[time-nuts] Re: The amazing $5 timestamper, part 3 (and: how do you calibrate an LPRO-101?)

[Pluess, Tobias]

Hi Jeremy

when buying LPROs from eBay, be careful! I did this twice and I bought it
from the same seller. However, one of the "LPROs" I got is actually not an
LPRO but an SLCR-101. I have not found much information about this, but it
appears to be a cheaper version of the LPRO.

As far as I know, some of the LPROs have a small hole in the case where you
can insert a really small screwdriver and adjust some internal
potentiometers, but not all LPROs have this. Mine has this little hole.
The SLCR does not have an adjustment hole. You have to open the case to
adjust things.

Best
Tobias
HB9FSX

On Thu, Jun 17, 2021 at 1:11 AM Jeremy Elson <jelson at gmail.com> wrote:

> Fellow nuts,
>
> This week, I've been working more on my "$5 timestamper" based on the
> STM32G4 chip. I've finally been able to use it to get some nice results
> comparing the frequency of a couple of LPRO-101 rubidium frequency
> standards I have to SI seconds via a GPS receiver.
>
> My original email to this list on my new timestamper, in February, had a
> version of this experiment. Unfortunately it was flawed because there were
> still some bugs in the analog front-end of my timestamper that I had not
> yet discovered. As a result, there were discontinuities in the timestamps
> when the clock line going into the timestamping chip had noise (generating
> extra pulses) or wouldn't quite be high enough voltage to go over the
> chip's threshold (causing missed pulses). These have been fixed, as I
> reported in my second email on the timestamper (in April).
>
> I moved recently, and now that I have GPS set up in my new lab I was
> finally able to redo my February experiment to measure the frequency of two
> LPRO-101 rubidium standards I bought on eBay for about $200 each. The
> seller ("test_tool") claimed to have calibrated both before sale. However,
> I discovered the performance of one of them was almost two orders of
> magnitude better than the other. The test setup was:
>
> 1) The device-under-test (LPRO-101) was used as the 10mhz reference clock
> for my timestamping board.
>
> 2) An early eval board of a ublox M10 GNSS (EVK-M101) with a decent sky
> view, was configured to listen to 3 constellations (GPS, Galileo, GLONASS).
> I did not use location surveying so the accuracy is probably less than it
> could have been but the reported 3D position was quite stable.
>
> 3) The PPS output of the uBlox M10 was attached to one of the timestamper's
> input channels.
>
> I did this with two LPRO-101 units. The resolution of the timestamper is
> currently ~6ns, i.e. the inverse of the 170mhz clock speed of the chip. (On
> my todo list is to create another revision of my board with the higher-end
> STM32G4 chip that will get the timestamper resolution down to 184ps.) I
> plotted the error in the timestamps of the PPS signal with time, i.e. the
> x-axis is the time the experiment has been running in seconds; the y-axis
> is the difference between the actual timestamp and what the timestamp
> "should have been" if the timestamps were actually received exactly 1
> second apart. Ideally it would be a flat line indicating no frequency
> difference.
>
> The better of the two units showed a frequency error (t=10000s) of about
> 4e-11, which (as I understand it) is typical performance for an RbXO:
>
>
> https://www.circlemud.org/jelson/time-graphs/2021-06-16-gpspps-rubidium-unit2-test2-after-warmup.txt.time.plot.png
>
> The other unit was about 20x worse, about 1e-9:
>
>
> https://www.circlemud.org/jelson/time-graphs/2021-06-16-gpspps-rubidium-unit1-test2.txt.time.plot.png
>
> I'm very pleased with the performance of my timestamper, which seems to be
> working perfectly, and I look forward to getting it "cooked" enough to be
> able to share with all of you.
>
> I'm less pleased with the performance of one of the frequency standards,
> which leads me to my question: has anyone calibrated one of these things
> before? The manual I found online says I should be able to turn a
> calibration screw on the cover, but the units I have don't seem to have any
> exposed screws!
>
> Regards,
> -Jeremy
>
> PS: If needed, the raw data behind the two graphs below is here:
>
>
> https://www.circlemud.org/jelson/time-graphs/2021-06-16-gpspps-rubidium-unit1-test2.txt
>
>
> https://www.circlemud.org/jelson/time-graphs/2021-06-16-gpspps-rubidium-unit2-test2-after-warmup.txt
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