[time-nuts] E1938 oven design

Bob kb8tq kb8tq at n1k.org
Sun Jun 11 15:56:39 EDT 2017


Hi

I certainly saw the “positive gain at this setting” going to “negative gain at that setting” result on a lot 
of OCXO designs. I never had the patience (or a stable enough system) to get into the millions 
or even 100K’s on a single oven.  As a practical result, a gain of -500 is not really any better or worse
than a gain of +500. It *can* be a bit confusing to set up though ….

Bob
 
> On Jun 11, 2017, at 1:43 PM, Richard (Rick) Karlquist <richard at karlquist.com> wrote:
> 
> 
> 
> On 6/11/2017 8:59 AM, Attila Kinali wrote:
> 
>>> The exact insulation is relatively unimportant.
>>> We even tried still air using a knife edge
>>> cradle.  Didn't make much difference.
> 
> 
>> What is a knife edge cradle?
> 
> We wanted to test still air as insulation.
> We couldn't just replace the insulation with
> "nothing".  There had to be some kind of mechanical
> support to keep the oven mass suspended inside the
> outer case.  Our ME built a skeleton framework
> made of plastic to support it.  In order to minimize
> conduction thru the plastic, he designed in knife
> edges where the plastic came into contact with the
> oven mass.
> 
> 
>> One big question remains: How did you set the ratio
>> between face and rim heater? Was it determined at design
>> time and then just set the same for all units or was
>> it part of the production test?
> 
> During proof of concept phase in R&D, I peaked up the
> thermal gain on each unit by trial and error.  I could
> usually get a run with positive thermal gain, then
> increment the ratio, and get a run with negative
> thermal gain.  I could then interpolate to get the
> ratio that should give "infinite" gain.  Maybe one
> or two more runs after that would get me to where
> the gain passed through infinity at some ambient
> temperature, and was well into the millions over the
> whole range.  At extremely high thermal gain, the
> gain is not constant over ambient temperature.
> 
> I collected data on a number of units and then used
> the average ratio as the production setting.  I
> checked production units from time to time and they
> typically ran well in the 100's of thousands for gain.
> 
> This compromise was workable because we individually
> programmed the oven set point to the exact turnover
> temperature.  This is a lot easier because it doesn't
> require environmental chamber runs.  The E1983A software that
> I "leaked" to the time-nuts community I believe has a
> command that can be used to search for the turnover.
> 
> 
> Rick N6RK
> 
>> 			Attila Kinali
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