[GPS_Standard] GPS and Rb standard

[Geoff Blake]

Bert wrote:

>Hi Bob,

> Yes, I have read the article. The author does mention that the
> Rb oscillator can be disciplined using GPS-derived systems (such
> as mine). While this is true, I don't see the real benefit of
> doing this. Here is why.

> Rb oscillators have a good long term accuracy, so they drift
> much less over a long period (months, years). They perform
> much better than OCXOs in free-running applications. But this
> comes at a price: around $300 on the used market. So their prime
> target application is a standalone, non-disciplined reference.

> In GPS-disciplined applications, the Rb oscillator's inherent
> long term stability is lost since you adjust the oscillator
> every hour or so. Why would you pay this much when you can get
> the same long term performance for 1/4 to 1/3 the price with an
> OCXO?

> Rb oscillators have other drawbacks. Their short term stability
> and phase noise characteristics are worse than good OCXOs (such
> as the HP units). The Rb lamp has a limited operational life,
> usually guaranteed to be at least 10 years when purchased new.
> Do you know how much life is left out of that unit you see on Ebay?
> Also, Rb oscillators are fancier devices and adjusting them
> is not as trivial as tweaking a potentiometer (I have not tried
> it myself but this my understanding, so don't quote me on this).
> There are a few adjustments and doing a good job requires
> knowledge of the inner guts, field adjustments, etc....

> There is a good summary of the relative performance of different
> oscillator types at the following IEEE link:

> http://www.ieee-uffc.org/freqcontrol/quartz/vig/vigcomp.htm

> I hope I have answered your questions.


Hi Bert, Bob,

[This is the second iteration of posting - seems that I was too
fast for the system :-) ]


I am a newbie to the list, but professionally I have been around
frequency standards for a few years now and hope that I have
something to offer.

At first view I would agree with you regarding the frequency
control of a Rb (or Cs) standard. However I would still use a GPS
(or other) receiver system to MONITOR any form of frequency
standard.

Currently, at the day job I have a R&S XSRM (a Rb standard which
disciplines a 5MHz OCXO) which is controlled by the accompaning
60kHz receiver via a rather long loop. I believe that the time
constant of the (digital) loop is several days! The system itself
attains an accuracy of somewhat better than 2 parts in 10^11, but
has eaten two lamps in its 30 years or so of use.

However, if I had a Rb standard at home I would use a GPS to
provide a phase comparison signal to monitor the stability of the
Rb standard.



Geoff

-- 
    Geoff Blake G8GNZ      located  near  Chelmsford,  Essex,  U.K.
    Please reply to:      geoff(at)palaemon(dot)demon(dot)co(dot)uk
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