[time-nuts] Home made GPS disciplined atomic clock
SAIDJACK at aol.com
SAIDJACK at aol.com
Sun Jan 25 19:31:38 UTC 2009
Hi Bruce,
since the LPRO has significantly worse STS (according to this thread) than
the Tapr Tbolt itself, then using the LPRO would only make sense if GPS is not
available, and the unit is in holdover. This is similar to what you mentioned
with the long time-constant. One would not want the LPRO to make the Tbolt
worse than it is when perfectly locked to GPS.
Our Fury and FireFly-II units allow an external 1PPS input to be connected,
and the switchover will automatically happen if the internal GPS goes into
holdover.
Using the LPRO on the external 1PPS, and selecting auto-switchover would
give the best of both worlds: the excellent ADEV over all measurement intervals
when GPS is available, and the Rubidium stability when GPS is out for longer
time periods.
Another advantage of this is that when the Fury/FireFly-II is using the LPRO
1PPS it will act as a cleanup-filter for the LPRO, and one would not lose
the better STS of the Fury/FireFly OCXO.
I am not sure if the Tbolt has an external 1PPS fail-safe backup input, I
could not see one on the PCB.
bye,
Said
In a message dated 1/25/2009 11:23:53 Pacific Standard Time,
bruce.griffiths at xtra.co.nz writes:
Given the large PPS output jitter wrt to the OCXO output frequency, this
is probably a bad idea.
There's nothing wrong with the idea of using a rubidium standard, you
just need to cleanup its output first by phase locking a low noise OCXO
with a suitable loop time constant to the rubidium output first. Use the
cleaned up output as the 10MHz signal for the Thunderbolt and lock the
rubidium standard to GPS using the thunderbolt with a suitably long loop
time constant.
This should result in low phase noise and drift during holdover.
Bruce
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