[time-nuts] Making a HP 10811 better

[Magnus Danielson]

Bob Camp wrote:
> Hi
>
> There are a couple of problems you will run into.
>
> The first is that if you do a very good job with the dewar flask, the "dead
> heat" (power of the oscillator, regulator etc) can indeed raise the
> temperature of the OCXO beyond the oven control temperature.
>   
I.e. there needs to be a leak into the surrounding, otherwise the 
heaters will turn totally off without achieving the chill-down effect, 
so it will almost behave like a bang-bang system with quick ramps in 
temperature and then long effects of unregulated cooling down.

So, the regulator needs the chill-load to create a heating balance.
> The second is that the gain of the control loop is in part determined by the
> thermal resistance. Increase the thermal resistance by a very large amount
> and the control loop gain goes way up. The control loop gain is probably
> pretty high already, so increasing it by a large amount is likely to make it
> unstable.
>   
In moderate levels it could be good thought... no?
> The next thing is that control loop gain and effective thermal gain to the
> crystal are not the same thing. Even if it is stable, a large increase in
> control loop gain probably will decrease the effective thermal gain. The
> reason is a bit complex. The simple answer is that the thermistor is not
> mounted on the crystal blank, thus it "sees" something different than the
> crystal.
>   
Do elaborate. I am curious. I am aware of thermal gradients and issues 
with geometry.
> With the large increase in gain, since it's a simple controller, you will
> need to re-set the control point. There's no handy integrator in there to
> crank the offset out for you. 
>
> Simply put, you design a dewar flask / vacuum bottle OCXO in a very
> different way than one that's conventionally heated. Miliwatts of
> dissipation do mater in a vacuum design. 
>   
OSA 8600 and 8607 has one side of the flask still open. There is a plug 
there, but it isn't near as good isolator as the flask. It needs to leak 
overflow heat that way. Consider that on these most of the electronics 
sits on the side of the flask, so only the actual oscillator core and 
heating transistors is inside the flask.

The 10811 has more stuff inside it.
> Reducing drafts is a good thing. Moderating ambient variation hour to hour
> is a good thing. Burying an OCXO in the back yard works, dunking it in a big
> barrel of water also works. Both require you to remember the waterproof bag
> before you take the final step :)....
Oh, we always toss our oscillators into the water when powered, didn't 
you know? ;) That's why we throw a towel at it.

Reducing *quick* changes in ambient temperature or breeze, is what comes 
cheap and does not significantly changes the equation.

Cheers,
Magnus