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

SAIDJACK at aol.com SAIDJACK at aol.com
Sat Nov 20 02:36:11 UTC 2010


Hi Dave,
 
forgot to mention:
 
The PRS-10 also has a limited temperature range only up to +65C, so  
military applications are a no-go. A good DOCXO will have +75C or even +85C  
capability.
 
Also, the spec for the PRS-10 is 1.18E-012 per Degree C temperature change, 
 and the units I mentioned before with the DOCXO are available  in better 
than 2E-012 per Degree C over a wider temp range, so are very  similar in 
performance over temperature.
 
bye,
Said
 
 
 
In a message dated 11/19/2010 18:12:13 Pacific Standard Time,  
SAIDJACK at aol.com writes:

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
>   

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