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

[Didier Juges]

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