[time-nuts] Cheap Rubidium

Bob Camp lists at cq.nu
Wed Dec 23 17:52:23 UTC 2009


Hi

If you can achieve <= 1x10^-13 at 10,000 to 100,000 seconds, that's quite a
bit better than any OCXO has a right to be doing. Yes I know that there is
one in the entire universe that gets close.

Even if you just do < 1x10^-12 at 1,000 seconds, that's better than a whole
lot of OCXO's will do at that tau.

The place where the OCXO does come in is < 100 seconds. Between 1 and 100
seconds you can get a number of OCXO's that will run <2x10^-12 over the
entire range. You can fine a lot more that will do < 1x10-12 over that range
than you can find that will do < 1x10^-12 at 1,000 seconds. The rubidium is
struggling to get to 1x10^-11 at 1 second and may or may not get to 1x10^-12
at 100 seconds. 

To get the "best", you need both the medium term of the rubidium and the
short term of a good OCXO. 

Once the rubidium starts to "turn up" from it's floor, you may need GPS. Of
course it takes GPS quite a while to get to well below 1.0x10-13 unless you
are comparing two GPS's to each other. 

The reason for needing more than one is that you really won't know what's
going on unless you compare them to each other. That assumes you don't have
an active hydrogen maser running full time in the basement (I don't ...).
Three is a nice number, four is reasonable if one is running as an offset
oscillator chain for beat note frequency measurements. 

Past four it's just experimenting with this and that. If you need 10 samples
at 100,000 seconds it can take a while just to see what you have done...

So much fun ...

Bob

 

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of WB6BNQ
Sent: Wednesday, December 23, 2009 8:08 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Cheap Rubidium

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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