[time-nuts] Disciplining Rubidium

Tom Duckworth tomduck at comcast.net
Thu Apr 24 05:09:54 UTC 2008


Ulrich,

In the 'bad part...' you mentioned, I wasn't referring to a Rb oscillator
but rather the GPS received signal from the satellites. The GPS signal from
multiple satellites over a long period of time can be a good 'Transfer
Standard'. The atmosphere (moisture, temperature, pressure) are the greatest
contributors to GPS short term received stability. By collecting and
averaging out the short term (second, minute, day, month) variations over a
longer period of time, the variations are smoothed out. This averaged metric
is then used to steer the OCXO. No Rb involved. 

Tom
Tom Duckworth
510-886-1396
 

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Ulrich Bangert
Sent: Wednesday, April 23, 2008 9:44 PM
To: 'Discussion of precise time and frequency measurement'
Subject: Re: [time-nuts] Disciplining Rubidium

Tom,

> I think the best of all worlds would be a double-ovenized 
> SC-cut OCXO running at 5 MHz (lower mass). These OCXOs have 
> the lowest phase noise and best Allen variance short term 
> stability (1-100 seconds) of any xtal or Rb. Then have this 
> OCXO disciplined by the GPS

this is the good part of the idea.

> with an ephemeris of variations 
> constantly collected, statically averaged over a long period 
> (at least 1 month), and the calculated average used to adjust 
> the OCXO frequency.

this is the bad part. Have a look to rb specs concerning environmental
changes specially temperature or measure tempco of a rb yourself to see
that observation times of this order make no sense!

Best regards
Ulrich Bangert

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Tom Duckworth
> Gesendet: Donnerstag, 24. April 2008 06:24
> An: 'Discussion of precise time and frequency measurement'
> Betreff: Re: [time-nuts] Disciplining Rubidium
> 
> 
> John,
> 
> I think the best of all worlds would be a double-ovenized 
> SC-cut OCXO running at 5 MHz (lower mass). These OCXOs have 
> the lowest phase noise and best Allen variance short term 
> stability (1-100 seconds) of any xtal or Rb. Then have this 
> OCXO disciplined by the GPS, with an ephemeris of variations 
> constantly collected, statically averaged over a long period 
> (at least 1 month), and the calculated average used to adjust 
> the OCXO frequency.
> 
> Tom
> Tom Duckworth
> 510-886-1396
>  
> 
> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf > Of John Miles
> 
> Sent: Wednesday, April 23, 2008 8:02 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Disciplining Rubidium
> 
> Adding to Tom's worthy list:
> 
> 11) Short-term phase noise; the GPS-Rb sources don't seem to 
> be as clean as the better GPS-OCXO packages.
> 
> -- john, KE5FX
> 
> > > More precisely, if I had two black boxes, one containing
> > > a GPS-Rb-XTAL setup and another containing GPS-XTAL,
> > > what measurement would you make from outside the boxes
> > > to distinguish from one another?
> >
> > Ah, clever question. Here's ten ways to distinguish them...
> >
> 
> 
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