[time-nuts] NCOCXO anyone?

Bob Camp kb8tq at n1k.org
Mon Jul 25 08:10:36 EDT 2016


A typical double oven design runs about 10 mw or so in the circuitry 
in the inner oven. The “stuff” inside the outer oven likely doubles that number. 
The rest of the circuit is put outside the outer oven to reduce heat rise. There
could be another 40 mw in that part of the circuit. Everything else is controlled 
power to heat things and will go to zero at the highest temperature. 

If you run the inner oven at an offset of 3 to 5 C from the outer, the inner
heater will likely not pull more than 40 mw and may pull quite a bit less.


> On Jul 24, 2016, at 4:45 PM, James Flynn <james.flynn at csun.edu> wrote:
> Richard (Rick) Karlquist <richard at ...> writes:
>> The last thing you want in an oven is a lot of
>> added thermal overhead, especially in a double oven
>> where you already fighting against running out of
>> temperature range.  If you want to ovenize everything
>> but the kitchen sink, put it in it's own oven that is
>> separate from the crystal oven.
> The thermal overhead is quite small compared to the power required for 
> the outer oven to hold temperature.  80mW maximum as opposed to about 1 
> watt.
> I did a comparison of the temperature rise of running the unit 
> essentially as a single, inner oven with the rest of the electronics 
> mentioned in the outer "box", and then turning on the outer heater.  The 
> Q from the inner oven was far and away the dominant factor.
> Putting the DAC inside the outer oven was the obvious solution for me, 
> as opposed to putting them outside in ambient.  I did consider thermal 
> compensation, but again the outer oven would be able to keep things 
> within a fraction of a degree over the range of normal ambient in the 
> lab. 
> Separate ovens seems to be inviting noise getting into the signal lines 
> between the two ovens.  
>> I also don't like intermingling digital signals with the
>> analog oscillator signal.
> Not sure what you mean by "intermingling". There are separate ground 
> returns for the oscillator output (transformer isolated) and the digital 
> signals. There is a low pass filter between DAC and oscillator control 
> to minimize noise getting across. Also digital circuits and oscillator 
> have their own individual precision regulators.
>> Have you measured the thermal gain of your outer oven?
>> I suspect it's not much.  You could use inner oven
>> current draw as a proxy.
> I did a while ago and remember it was on the order of 50 - 100. But I 
> have changed the design somewhat and should do it again when the 
> experiment gets going.
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