Xtal oscillator using LM311

[GGAPBE96]

Hi there,

I'm trying to design a crystal oscillator by refering the document.

**broken link removed**
(Please see "9.0 Typical Applications" 100kHz crystal oscillator)

However,I can't understand the oscillation mechanism.
How does the oscillator work??

I think 100kHz crystal and 10pF makes positive feedback.
What does C2 do??

 

[FvM]

C2 provides the crystal's specified load capacitance and also works as a voltage divider. If You don't understand the circuit
or doubt it's not suitable, use a different one from a text book.

P.S.: I think a crystal oscillator with a high-pass feedback is at risk to fall into overtone oscillations, if the modes are not
well supressed in crystal design. I would prefer an unbuffered CMOS gate as crystal oscillator.

 

[GGAPBE96]

FvM, thank you so much.
So far, overtone oscillation does not occur.

Sorry,I mistook my question..
What I wanna ask is not the role of C2 but the role of C1.
It seems to be irrelevant to positive feedback.

 

[LvW]

............
Sorry,I mistook my question..
What I wanna ask is not the role of C2 but the role of C1.
It seems to be irrelevant to positive feedback.

Yes, but don't forget negative feedback.
R3 provides 100% feedback for dc. That's necesssary for opamp biasing.
However, for frequencies in the region of oscillation the negative feedback must not dominate - otherwise the circuit wouldn't oscillate.
Thus, the inverting input has to be ground referenced for these frequencies.
That`the task of C1.

By the way: As C2 is connected in series with the crystal, the crystal operates in its inductance region (series resonance with C2).

 

[GGAPBE96]

Thank you for the quick and kind explanation.
Now I understand the circuit.