[time-nuts] Rb vs.Crystal OCXO

Magnus Danielson magnus at rubidium.dyndns.org
Fri Apr 25 18:44:33 EDT 2014

On 04/25/2014 10:30 PM, Tom Van Baak wrote:
>> Hi Everyone,
>> I'm newer to this forum but I really enjoy reading the discussions. I have
>> a pretty basic question.
>> I'm wondering why one would chose an Rb Oscillator over a traditional OCXO?
>> It does not immediately appear there is a phase noise advantage in the Rb..
>> Thanks,
>> ~Shane
> Welcome to time-nuts. Good question.
> In some applications the exceptional long-term frequency accuracy and stability you get with Rb is more important than the better phase noise, smaller size, lower power, better short-term stability, longer lifetime, or cheaper price that you get with an OCXO.
> When you buy a new OCXO, it might be accurate to 4 or 5 or 6 digits, out-of-the-box. You have to mechanically (screw) or electronically (EFC) calibrate it to get closer. If you want, say, 9 digits of accuracy, then you may have to re-calibrate it every day or at least once a week.
> When you buy a new Rb, it might be accurate to 9 or 10 or 11 digits, out-of-the-box. If you want, say, 9 digits of accuracy, then you may have to calibrate it every couple of years.

Size, weight and cost may also be issues.

In todays age of modernization, a well-sized OCXO or rubidium isn't an 
option, and well, smaller OCXOs exist while rubidiums starts to become 
really scarse below a certain volume, and when the volume come downs 
further, the CSAC is the only atomic reference and then you only see 

Cost-wise some high-performance OCXOs gives the rubidiums a match 
performance-wise (even if they need a bit more trimming) but not 
price-wise or even volume-wise.

Them some telecom rubidiums just isn't very good at phase-noise, but 
have fair stability for the size and cost.

In the end, there is no one choice which always fits, it *really* 
depends on what you need it for, and the requirements, then it plays 
itself out as you look through the options.


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