[time-nuts] Question about GPS 1PPS ADEV

[Matthias Welwarsky]

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?

Best regards,
Matthias

https://hamsci.org/sites/default/files/publications/2020_TAPR_DCC/
N8UR_GPS_Evaluation_August2020.pdf
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