[time-nuts] MCXO and dual mode

Bob kb8tq kb8tq at n1k.org
Tue Jun 6 22:59:17 EDT 2017


> On Jun 6, 2017, at 10:15 PM, Richard (Rick) Karlquist <richard at karlquist.com> wrote:
> 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.

The circuit that Stan Shadowski presented is a fundamental / third overtone dual. The example
below is based on that circuit. 

Let’s say both modes are running into a 32 pf load and it is a single capacitor. 

The capacitor changes due to aging by 1 pf, you now are at 33 pf load.

The fundamental changes frequency ~ 3X as much (in ppm) as the third overtone.

The beat frequency shifts since the two modes do not tune identically. 

Beat frequency shift = temperature error.

Yes the example is a little contrived. The real numbers would depend a bit on the design of
the crystal used. 


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