[time-nuts] Why discipline Rubidium oscillator?

[Dana Whitlow]

And even without problems like external magnetic fields, Rb oscillators do
drift with
age. Over a period of several years they may drift as much as ~1E-9, which
is a *huge*
error for serious time nuts.

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 frequency
suddenly
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 in
this group
can explain* that* behavior (that is, held for a week before resuming old
habits), I'd
love to learn about it.

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
> > _______________________________________________
> > time-nuts mailing list -- time-nuts at febo.com
> > To unsubscribe, go to https://www.febo.com/cgi-bin/
> mailman/listinfo/time-nuts
> > and follow the instructions there.
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/
> mailman/listinfo/time-nuts
> and follow the instructions there.
>