[time-nuts] Unique TBolt GPS characteristics

WarrenS warrensjmail-one at yahoo.com
Sun Jan 25 21:29:47 EST 2015

Another unique thing about the TBolt engine is how fast it can calculate a
Freq change in its 10MHz osc.
Over short times periods, it can be 100 times better than a standard 1PPS
GPS engine.
It would be interesting to compare it to a high end dual freq GPS.

With the Tbolt in manual hold over mode, and using Lady Heather for the
It takes less than 10 seconds, to measure the freq offset to well under
Typically about 1e-11 freq error in 3 seconds
When setup to use an external freq input, it makes a fast 11 plus digit,
10 MHz freq counter.

The PPT output (osc Freq offset) of the Tbolt, responds to any freq changes
in the next 1 sec output.
How it does this is a mystery to me, because it is not using phase error.
The limit is the noise which is very low.
In a well set up Tbolt, the 1 second PPT freq noise is about 10ppt RMS
(50e-12 PP).
5e-12 with 5sec filter, 3.5e-12 with 10 sec filter and 2.5e-12 with a 50
second filter.
Attached is an old  plot showing the noise floor difference between the
TBolts phase and Freq outputs.

The Uncertainty of a non-sawtooth corrected 1 sec GPS can be around 10 ns
plus, with sawtooth correction that can go down to near 1ns uncertainty
The GPS signal its self has up to a 10 ns uncertainly, so using the 1PPS
signal out of a common GPS engine you get at best a 1e-8 to 1e-9 freq
uncertainty using two readings 1 second apart.


"This operation is very typical of all of the cell site GPSDO's. The only
part that is unique to the TBolt is the ability to fiddle the loop
characteristics a bit."
And the fact that the GPS's CPU clock is derived from the 10MHz and
therefore aligned to the PPS so there is no hanging bridge and sawtooth
correction is not required.

I am not aware of any other GPSDO implementing that scheme, which is very
elegant in its simplicity.

Didier KO4BB

It is indeed an elegant solution. Based on looking at 1 pps outputs on a
group of them over a year or more,  It's actual impact on the control loop
function is pretty minor compared to a properly executed sawtooth correction
process. It would have a significant advantage if compared to a GPSDO that
does not use sawtooth correction.

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