Hi Pretty much all Rb's tune by changing the magnetic field.
Bob On Jul 28, 2010, at 12:52 AM, [email protected] 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, [email protected] > 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. > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
