[time-nuts] Thunderbolt Loop Damping
holrum at hotmail.com
Mon Jul 7 23:49:39 EDT 2008
The Thunderbolt attempts to align the specified PPS and OSC edges with the GPS satellite time signal. The PPB parameter is its estimate of the error of the oscillator edge from where it needs to be. They specify it in mysterious parts-per-billion, but never really say billions of what. They do say positive values mean the 10MHz signal is running slow relative to GPS time.
My ADEV plotter takes the PPS error in nanoseconds and adds 1.0 to it to get a per-second phase error. To get comparable values from the OSC PPB value, divide it by 1E9 and multiply by 100. Add that to 1.0 seconds to get a per-second phase errorof the OSC signal. The resulting ADEV plots of PPS and OSC PPB are off by a decade below 20 tau, and match very closely above 100 tau. The OSC PPB is worse below 100 tau (4.5e-9 vs 4.5 e-10 at 1 tau, both around 4e-11 at 100 tau) by 10,000 tau the OSC PPB is slightly better than the PPS signal (both in the 5e-13 range).
As far as messing with the discilining constants, I figure Trimble has them fairly well optimized for their OXCO. They probably only need to be changed if you graft a different OCXO onto the unit.
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