[time-nuts] Home made GPS disciplined atomic clock

SAIDJACK at aol.com SAIDJACK at aol.com
Sun Jan 25 14:31:38 EST 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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