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

[Bob kb8tq]

Hi

> On Apr 16, 2022, at 11:55 AM, John Ackermann N8UR <jra at febo.com> 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
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