[time-nuts] Temp control of LPRO

[Neville Michie]

Hi Bill,
the lamp and detector of the LPRO are both thermostat controlled.
At higher case temperature the power to heat them decreases as do the
temperature gradients in the device. From 17watts at 32V supply at 0*C
it can be reduced to 7.5 watts at 18 v supply and 45*C.
Although spec'ed to 70*C there is a MTBF penalty for higher  
temperature operation.
If your shack never gets over 40*C it is possible to control the LPRO  
to 45*C
by controlling its self heating with a tiny fan.
I built the LPRO into a thick walled aluminium box which is insulated  
with foam plastic,
through the box is a duct with fins that the tiny fan blows through  
and by switching the fan I get
control much better than 0.1*C.
I am looking forward to measuring its performance in frequency  
stability.
cheers, Neville Michie


On 11/09/2008, at 4:35 AM, WB6BNQ wrote:

> Hi Neville,
>
> In all seriousness, I thought the Rubidium physics package is  
> heated for
> reasons.  Now that you are heat sinking it, what has happened to  
> the current draw
> for the whole unit ?  Or am I misunderstanding something ?
>
> Bill....WB6BNQ
>
> Neville Michie wrote:
>
>> Hi,
>> I have just commissioned a temperature control for my LPRO rubidium
>> oscillator.
>> When I read the specs I noticed that at 18V supply and about 40*C the
>> unit only requires about
>> 8 watts. That is getting close to what my stand-alone power source
>> can maintain.
>> Running at a higher temperature takes a little load off the ovens on
>> the lamp and the detector,
>> but it does have a penalty in MTBF. At 40*C I thought the trade-off
>> was worth it.
>> Now the local crystal oscillator is not temperature controlled and
>> its control voltage drifts widely.
>> So I bolted the LPRO to a plate of aluminium about 1/4 inch thick. On
>> the other side of the aluminium plate
>> I bolted heat sink fins about 3/4 long over the whole length.
>> The whole unit mounts inside a piece of about 3X4 aluminium
>> extrusion. At one end in a little plenum
>> chamber I mounted a12V  1 watt 40mm brushless DC ball bearing fan, in
>> the AL plate I drilled a
>> deep well in which a bead thermistor lives.
>> The whole thing is insulated in 1/2 inch plastic foam sheet.
>> When the fan running on 12V the temperature is held to 4 *C above
>> ambient. At 8 volts this rises to about 7*C.
>> The fan is quite unobtrusive on 8 V.
>> The thermistor is in a bridge circuit and switches the fan in a PWM
>> control mode with a 50 second cycle time.
>> The surface of the plate has less than 0.1*C variation over a day
>> with no sign of the temperature cycling.
>> The oscillator control voltage now rises on startup and then becomes
>> rock steady.
>> The next step is to get on with the building of the gear to measure
>> how well the LPRO now performs
>> at constant temperature.
>> Cheers,
>> Neville Michie
>>
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