[time-nuts] ***SPAM*** Re: Why discipline Rubidium oscillator?

[Dana Whitlow]

Oops, I should have mentioned:  The unit was a Symmetricom MHM-2010 (I hope
I got that right), except that it was built before the company was bought by
Symmetricom (which was later bought by Micro-Semi in turn).

This is an active maser, meaning a self-sustaining oscillator whose gain
medium
was a volume of hydrogen atoms (at low pressure) maintained in a population-
inverted state by squirting a thin stream of state-selected H atoms into a
glass
bulb, and simultaneously pumping on the bulb to maintain a low pressure.
The
inside of the bulb was treated so that the collisions with the surface did
not
usually cause a quantum state change of the H atom involved.  I've read that
the average excited atom typically "survived" a large number of such wall
collisions before being "consumed" by contributing a quantum of energy to
the oscillating mode; this has always amazed me.

So the primary frequency-determining mechanism is the collision-broadened
line width of the gain mechanism. However, the cavity resonance exhibits a
noticeable frequency-pulling effect, and our maser has a feedback loop that
strives to keep the cavity tuned to the center of the medium's gain
profile.
But I think this loop is not a tight loop, ergo not completely successful.

Anyway, a small amount of RF power (a fraction of a pW as I understand it)
is extracted from the cavity as the useful output.  This drives a frequency
synthesizer to make a useful standard frequency output.  The "divide ratio"
of that synthesizer is adjustable in fine steps, with one step being a
fractional
frequency change of about 7E-17.

I've long wondered what causes the slow frequency drift, typically amounting
to about 3E-14 over a time span of several months.

Dana


On Tue, Nov 21, 2017 at 1:39 AM, Mike Cook <michael.cook at sfr.fr> wrote:

>
> > 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.
>
>
> >
> > Dana
> >
> >
> > On Mon, Nov 20, 2017 at 1:40 PM, 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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