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[time-freq] Re: time-freq Digest, Vol 22, Issue 1

K&J Aggett kj.aggett at optusnet.com.au
Fri Feb 8 22:10:27 UTC 2008


Hi Randy,

Another approach to the subject is to understand that, all things being 
equal, the cesium, GPSDO and rubidium standards all rely on a crystal 
oscillator to provide short term stability.  The atomic parts of the 
cesium and rubidium standards "only" provide long term stability, while 
the GPSDO requires satellite lock for its long term stability.  Both 
atomic standards required, in my time at least, careful analogue 
adjustments in order to achieve good performance.  As the "owner" of one 
cesium, these adjustments would take from hours to days to show the 
results, being checked against other cesium standards by radio.  
Rubidium's, as John has said, are less stable than cesium standards.  It 
was possible for the control systems of either standard to lose control 
and it might take hours/days before you found out.  A GPSDO can also 
require adjustment for optimum performance, but nowhere near as complex.

As far as I am concerned, GPSDO's have distinct advantages.  I am not in 
the position to say whether or not a GPSDO is a better standard than a 
rubidium, but I can say that you can easily find out when you lose 
satellite lock.  Both cesium and rubidium tubes have a finite life 
which, for a GPSDO, is someone else's problem.  Further, for not much 
more than the cost of a suitable crystal oscillator and GPS system, 
anyone can build a GPSDO standard.


Hope this perspective helps,

Keith.
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 8 Feb 2008 09:15:08 -0800
> From: "Randy" <hamradio at oz.net>
> Subject: [time-freq] Rubidium standards
> To: "'TAPR time and frequency projects'" <time-freq at lists.tapr.org>
> Message-ID: <030301c86a76$283d2480$6601a8c0 at Laptop>
> Content-Type: text/plain;	charset="US-ASCII"
>
> I see advertised once in a while Rubidium frequency standards that include
> the following statement:
>
> "The Rubidium standard is carefully calibrated to a GPS Disciplined
> Oscillator."
>
>
> I thought those standards were based on the atomic physics of Rubidium and
> that was what determined the frequency.  Why do they need to be "calibrated"
> at all?  This also suggests that a GPS-disciplined oscillator is better than
> the Rubidium standard.  It that true?  Are the HP Z3801 oscillators, for
> example, ordinarily more accurate than a good Rubidium standard?
>
> Thanks!
>
> Randy Stegemeyer, W7HR
> Port Orchard, WA
>
>
>
>
>
>
>
> ------------------------------
>
> Message: 2
> Date: Fri, 08 Feb 2008 12:23:56 -0500
> From: John Ackermann N8UR <jra at febo.com>
> Subject: Re: [time-freq] Rubidium standards
> To: hamradio at oz.net, 	TAPR time and frequency projects
> 	<time-freq at lists.tapr.org>
> Message-ID: <47AC902C.60205 at febo.com>
> Content-Type: text/plain; charset=US-ASCII
>
> Randy said the following on 02/08/2008 12:15 PM:
>   
>> I see advertised once in a while Rubidium frequency standards that include
>> the following statement:
>>
>> "The Rubidium standard is carefully calibrated to a GPS Disciplined
>> Oscillator."
>>
>>
>> I thought those standards were based on the atomic physics of Rubidium and
>> that was what determined the frequency.  Why do they need to be "calibrated"
>> at all?  This also suggests that a GPS-disciplined oscillator is better than
>> the Rubidium standard.  It that true?  Are the HP Z3801 oscillators, for
>> example, ordinarily more accurate than a good Rubidium standard?
>>     
>
> Hi Randy --
>
> An Rb standard is based on atomic physics, but it's not intrinsically
> accurate because there are various effects that can move its frequency.
>  So, while a Rube is much more stable than a crystal, it still needs to
> be set to the correct frequency and checked every now and then.
>
> Even a Cesium standard, which is by definition "correct" has a certain
> amount of uncertainty and will be in error by some amount; if you have a
> good source (like a GPS or GPSDO), you can fine tune it to more closely
> match the national standard.
>
> A good GPSDO is a very good all purpose standard, with the short term
> stability of a crystal and the long term accuracy of GPS.  Its weakness
> is that if it loses its GPS signal, it will quickly revert to normal
> crystal performance.  An Rb or Cs doesn't need an external reference to
> retain its accuracy, and that self-contained nature can have some
> advantages.
>
> 73,
> John
>
>
>
> ------------------------------
>
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>
> End of time-freq Digest, Vol 22, Issue 1
> ****************************************
>
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