[time-nuts] Why discipline Rubidium oscillator?

[Bob kb8tq]

Hi

It’s very much a “somewhere near that number” sort of thing with an Rb. The 
“thing” you are looking at is quantum mechanical in nature. Unfortunately that
by its self does not make it perfect. A beam tube (as opposed to a gas cell) 
isolates things better. 

A 5061 is a beam tube device. A 5065 is gas cell based. It is very important to note that
accuracy and stability are two different things …. The beam tube is more accurate. 
The gas cell is more stable (over some range of tau). 

A normal Rb standard has a bit of this and that in the bulb. These other gasses
help in various ways. They each also add a bit of “pull” to the frequency one way
or the other. They get you away from your “magic number” but the benefits they
deliver are worth the trouble. The exact gas mix gets into the “secret sauce” of
the Rb manufacturer. They each optimize things a bit differently. The walls 
of the bulb get into the act ….

Beam standards are actually a bit old these days. The more modern approach 
would be a fountain (toss the ion straight up and let it fall back to you). An even 
more modern approach would be a trapped ion standard. The amount of money
involved goes up dramatically with each of those steps. You get rid of this and 
that subtle effect with each improvement. Accuracy gets better and better. 

Lots of choices !!!

Bob

> On Nov 20, 2017, at 3:28 PM, Jerry Hancock <jerry at hanler.com> wrote:
> 
> Bob, I was referring to the rubidium standard of 6834682610.904 Hz.  For some reason I thought it was closer to 9Ghz.
> 
> I assume then rubidium standards oscillate (if that is the correct term) somewhere around that number but not exact or is it in the detection where things fall down?
> 
> 
> 
>> On Nov 20, 2017, at 11:40 AM, Bob kb8tq <kb8tq at n1k.org> wrote:
>> 
>> Hi
>> 
>> 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 ….
>> 
>> Bob
>> 
>> 
>>> 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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