[time-nuts] Mini ovens packaging

Bob Camp kb8tq at n1k.org
Mon Aug 15 21:13:26 EDT 2016


HI

> On Aug 15, 2016, at 7:07 AM, Guillermo Sobreviela Falces <Guillermo.Sobreviela at uab.cat> wrote:
> 
> Hi Attila and Magnus! It is a pleasure to meet again (this time via time-nuts)! :D
> 
> Thank you very much Didier and Bob for your feedback (Also Attila and Magnus), I attach some characteristics of the desired oven:
> 
> -        Temperature stability of 0.1ºC, what means temperature control included.

To be meaningful, the stability number when run over the entire range needs to be looked at. If the range is still -40 to +125, that is a 165C range. 
A stability of 0.1C implies a thermal gain (reduction in change) of 1,650. That is significantly higher than the “few hundred” one might expect out of 
evan a fairly good component oven. 

> -        Inner cavity of 2cm x 2 cm x 1 cm.

If the stability is inclusive of gradients over that cavity size, the task just became more difficult. That goes up a bit more if power is being 
dissipated by parts inside that volume. 

> -        In addition to the connectors needed for the oven I need 3 pins reserved for the system.
> -        The system must be included into the oven via a chip socket, or pasted inside with silver paint and wire bonded to the oven connections.
> -        PCB compatible oven
> 
> I have been looking for a Peltier junction in order to reduce the oven temperature to 50ºC (Reducing the temperature is good for my system). But the temperature stability is a must and I am not sure of the precision level I can reach with a Peltier junction.

There are two basic problems with junction based devices:

1) They pump energy poorly. Getting a 160 degree delta likely requires a stack of a large number of devices. The power required goes up with ever level in the stack.

2) All of the power from the stack ultimately needs to be discharged. Water cooling often comes into play.

> 
> Also, I have been designing a temperature detector based in a power divider using a resistor and a RTD that will trigger an OPAMP (comparator). This OPAMP will activate/deactivate a high power BJT or FET transistor that will warm the oven. In order to reduce the thermal inertia I thought to make an escalated temperature detector as the shown in the attached picture. The main idea is have a fast heating and reduce the warming current when the temperature is approaching the final equilibrium temperature (120ºC).

Building the entire oven into a MEMS is a commonly used solution to this sort of problem. 

> 
> Another problem is related to the shell of the oven. It should be metallic in order to equilibrate the temperature, but, would it be interesting to use an external shell made of an isolator material in order to reduce thermal loses or will it difficult the thermal stability?

If the *guesses* above are correct, this is not a simple oven design. There are a lot of factors that will need to be considered. One not mentioned so far is the number of units you need to produce. That will impact the design as much as anything else. 

> 
> After this discussion, would it be possible to buy a system that meet the characteristics described above? I am not sure about I would be able to design and test this system on time, but I need this kind of oven with this temperature stability as soon as possible (This is why I am looking for a commercial solution).

You need a custom design.

Bob

> 
> Greetings and thank you all in advance!
> 
> 
> ________________________________
> De: time-nuts <time-nuts-bounces at febo.com> en nombre de Magnus Danielson <magnus at rubidium.dyndns.org>
> Enviado: sábado, 13 de agosto de 2016 15:57
> Para: time-nuts at febo.com
> Cc: magnus at rubidium.se
> Asunto: Re: [time-nuts] Mini ovens packaging
> 
> Hi Guillermo!
> 
> Nice to see you ask questions here!
> 
> (Attila and I met Guillermo at the EFTS and had many nice discussions
> and nice time to hang out. He does his PhD in MEMS systems, which Attila
> refers to. I also met him at IFCS in New Orleans when he came up to me
> and Dr. Rohde and asked questions on phase-noise measurements. I hope he
> got that resolved eventually.)
> 
> On 08/12/2016 11:59 PM, Attila Kinali wrote:
>> Hey Guillermo!
>> 
>> How is it going?
>> 
>> On Thu, 11 Aug 2016 09:05:02 +0000
>> Guillermo Sobreviela Falces <Guillermo.Sobreviela at uab.cat> wrote:
>> 
>>> I am currently looking for a temperature compensation system for an IC
>>> (Temperature range from -40°C to 120°C and chip area of 1cm x 1cm).
>>> This compensation system has to be external to the IC and, as the power
>>> consumption is not the main problem, I have been looking for a crystal oven.
>>> The ideal solution will be a PCB compatible oven, but it also can be an
>>> external element.
>> 
>> I guess this is for one of your MEMS chips?
>> 
>> What is the spec of the board's environment temperature?
>> How big may the oven be?
>> 
>> And I would agree with Bob that building your own oven would be
>> actually a good idea. It is not that difficult. You need a rough
>> estimate of what your thermal mass inside the oven will be, an
>> estimate on what the thermal resistance between the inside and
>> outside is, then can apply standard control theory to design
>> a PI control loop to keep it stable.
> 
> I agree with Bob and Attila, try to build your own, to get something
> working isn't all that difficult, and I think that in the end you will
> benefit from having the knowledge and additional system insight.
> 
> A good test for the trimming of the stability is to turn it on while
> cold. I learned this the hard way on a sample that a supplier had sent
> me, it oscillated wildly on turn-on, which was obvious on the current.
> The remaining oscillation caused the ADEV to have a bump, which was the
> first feature I discovered.
> 
> For crystals, the oven is set to the turn-over point so that the oven
> variations has least impact on the frequency. Do you have a suitable
> turn-over point?
> 
> Another aspect to reflect on, and here I refer to Rick Karlquist's
> paper, is the temperature gradient sensitivity. Do your device have
> frequency sensitivity to temperature gradients in any directions?
> 
> Some crystal devices have achieved better performance by putting the
> temperature sensing thermicaly close to the crystal, it's a generic hint
> but it can be good to think about how it can be achieved in the long run.
> 
> Cheers,
> Magnus
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