[time-nuts] DOCXO vs. Rubidium medium-term stability

[SAIDJACK at aol.com]

Hello Dave,
 
as some folks have already mentioned here, the best solution for you will  
depend on your specific requirements in terms of how much warmup time you 
have  before GPS is gone, and how much drift your solution can handle.
 
The PRS-10 is a good unit, but requires cooling, a large amount of  power, 
only has a single 10MHz and 1PPS output,  and it has a somewhat  noisy 
output in terms of phase noise and short-term-stability. It also costs  about 
$1500, is quite large, and does not provide a GPS receiver, nor one  especially 
optimized for timing.
 
You may want to look at the Fury or FireFly-IIA GPSDO units, these are  
lower cost, include the complete GPS sub-system, achieve performance similar to 
 the PRS-10 after sufficient warmup, are much smaller, lower power, the  
FireFly-IIA has a built-in isolated distribution amplifier, and don't have an 
Rb  lamp life limitation. Typical Fury DOCXO units can achieve better than 
1us drift  over 24 hours after they have fully stabilized, which is better 
than  many Rubidium references.
 
If you are looking for drift in the <10us range per day, you will need a  
double oven SC-cut OCXO.
 
You didn't mention if your application was airborne, in that case you may  
need a low-g sensitivity oscillator to avoid loss of short term stability 
and  increased phase noise due to aircraft vibration and acceleration. 
Rubidiums are  especially sensitive to airborne vibration such as caused by 
Turboprops,  Rotorcraft, etc.
 
Without having your specifications for the warmup time, thermal changes,  
and the desired drift, it is difficult to say if a single oven, double oven,  
Cesium, or Rubidium based unit would work for you.
 
Lastly, unless you are underwater or under-ground, GPS should be  available 
with a modern, good jamming-resistant receiver, and if it is a modern  
GPSDO will perform as well or better than a modern Rb.
 
bye,
Said
 
 
 
 
In a message dated 11/19/2010 14:43:43 Pacific Standard Time, lists at rtty.us 
 writes:

>  
> I had assumed that a Rubidium oscillator would give me the best  stability
> over the course of 8-12 hours. Obviously a Cesium would be  better but 
those
> are impractical due to cost and power constraints.  I've begun evaluation 
of
> a Rb oscillator but now I'm being told by  some people that a good DOCXO 
is
> likely to give me similar medium term  stability (with obviously better
> short-term stability).
>  
> Anyone here have thoughts on this? Obviously I'd rather go with a  DOCXO 
for
> cost and power reasons if there's no performance benefit to  be had using 
a
> Rb osc. The Rb unit I'm testing  
(http://www.thinksrs.com/products/PRS10.htm)
> seems to perform well but  I am interested in hearing others' thoughts.
> 
> Thanks,
>  
> Dave
> 
> ----------------------
> 
> Dave  Jabson
> Systems Engineering Manager
> Quasar Federal  Systems
> 5754 Pacific Center Blvd, Suite 203
> San Diego, CA  92121
> 858-412-1706
> www.quasarfs.com
>