Hi > On Jun 6, 2017, at 10:15 PM, Richard (Rick) Karlquist <[email protected]> > 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. Bob > > [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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
