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 <tplu...@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 <jel...@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 > > _______________________________________________ > > time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe > send > > an email to time-nuts-le...@lists.febo.com > > To unsubscribe, go to and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send > an email to time-nuts-le...@lists.febo.com > To unsubscribe, go to and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.