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Crystal-oscillator design (Pierce Configuration)

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pierce configuration

Hi all,
I am designing a crystal oscillator in parallel configuration at 28MHz.
(The series resonance of the crystal is at 28MHz).
When i connect the crystal equivalent circuit (LCR) to the oscillator
(consist of an inverter with a resistive feedback),
the circuit oscillates at much lower frequencies than the expected (28MHz).
The problem seems to be with the value of the feedback resistor.
It sould be high enough (more than 100 KOhms), but only when it drops down to some kOhms (5KOhms) the circuit oscillates near the crystal resonance freq.
Is this phenomena reasonable? Has to do also with the inverter?
Can you point me some references for crystal oscillator designs.

Thank you very much in advanced!!
 

pierce crystal oscillator

**broken link removed**

search this .
 

oscillator 28mhz

Is it only simulation? I guess some hspice simulation isn't right for oscillator.
 

pierce crystal oscillator design

Did you check the Phase and Gain in 28MHz ?

If Two things meet the oscillation condition, Crystal will oscillate using spice.

Maybe need long Time.
 

crystal oscillator design

Inverter-based oscillator can sometimes operate as a relaxation oscilator dominated by RC constants instead of LC resonance. As a simple test, increase the size of the inverter transistors. Also consider the resistor in series with the output of the inverter and crystal. You can reduce this resistor till getting enough energy going into the crystal.

Make a magnitude/phase plot of the open loop gain with the crystal connected.
 

spice pierce oscillator open loop gain

I am simulating with spectre, and it doesnt seem to have any problem with the convergence. The circuit oscillates but not in the expected frequency (crystal series resonance freq). The resonance freq is mostly dependent by the resistor value (RC). I am trying now, by changing the inverters size to make the circuit oscillation freq robust to the resistor value. By increasing the tranzistors size of the inverter, can i make this happen?

Thanks.
 

oscillator design pierce

Yes, change transistor sizes. But increase the feedback resistor. Decrease resistor in series with crystal.
 

crystal oszillator pierce

Thank you Humungus.
Now we are playing!!!
 

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