[time-nuts] Fury - Rubidium - PIS
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:
> In a message dated 7/27/2010 20:11:00 Pacific Daylight Time, smace at intt.net
> 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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