[time-nuts] NCOCXO anyone?

[James Flynn]

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.