[time-nuts] Two types of GPDSO / Rubidium

[Bob kb8tq]

Hi

The first gotcah is that the distinction is not very clear. A TBolt varies the frequency of
the OCXO in the GPSDO. GPSDO’s that use Rb’s can and often do vary the frequency. 

Indeed some devices use a DDS to generate an output rather than varying something else. 
I suspect that is what you are bumping into. In some cases a DDS feeds a cleanup loop or filter
 that restricts the output range. 

If you are after low noise at 10 GHz, the multiplier chain needs to be carefully designed. There
is a lot more to it than you might think. The first and most basic question would be “is it a fixed
frequency output or tuned?”. From there you can head off in a number of directions. 

If you need to be able to change frequency from 1 MHz to 10 GHz in a few nanoseconds, then 
there is no alternative to using a DDS. If hat is part of the requirement then you go from there. If
you want only 10 GHz +/- 0.001Hz then there are other, more quiet / lower spur ways to do the job.

Indeed it goes on and on …..

Bob



> On Sep 25, 2019, at 4:45 PM, Taka Kamiya via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 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?
> 
> --------------------------------------- 
> (Mr.) Taka Kamiya
> KB4EMF / ex JF2DKG
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