[time-nuts] Cheap Rubidium

[WarrenS]

Yep
There are all kinds of ways to do it wrong and screw it up, 
but it'll work fine and be decades better than non active with a little care if you know what you're doing.
And you forget to warn me that PID Temperature controllers can oscillate, so should not use them.

ws
*************Hi

The problem with a variable speed DC fan close to the rubidium is that you
change the magnetic field when you change the fan speed. Since the rubidium
is sensitive to the magnetic field, you tune it.

The gotcha with a simple controller is that you may control a point to a
specific temperature, but in this case you want to control the entire
device. Gradients and the change in gradients matters quite a bit.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of WarrenS
Sent: Wednesday, December 23, 2009 9:39 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Cheap Rubidium


OR you could use a small PC type IC cooler fan blowing at the heat sink from
a short distance away, driven from a Lady Heater type variable speed fan
active PID Temp controller.
That has NO trouble holding 0.01 Deg C at where ever you place the Temp
sensor, over normal day / night room temperature variations, even with it
all out on an open table without a box, insulation, OR swimming pool. 
ws


*****
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"<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
>>>>
>>>>
>>>> _______________________________________________
>>>>