[time-nuts] MCXO and dual mode

Richard (Rick) Karlquist richard at karlquist.com
Tue Jun 6 22:15:26 EDT 2017

On 6/6/2017 3:16 PM, Bob kb8tq wrote:
> Hi
> If you do the classic MCXO with two oscillator circuits and one resonator, the issue is
> pretty simple. You have a load capacitance on the fundamental. You have a load capacitance
> on the third overtone. Even if it is the exact same capacitor, the tuning sensitivity on
> the fundamental is different than the sensitivity on the third overtone. As the load impedance
> changes (parts do drift) the delta between the two modes will show up as an offset between
> them. If you run through the math, it gives you a delta temperature. How much? How fast? Obviously
> that depends. When I brought this up at the time with the authors of the paper, the reply was that
> a recalibration of the MCXO was provided for for this reason.
> Bob

I don't understand what you are talking about here.  The tempco
difference between modes is unrelated to load capacitance.  The
dual mode idea would work just as well if the oscillators
operated at series resonance.

[I attended this talk in person ~25 years ago; it got a lot of

The reason why the SC cut mode C and mode B dual mode patent
from HP fell out of favor was the problem with activity dips
in mode B.  Otherwise, it was a great idea.  It would still
be fine for an OCXO, where you just avoid activity dips.
However, the circuit design is very complicated.

Rick N6RK

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