[time-nuts] Poor man's oven
attila at kinali.ch
Wed Jun 7 16:09:54 EDT 2017
On Tue, 6 Jun 2017 19:07:29 -0700
"Richard (Rick) Karlquist" <richard at karlquist.com> wrote:
> In the E1938A oscillator, we used a PIDI^2 loop. IOW, a PID
> plus a double integrator. This was Len Cutler's idea.
> Once the constants were dialed in, this worked phenomenally
> well in terms of transient response. Even dumping in liquid
> nitrogen full throttle into the environmental test chamber
> barely wiggled the crystal temperature/frequency.
The E1938 is kind of special in several ways, IMHO.
Beside having a nice zero-gradient design (I really love
this idea :-) it has also a quite large surface vs volume.
This means that there is a lot of heat flow out of the
can and at the same time the (face) heater is large, which
makes the PI loop better behaved. Also the thermal mass of the
crystal holder is quite small. Especially compared ot the 10811.
Due to the flat puck design, I assume that the majority of the
heat going to the crystal holder is due to radiation (unless I
missed some insulation around the crystal). Radiation, albeit being
a high "resistivity" transport mechanism, has very low inherent
"capacitance". And thus the delay associated with this transport
mechanism is low.
There are probably a lot more small design decisions in the E1938
that make it such a superb oven. More than I probably will ever
be able to figure out. And, I still am astonished how well it works.
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering. -- The Doctor
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