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, [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.
