[time-nuts] True Position GPSDP + Rb X72

Mark Sims 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.

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