[time-nuts] Why discipline Rubidium oscillator?

Bob kb8tq kb8tq at n1k.org
Mon Nov 20 14:40:07 EST 2017


There is no direct relation for an Rb to 10 MYz. Cs beam tubes are what have a direct relation. 
Even then, the qualifier is “under standard conditions”. They are sensitive to magnetic field. Rb’s
also are sensitive to magnetic field. Both can be tuned by varying the field. In the case of an Rb
that also takes care of a multitude of other issues.

In the case of Rb, there is a distribution of cells coming out of the manufacturing process. Some
are pretty close to the “right” frequency. Others are way off (as in 100’s of KHz or more). All of them
are capable of meeting the required specs. DDS techniques allow those cells to be used in a 
production part. That increases the yield and thus drops the production cost. 

Since you now magically have a DDS in the Rb, you can do all sorts of interesting things. If you
suddenly need a 9.99900 MHz standard …. here it is … If you need to do temperature compensation 
via a lookup table … it just takes a bit of testing and some code to make it happen. Indeed, the DDS
does also give you some issues. Without some sort of cleanup oscillator, you will have spurs and 
phase noise on the output.

Lots of fun ….


> On Nov 20, 2017, at 1:34 PM, Jerry Hancock <jerry at hanler.com> wrote:
> I know this is going to sound dumb as I know many GPSDOs had rubidium oscillators in them.  I can see why, in that during holdover, they would tend to be more stable vs others, but given that there is a direct mathematical relationship between the rubidium frequency and potentially the 10Mhz desired output frequency, why do they have to be disciplined or better yet, what advantage does it bring?  Also, I can see how you discipline a DOCXO with the external voltage, how do you discipline a rubidium?  Pulse stretching?  
> I guess I don’t understand how the technology works, but it seems like an RF signal is swept that would be used to detect a dip at a pretty well defined frequency.  This dip can be used to discipline the oscillator to something like 9Ghz or a factor of what, 900+ times better than 10Mhz.  So wouldn’t that be able to get your desired 10Mhz to 10,000,000.001 or pretty much my level of measurement?  Or does is the dip not quite that precise?  If you can point me to a write-up on this I’ll go away.
> Thanks to Gilbert for providing me with at least one rubidium oscillator that is working out of 5 though 2 others seems to stay locked for a few hours during my testing.
> Jerry
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