[time-nuts] MCXO and dual mode (was: Poor man's oven)

Bob kb8tq kb8tq at n1k.org
Tue Jun 6 18:16:13 EDT 2017


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. 


> On Jun 6, 2017, at 5:40 PM, Attila Kinali <attila at kinali.ch> wrote:
> On Mon, 5 Jun 2017 20:21:10 -0400
> Bob kb8tq <kb8tq at n1k.org> wrote:
>> That paper is the basis for the MCXO. It is an interesting way to do a TCXO. 
>> The drift between the two modes makes it a difficult thing to master in an OCXO.
>> Plating a pair of electrodes (one pair per mode) is also an approach that has been
>> tried. 
> That's the first time I hear of modes drifting respective to eachother.
> Do you have any references I could read on this?
> I always wondered why the MCXO approach was not used more often.
> Or why none of the OCXOs used a dual mode approach to sense
> the temperature of the crystal directly instead of using a
> thermistor. 
> 			Attila Kinali
> -- 
> 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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