[time-nuts] Re: Question about GPS 1PPS ADEV

[John Ackermann N8UR]

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.

Best,
John
----

> Best regards,
> Matthias
> 
> https://hamsci.org/sites/default/files/publications/2020_TAPR_DCC/
> N8UR_GPS_Evaluation_August2020.pdf
> 
> 
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe send an email to time-nuts-leave at lists.febo.com
> To unsubscribe, go to and follow the instructions there.