[time-nuts] Fury - Rubidium - PIS
SAIDJACK at aol.com
SAIDJACK at aol.com
Wed Jul 28 04:52:53 UTC 2010
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 bandwidth 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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