Try an insulated water cooled metal block for the baseplate heatsink
The water can be recirculated and heated/cooled as required.
With a recirculating system and say 25W dissipation with no temperature
control you will only need about 5 cubic meters (5 tons) of water to
maintain a temperature rise of less than 0.01C for 3 hours. Assuming
that the 25W rubidium dissipation is the only significant source of
heating for the water.
That means you need a well insulated swimming pool in your basement.
Controlling the temperature of a smaller amount of recirculating water
is probably a simpler proposition.
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"<[email protected]>
To: "Discussion of precise time and frequency measurement"<[email protected]>
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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