[time-nuts] Cheap Rubidium

WB6BNQ wb6bnq at cox.net
Wed Dec 23 13:10:42 UTC 2009


**************

I meant to say TUB not tube in the third paragraph.

**************

WB6BNQ wrote:

> Bob,
>
> I don't know why you would want 4 Rb's running, not to mention 8 or ten of them.  I don't see the need for a follow up OCXO either.  If you had such a good OCXO, it would not make much sense to also have the Rb.
>
> You could put an oil bath together for a better thermal mass.  Use a mineral oil much like Johnson&Johnson Baby oil.
>
> You would probably want at least two gallons (if not more) per Rb and make sure you have a heat sink on the Rb to help spread the heat in to the oil.  If you have a big enough tube then you could have all four in the same pot.
>
> You would want to stir the oil constantly at a slow rate.  A small 2 1/2 or 3 " 12 volt computer fan, suspended in the oil, would be good for that.  You can adjust it's speed by varying the voltage to the fan.  The Johnson&Johnson Baby type mineral Oil would not hurt the electronics, but it would be messy in some
> respects.
>
> Bill....WB6BNQ
>
> Bob Camp wrote:
>
> > Hi
> >
> > Water might work. It would take quite a bit of it.
> >
> > Here's my "wild guess" level math:
> >
> > 1) The basement moves 0.1 to 1 C short term / over a day.
> >
> > 2) I want to get to < 0.01
> >
> > That takes the time constant out to >= 10X the time I'm interested in.
> >
> > 3) The time period of interest is 3 to 30 hours.
> >
> > That gets to a time constant of at least 10 days.
> >
> > At the same time you have >10 watts coming out of the gizmo. You can't put the thermal mass inside a vacuum  bottle.
> >
> > I suspect that some combination of thermal mass and active stabilization will be needed.
> >
> > So much fun ....
> >
> > Bob
> >
> > On Dec 23, 2009, at 2:18 AM, Don Latham wrote:
> >
> > > sheesh! How about a right-sized water jug?
> > > Don
> > >
> > > ----- Original Message ----- From: "Bob Camp" <lists at cq.nu>
> > > To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> > > Sent: Tuesday, December 22, 2009 8:23 PM
> > > Subject: Re: [time-nuts] Cheap Rubidium
> > >
> > >
> > >> Hi
> > >>
> > >> I agree that if you simply bolt the rubidium to an old engine block and toss a blanket over it, you might get some pretty good thermal stability in the "hour to couple hours" time period. That's certainly a better approach than putting some kind of DC heater (and it's varying magnetic field) near the rubidium.
> > >>
> > >> I'm still wondering if they do indeed hit 1x10-13 (as in almost 1x10-14) or not. I suspect not. I'm sure that they do indeed get into the 1x10-13's, just not sure they get to the bottom of that region.
> > >>
> > >> Bob
> > >>
> > >>
> > >> On Dec 22, 2009, at 8:26 PM, Bruce Griffiths wrote:
> > >>
> > >>> Bob Camp wrote:
> > >>>> Hi
> > >>>>
> > >>>> If I randomly pick up a FE 5680A data sheet, I find that it's short term stability is 1.4/sqrt(Tau) x 10-11.  Since I never doubt anything I see on a data sheet, this immediately tells me I should get 1.4x10-12 at 100 seconds, and I only have to wait for 10,000 seconds to get to 1.4x10-13.
> > >>>>
> > >>>> Since the temperature performance is at the 1x10-12 / C level, I would need a room that's stable to *much* better than 0.1 C over a 3 hour period to get there. I suspect that 0.01C might not be good enough ...
> > >>>>
> > >>>> So here's the question:
> > >>>>
> > >>>> Has anybody run any of the cheap rubidiums (FE or Efratom)  in a *very* stable temperature environment to see how close they get / what the floor is?  I've run through a lot of data on the web, but I haven't really found what I'm looking for.
> > >>>>
> > >>>> Thanks!
> > >>>>
> > >>>> Bob
> > >>>>
> > >>>>
> > >>> Figure 7 on the FE5680 page (also on the data sheet) indicates that you may need somewhat less than 3hours to achieve  ADEV ~1E-13.
> > >>> 0.01C stability should be adequate.however its not necessary to control the room temperature to this stability if the FE5680 is in an enclosure with a sufficiently high time constant whilst having a sufficiently low thermal resistance so as to avoid overheating the FE5680.
> > >>>
> > >>> Bruce
> > >>>
> > >>>
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> > >>
> > >>
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