[time-nuts] 10811 foam enclosure

Tom Van Baak tvb at LeapSecond.com
Sun Jan 9 23:59:39 UTC 2011

> Being a Shera Fan I finally broke down and bought a Tbolt. I experimented  
> with a foam enclosure with roughly 1 centimeter on all sides. What I found 
> that  it only increased the board temperature by 19 C. It was still as 
> sensitive to  ambient. Thanks to Lady Heater it even shows an increase of 40 mC 
> when I walk up  to it with one minute delay.. Changing to an aluminum 
> extrusion the increase is  only 7 C and the response is much slower. In its final 
> assembly it will be  inside a larger metal mass, I do not think that the 
> higher temperature of  56 C is conducive to longer life. Opinion: putting foam 
> around a 10811 will only  give you a warm feeling unless you make it much 
> larger than shown in the picture  of of KH6GRT.

Nice set of experiments. Thanks for doing them and reporting
the results. I also found that foam insulated enclosures do not
help with ~hourly HVAC cycling. I reasoned that over the span
of half an hour there is more than enough time for the delta T
of the outside world to infect the inside temperature; thus the
foam does little or no good. If the temperature changes are
more rapid however the foam might start showing advantage.

> Based on the above test results I feel mass is more important. So I took a  
> aluminum enclosed dewar weighing 943 gr. and did first put a resistor in it 
> and  heated it up with 2 W. Temp. did rise to 70 C. Next I disassembled my 
> worse  10811, which is very easy, since it is a nice compact unit and 
> inserted it in  the dewar. Monitoring temp with my YSI it shows 65.2 C with 24 C 
> ambient. Power  dissipation is 1.575 W. Will track it over time before I 
> replace the 10811 with  one of my better ones for frequency tests.
> Questions to our experts: 
> A) will Removing the foam  mess with the temp. control loop 

It seems this would change the time constant so the loop may
no longer be optimized. But by how much is hard to say.

> B) How low can total power into the 10811 be before temp control can become 
> a problem, I think I am ok now,  but once in a chassis with Shera and 
> power  and battery the temp. surrounding the dewar may become a problem.
> C) What is the preferred orientation of the unit.

If your instrument is fixed I don't think it matters. You'll see 10811
mounted in several different orientations in hp gear. The key thing
is not to bump or tilt the instrument (or drop it ;-). I get better results
on a concrete floor than I do in a rack or on a shelf due to vibration
and shake.

> I asked already once before if any one has data on the 1 1 PPS output of  
> the Tbolt over time, since I am considering it as an alternative to a stand  
> alone GPS receiver, and if there are GPS receivers that outperform a Tbolt 
> what  is the recommendation. It will be used to control a Rb.
> Bert Kehren

How about using the Rb as the TBolt LO?

I'll send you some raw TBolt 1PPS data.


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