[time-nuts] Why would I want a rubidium

John Ackermann N8UR jra at febo.com
Mon Oct 5 17:34:29 UTC 2009


Here's a reason why you might "want" (as opposed to "need"!) another 
standard beyond the GPSDO.

An Rb or Cs is self-contained.  The GPSDO relies on an external factor, 
the presence of GPS.  Holdover mode may be good to keep thing close for 
a while, but over the long term the GPSDO without GPS is no better than 
its crystal oscillator.

The Rb is a secondary standard and therefore isn't "correct" by 
definition, but it has a low aging rate and gives you something 
independent of GPS to use for measurements.  Monitor it against GPS for 
a while to learn its offset and drift characteristics, and then you can 
extrapolate its performance out over a much longer time than you could 
with an OCXO.

You may not require that independence, but it gives you additional 
measurement capability.  For example, comparing the output of two GPSDO 
may not be meaningful because their frequencies could be correlated by 
their common view of the GPS constellation.  Using an Rb reference would 
eliminate that common mode error and reveal information about the 
GPSDO's short and medium term stability that would otherwise be hidden.

John
----
Joseph Gray wrote:
> I know I don't "need" any of this stuff. I was just wondering what I
> could do with a rubidium vs what I already have.
> 
> On Mon, Oct 5, 2009 at 10:58 AM, J. Forster <jfor at quik.com> wrote:
>> From an engineering point of view:
>>
>> Very few people really "NEED" a Rb or Cs as they are not really doing
>> anything that requires that level of precision in frequency or accuracy in
>> time (me included). These days a BC-221 or WWB is not good enough, but 1
>> Hz at X-Band is for almost all practical uses.



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