[time-nuts] True Position GPSDP + Rb X72
holrum at hotmail.com
Wed Sep 20 10:43:51 EDT 2017
GPS disciplined rubidium oscillators are generally not a good idea. Rubidiums tend to be quite a bit more noisy than OCXOs. Their advantage is their long term frequency stability. The GPS system in a GPSDO compensates for the OCXO drift, so the only advantage of a GPS disciplined Rb is if you lose the GPS signal for a long time.
Also, remember Mark's Law of Rubidium Oscillators... the small the oscillator, the crappier it is. The X72 is a very small Rb oscillator and , guess what, it's rather crappy. Noisy, temperature sensitive, not all that good frequency stability. A decent OCXO can out-perform it.
Lady Heather now supports the X72 (and SA22.c). It has a disciplining routine that can lock it to a 1PPS input. It uses the X72's built in time interval counter that has a 16.667 ns resolution (due to noise and synchronizing issues more like 33 ns).
Later firmware versions of the X72 and SA22.c have a built in 1PPS disciplining routine, but I am not too impressed with it. It seems rather temperamental (or just plain mental) and I have seen it go off into la-la land and refuse to lock.
More information about the time-nuts