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 readout, It takes less than 10 seconds, to measure the freq offset to well under 1e-11. 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. ws ************************* "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." bob 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. Bob
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