[time-nuts] TIC resolution impact on GPSDO's performance

[Dr Bruce Griffiths]

Didier Juges wrote:
> This discussion is fascinating, and as always it has prompted a number 
> of other questions for me.
>
> I understand the sawtooth correction is provided to allow correction of 
> 1 PPS timing errors when the processor clock is non-coherent with the 
> GPS signal (at least not intentionally), because the processor clock is 
> running at a finite frequency and provides granularity to the PPS 
> adjustment capability.
>
> I wanted to know if this was available in my Trimble Thunderbolt GPSDO.
>
> I have looked over the entire Thunderbolt manual and I have not seen a 
> mention about sawtooth correction, so I am not sure if it is available.
>
> Then I realized the Thunderbolt is different from most GPSDO that have 
> been discussed, in that it uses the 10 MHz from the OCXO as the clock 
> for the GPS receiver, so in theory, the sawtooth correction should not 
> be necessary on this type of receiver since the processor clock and the 
> GPS signals are coherent (maybe I should not use that term...) when the 
> receiver is tracking.
>
> Yet, the Thunderbolt spec for timing accuracy  on the 1 PPS output is 
> +/- 20 nS at 1 sigma, just like an *ordinary* GPSDO with stand alone 
> receiver and what would be about a 25 MHz clock (please confirm).
>
> The Trimble software also displays values referred to as "Timing Output" 
> for the 1 PPS and the 10 MHz. The PPS value is currently hovering around 
> -1.xx to -2.xx nS and the 10 MHz value is around +/- 0.0x ppb. I am not 
> sure what that is based on. The manual simply refers to those as 
> "Estimate of UTC/GPS offset", but does not give any detail of how they 
> are computed or what they mean.
>
> The Thunderbolt User's Manual says that the 1 PPS output is the OCXO's 
> 10 MHz divided down. Yet, the block diagram shows the 1 PPS coming from 
> the CPU and support circuit, not directly coming from the 10 MHz. I 
> believe the 1 PPS is the 10 MHz divided down, except when the receiver 
> is doing jam-sync (after power up or recovery, when the GPS_PPS and 
> HW_PPS are far away from each other)
>
> Going back to what Poul-Henning just said, I believe that for the 
> Thunderbolt, there actually may be three PPS signals:
>
> GPS_PPS: what the receiver thinks the PPS should be (a theoretical signal)
> HW_PPS: the best approximation of the GPS_PPS that the receiver can do
> OCXO_PPS: the 10 MHz divided down
>
> In jam-sync mode, the Thunderbolt forces the HW_PPS to within 100nS of 
> GPS_PPS (the closest it can get using software delays and programmable 
> dividers I guess, using a 10 MHz clock) without touching the OCXO, so in 
> that mode, HW_PPS and OCXO_PPS are obviously not the same. Once the two 
> are within 100 nS, the Thunderbolt switches to discipline mode and fine 
> tunes the OCXO to get the HW_PPS as close to GPS_PPS as possible (20nS 1 
> sigma by specification.) It would appear that the receiver should be 
> able to adjust its OCXO as close to GPS_PPS as the DAC will allow, 
> without the artificial limit of the processor clock period. So, there 
> should not be a need for sawtooth correction and there should be no 
> hanging bridges either.
>
> Then, what are the "Timing Outputs"?
>
> Does this make any sense?
>
> Didier KO4BB
>
>
>
>
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>   
Didier

Look at:
http://trl.trimble.com/docushare/dsweb/Get/Document-8425/

There is little evidence of sawtooth corrections in graph in the above 
document, however the data were taken when SA was switched on.
To confuse matters further the document actually talks about pulse 
position quantisation.

http://trl.trimble.com/docushare/dsweb/Get/Document-8424/
Shows differences between 2 Thunderbolts, again there is little evidence 
of quantisation but data may have been smoothed.

Have you any reasonably stable crystal oscillator that can be used to 
log the time interval between the Thunderbolt PPS output and the divided 
down oscillator output?
Plotting these results should quickly show evidence of any sawtooth error.

As you say there shouldn't be any sawtooth error if the receiver timing 
is derived from the 10MHz clock which is in turned locked to the PPS output.
However there are other sources of noise in a GPS timing receiver which 
could, in an older receiver design like this, easily be as much as 20ns rms.

Bruce