[time-nuts] ***SPAM*** Re: Why discipline Rubidium oscillator?
michael.cook at sfr.fr
Tue Nov 21 02:39:34 EST 2017
> Le 20 nov. 2017 à 20:53, Dana Whitlow <k8yumdoober at gmail.com> a écrit :
> In my pre-retirement job I rode herd on an active Hydrogen maser system,and even
> that has a clear drift tendency. Generally a couple or three times per
> year I had to make a frequency adjustment in the neighborhood of 3E-14. And still being
> privy to its performance, I was amused to note that its drift tendency was
> interrupted by the hurricane Maria. On the day of eye passage over the site the frequencysuddenly
> decreased by a few parts in 10^14, held about constant for roughly a week,then
> resumed almost its original value and drift rate thereafter. If anybody inthis group
> can explain* that* behavior (that is, held for a week before resuming old
> habits), I’d love to learn about it.
You don’t mention the make of the instrument, but I suspect the same basic technology is used by all.
To quote from the Oscilloquartz page on their CH1-76A product:
« The quantum device is used as a frequency discriminator in an automatic frequency tuning system of a crystal oscillator. »
They don’t however quote stability relative to air pressure. However…..
It is known that atmospheric pressure changes can induce OCXO frequency changes due to deformation of the crystal envelope causing stray capacitance changes.
As the eye of a hurricane has greatly reduced air pressure than normal, by as much as 15%, it could be related.
> On Mon, Nov 20, 2017 at 1:40 PM, Bob kb8tq <kb8tq at n1k.org> wrote:
>> 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
>> 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.
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