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

Fri Apr 1 12:19:27 UTC 2022

Hi

The simple answer is to use your three corner hat approach and
use a different source for each of the corners. Possibly a cheap 
Rb for one and a fairly good OCXO for another. Then make your
GPSDO the third corner. 

No, you donâ€™t eliminate all questions this way, but you do address
the common mode problem. 

To properly come up with accurate data then you are into the 
â€œwelcome to Time Nutsâ€ empire of â€œmany sourcesâ€ that you can 
compare to each other. You can then start sorting the good ones
from the bad ones. 

Yes, you could instead spend a lot of money on a commercial 
standard and just use that. Some do.

Bob

> On Mar 31, 2022, at 8:20 PM, Neal Pollack <nealix at gmail.com> wrote:
> 
> 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
>> â€œmoveâ€ forward in time by 15 or 20 ns over some time period.
>> 
>> The GPSDOâ€™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, itâ€™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, AndrÃ© 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 â€œcommon modeâ€ to all of the devices.
>>>> 
>>>> Bob
>>>> 
>>>>> On Mar 31, 2022, at 4:16 PM, AndrÃ© 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 canâ€™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, AndrÃ© 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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