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

[Jeremy Elson]

After taking the cover off the bad LPRO I finally found the calibration pot
and realized why I hadn't seen it before: the hole in the case that allows
access to the pot was covered by a calibration sticker.

However, turning it seemed to have no effect at all on the frequency. I
wonder if the unit is just broken.

Is there anywhere I can send the thing for repair?


On Thu, Jun 17, 2021 at 4:20 AM Pluess, Tobias <tpluess at ieee.org> wrote:

> 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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