[time-nuts] GPSDO Question - Clarification Please

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Sep 2 19:49:15 EDT 2007

Thomas Linbeck wrote:
> I am confused by the PLL vs. FFL lock discussion.  Does this concern the
> Schera board vs. the VE2ZAZ methods of time base discipline?  Can someone
> share their thoughts on the subject?
> Thanks!
> 73 de K4TEU (Tom(

The discussion is somewhat more general than the Schera (PLL) board
versus the VE2ZAZ (FLL) board.

The Schera board is not the only way to implement an inexpensive PLL.
However don't fall into the trap of locking with a short loop time
constant to the 10KHz output of a Conexant/Navman Jupiter GPS receiver,
it has no better stability than the receivers PPS output.
The Schera board is certainly not (at least with a good modern GPS
timing receiver) the optimum technique for implementing a low cost high
performance GPSDO.
That said, its performance should be considerably better than that of an
FLL implementation using the same OCXO.
It is of course possible, with modern components to implement a
technique somewhat akin to the Schera method with far fewer ICs.

When one is trying to lock an oscillator to a passive atomic standard
(Rubidium absorption cell, Caesium beam tube, mecury ion trap, etc) then
a frequency lock loop is optimum.
In this case a phase lock loop will not work as there is no signal for
the PLL to lock onto. An FLL (using the Pound or similar technique) can
also be used with a passive standard like a  Sapphire whispering gallery
resonator or a quartz passive quartz crystal.

When one has an active frequency standard (hydrogen maser, GPS,
Rubidium, Caesium standard) a PLL is usually the optimum technique.


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