[time-nuts] Fury - Rubidium - PIS

Bob Camp lists at rtty.us
Wed Jul 28 07:22:07 EDT 2010


Pretty much all Rb's tune by changing the magnetic field. 


On Jul 28, 2010, at 12:52 AM, SAIDJACK at aol.com wrote:

> Hi Scott,
> yes, Rb's tend to have many orders of magnitude smaller control ranges than 
> OCXO's, so it takes longer to phase lock. What's even worse is hysterisis, 
> eg.  we have seen units that jump back and forth between two control 
> voltage values.  This tends to happen in TCXO's and Rbs from what I can tell, not 
> so much in good  OCXO's.
> It seems that these units do not change their output frequency until the  
> EFC voltage change reaches some threshold, then the frequency jumps in a 
> large  value. OCXO's seem to react instantly to even the slightest LSB changes 
> on our  24 bit DAC. Many Rb's and TCXO's seem to have this spring-like 
> effect, and that  causes some "chasing" of the phase as the oscillators "ignores" 
> the EFC voltage  changes until they reach a certain level, at which time 
> they over-compensate and  the effect reverses. 
> I am wondering if adding some dithering noise would help prevent this  
> effect, where the noise bandwidt is below the Rb's control bandwidth so as not  
> to add ADEV instability...
> Does anyone know how popular Rb's adjust their frequency? Is it through  
> linear methods such as Varactor diodes? Or do they use an ADC to sample the 
> EFC  voltage, thus creating quantization errors that could be the cause of the 
> hysterisis we sometimes see?
> Brian, on a locked DOCXO unit (after a day or so typically) we  would like 
> to see below 5ns SD on the GPS TI. Good DOCXO units regularly achieve  
> <2.5ns. No reason a good Rb should not be able to achieve that as well. The  
> DOCXO unit running at the University of Colima for example regularly achieves  
> ~1.8ns SD. For some reason the TI indicator has not been visible for some 
> months  now though, but the plot still speaks for itself. Notice also the small 
> control  voltage range of less than 50 microvolts typically:
>   _http://resco.ucol.mx/Fury/gpsstat.htm_ 
> (http://resco.ucol.mx/Fury/gpsstat.htm) 
> bye,
> Said
> In a message dated 7/27/2010 20:11:00 Pacific Daylight Time, smace at intt.net 
> writes:
> After  _MANY_ hours of watching and playing with different 
> settings, I came to  the conclusion that you just need to be patient when 
> using Fury+Rb, don't  expect it to converge like an OCXO, but it will 
> over a few hours and give  you a good result as long as you have stable 
> power and shield it from air  streams. 
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