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

Bob Camp lists at rtty.us
Sat Nov 20 03:06:11 UTC 2010


Hi

If high temperature is an issue, keeping the Rb cool will be a major chore. The OCXO will be far more "happy" at 75 than the Rb will be at 65. 

Depending on just how mobile we're talking about, the OCXO may have some issues with 2G tip / acceleration. 

There's a lot to consider in a setup like this and without a bit more data we're going to head off into crazy land pretty fast.

Bob


On Nov 19, 2010, at 9:36 PM, SAIDJACK at aol.com wrote:

> 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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