Rubidium frequency references are OK but consideration needs to be given to the fact that they are trimmed onto frequency by a calibration source....so you really need a GPSDO to calibrate them ! Why have both if the GPSDO may be used (you need an external aerial)..
I use the rubidium (a FE-5680A) for day-to-day use and the GPSDO (a Rapco 1804M modified for 10MHz output) occasionally. This is mostly because I don't run the GPSDO continuously and my experience is that it takes 10 days to two weeks to
really settle down.
Eg I've had both on for a couple of weeks while I bed in a new OCXO for a Racal 1992. The rubidium locks in less than 5 minutes and drifts very slightly for a few minutes after that but then is stable.
Given adequate signal the GPDSO achieves "fine" lock within a few tens of minutes to a couple of hours. Actually I don't have an adequate antenna, just a puck style thing; a decent one is on the "would like" list but what I have has to do for now.
Fine lock means within a couple of parts in 10
10, the manual doesn't say exactly. However I checked last week and they were about 3 in 10
11 apart.
Had another look this evening and the relative phase of the GPSDO output and the Rubidium hasn't changed at all in the last two hours so we're now very close to each other, probably 1 in 10
12 or better.
Since I doubt very much that the rubidium unit has drifted in the last two weeks (they're extremely stable while they work) I'm assuming that it is reasonable to interpret this as
1) The GPDSO has been slowly converging on something better than 1 in 10
11 over that past two weeks.
and
2) The rubidium was spot on - at least as close as I can measure.
But, as I said above the kicker is that the rubidium can give me this in 10 minutes after switch on.
Of course I now need a third time standard to be really confident about the other two.................