[time-nuts] Two types of GPDSO / Rubidium

[Taka Kamiya]

This is a cross post from EEVBLOG.  I'm hoping there is someone who's familiar with this subject would help me out here.
I am hoping someone can help me understand this.  I've seen similar discussions take place both for GPSDO with Crystal Oscillators and Rubidium modules.  It appears there are two types of each.  
1)  fixed frequency type (less jitter)
2)  frequency agile type (more jitter)

I've read frequency agile Rb modules (ones you can change output frequency) is one kind of Rb (sa.22c and fe5650, etc), and there is another one that you cannot change frequency. (ie. T-bolt, PRS10, etc).  Words like phase noise and PLL are thrown out often in discussions.  I vaguely remember frequency agile types are less suitable if ultimate in stability is needed such as multiplying the output into GHz range.  This discussion was about 10GHz transverter.
Is this because frequency agile type has the ultimate output from PLL (subject to jitter) and fixed frequency type is from OCXO?  If this is the case, why frequency agile type even exist?  It's not like it can be used as a VFO (on radio).....

I'm sorry this is SO vague but that's the reason for this post.  I need to understand this.  There was a wiki page on this, but it doesn't go into this discussion deep enough.

Would someone help me gain knowledge in this?

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(Mr.) Taka Kamiya
KB4EMF / ex JF2DKG