Hi > On Apr 16, 2022, at 11:55 AM, John Ackermann N8UR <[email protected]> wrote: > > > > On 4/16/22 09:52, Matthias Welwarsky wrote: >> Dear list members, >> in 2020 John Ackermann published an evaluative survey of current day GPS and >> GNSS receivers (URL below). I have a question about figure 26, which shows, >> among others, the ADEV of a NEO-M8T against a Cesium reference, with >> quantization correction applied. The curve shows a significant "bulge" above >> 1e-10 between about 10s and 100s tau. >> I'm using a LEA-M8T in my DIY GPSDO, which I think is the same chipset in a >> slightly different package. I have attached an image worth about 3000 seconds >> of data, raw 1PPS phase difference against the LO used in the GPSDO. The >> GPSDO >> is locked, not in hold-over mode. >> The bulge between 10s and 100s is not really visible here. There is a slight >> bend, but not as pronounced. My explanation is that this is due to the LO >> being pulled by the GNSS receiver so that it is no longer fully visible. I >> reason that, were the LO more stable, more loosely coupled to the GNSS, I >> should see the bulge from figure 26. Would you agree? > > My theory of the ADEV flat spot when the M8T is qErr corrected is that > multiple things contribute to noise on the PPS output, and the qErr is only > one of them, and it consists of a noise source (clock granularity) that is > unrelated to any external analog process. > > Since Qerr has a fixed limit of ± XX nanoseconds (half the receiver clock > granularity), its contribution to the PPS noise decreases with longer > averaging times. Meanwhile, the other sources of noise such as ionosphere, > etc., are slower and become more pronounced as tau increases. At around 30 > seconds, the external noise factors become larger than the qErr. > > So at short tau cancelling out the qErr gets rid of a major noise source and > improves ADEV but as tau increases beyond ~30 seconds the qErr contribution > is gradually outweighed by the other noise sources and disappears, returning > the ADEV slope to its normal -1. > > Also note that in Fig. 26, at tau below 3 seconds it's possible that both the > M8T and F9T corrected plots are limited by the TICC resolution, which is > around 8e-11 @ 1 second on a good day. That's below these traces, but may > still be high enough to impact the measurement. > > Finally, it looks like you're comparing the raw PPS with an oscillator that > is steered by that same PPS. I have to think their correlation could lead to > possible and unpredictable errors. It would be better to have the OCXO > remain unsteered during the measurement.
The only way to make meaningful single comparison measurements of what’s going on is to compare to an external “free running” standard that has a stability adequate to the task. Normally this means an ADEV 5 to 10X better than what you expect to see on the device under test. Bob > > Best, > John > ---- > >> Best regards, >> Matthias >> https://hamsci.org/sites/default/files/publications/2020_TAPR_DCC/ >> N8UR_GPS_Evaluation_August2020.pdf >> _______________________________________________ >> time-nuts mailing list -- [email protected] -- To unsubscribe send an >> email to [email protected] >> To unsubscribe, go to and follow the instructions there. > _______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send an > email to [email protected] > To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
