency shift at all.
>
> Yes, it’s a bit of a contrived example, but it is why a common mode issue
> is going to be a problem.
>
> Bob
>
> > On Mar 31, 2022, at 6:38 PM, André Balsa <andrebalsa at gmail.com> wrote:
> >
> > Hi Bob,
> > I haven't taken a close look at the math behind the Three-Cornered Hat
> > method yet, but I would imagine there is a mathematical way to remove or
> at
> > least quantify the uncertainty ("jitter") introduced by the (separate,
> but
> > similar) GPS receivers in the three GPSDOs under test?
> >
> > On Fri, Apr 1, 2022 at 12:02 AM Bob kb8tq <kb8tq at n1k.org> wrote:
> >
> >> 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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