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

[Dr Bruce Griffiths]

David I. Emery wrote:
> 	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.
>   
The mix and divide technique (at the cost of increased complexity) also 
reduces the close in phase noise, which for a DDS at 10Hz offset from 
the carrier can be 20-30dB worse than a low phase noise crystal 
oscillator of the same frequency as the DDS output. Even for ultrastable 
OCXOs (and other frequency stable sources) that have drifted out of the 
adjustment range only require a DDS correction range of a few ppm to 
make them usable as frequency standards. Consequently it is relatively 
easy to significantly reduce the close in phase noise by 40dB or more 
with a well designed mix and divide technique whilst allowing a 
frequency adjustment range of 10ppm or more. Eventually the close in 
phase noise is limited by the dividers used. Regenerative dividers have 
far lower noise than digital dividers so they can be useful for the last 
mix and divide stage when the ultimate phase noise performance is desired.

Combining this with GPS carrier phase tracking techniques to discipline 
the frequency reference should result in near hydrogen maser 
performance, at least for short observation times.

Bruce