[time-nuts] Austron PRR-10 GPS discliplined Rb...

[David I. Emery]

On Sat, Jan 27, 2007 at 01:58:58PM +1300, Dr Bruce Griffiths wrote:
> > 	This means they can take a slightly off frequency but stable 10
> > mhz and make a precisely on frequency and even more stable 10 mhz locked
> > to GPS when GPS is available and open loop corrected  to the last GPS
> > offset values when GPS is not using both  measured frequency offset and
> > change of frequency offset with time (and I think temperature).


> This technique is suitable for use with any of the ultrastable
> oscillators on the surplus market whose frequency has drifted outside
> the efc and manual adjustment range. Since 48 bit DDS chips are readily
> available, adequate adjustment range and resolution is available to
> discipline oscillators that are 100ppm or more off frequency. 

	Apparently the Austron/Datum versions (they held a patent on
this) have adjustment in the better than 10^12 area but the PRR-10 is a
pretty old design and one could certainly do better with a modern NCO
chip.

	The PRR-10 and other Austron designs I am vaguely familiar with 
apparently use a 20 MHz VCXO to clean up after the DDS chip.   Given the
right choice of loop parameters (and the inherently narrow bandwidth of
the VCXO EFC) this should knock down DDS spurs a lot I should think. I'd
imagine spurs and phase noise out beyond a few hertz would be based
entirely on the VCXO phase noise performance and of course EMI issues in
the board design, not the DDS.   And most DDS's don't have all close in
spurs, though their close in phase noise I guess depends on analog
issues in the DAC and clock jitter.

> Another option is to use a low noise reference OCXO that has no
> frequency adjustments using either varactors or trimmer capacitors,
> perhaps enhancing the oscillators stability somewhat.

	I think that is what the Austron folks claimed is the big win
with the technique.   And indeed I imagine that various materials
effects in the magnetics of a Rb might well mean that is more stable if
left alone than tweaked with EFC changes to the C field coil current (I
am only speculating, and someone on this group probably knows much much
more than I do about such issues).

	And there is little doubt that the Telcos used PRR-10s with
cesiums as 10 MHz input on occasion or other standards which provided no
EFC ability to complete the loop.

	The PRR-10s (which have sometimes gone pretty cheap on Ebay) are
a kind of amusing surplus toy, however... they will take an external
input 10 MHz and tell you how far off GPS it is and what kind of Allan
deviation it exhibits.   I first met one (from Ebay) in 2003 that was a
casualty of the Enron collapse (no kidding, the GPS location in the
EEPROM was the Enron building in Houston according to Google Earth) and
played with it then to monitor another disciplined Rb here for a while.


-- 
   Dave Emery N1PRE,  die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."