[time-nuts] Improving the stability of crystal oscillators

Richard (Rick) Karlquist richard at karlquist.com
Thu Oct 11 12:22:09 EDT 2007

The 10811 has an ANALOG oven control loop.  The gain is set
to be just below the oscillation point.  This is due to the
stability limits dictated by the oven mass and (believe it
or not) the size of integrator capacitor that can physically
fit.  If you want to "soup up" at 10811 oven, externally wire
in a larger capacitor in parallel and change the resistors to
increase the gain.  The 10811 designers did the best they
could with what they had to work with, but you don't want to
blindly copy them in new applications.

BTW, do not use a "metalized" plastic integrator capacitor.
Must be "foil" type.

I am extremely happy with the PII^2D control loop on the E1938A
(I didn't design it, only tested it).  I can't imagine anything
being better.

Rick Karlquist N6RK

Bruce Griffiths wrote:
> If a purely proportional control loop has such great performance why
> does the 10811A use a PI temperature controller and the E1938A use a
> PII^2 D controller?
> Surely the finite offset between the setpoint and actual temperature
> achieved by a proportional controller is a source of long term
> temperature instability?
> If one uses resistive heating then some linearisation improves the
> performance as the heat from the heating element is proportional to the
> square of the voltage across the heating element.
> A state space controller may give improved performance but PI(10811A),
> PID and PII^2 D(E1938A) controllers seem to work well when used to
> regulate crystal oscillator temperatures.
> Bruce
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