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Pierce Oscillator Circuit Validation

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stefannm

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I have designed and had built a 4.9MHz pierce oscillator on an IC. I have it in the lab and everything looked okay for a while. The oscillator seems to start up and have decent frequency accuracy. I have noticed when I put my finger on the crystal pins, the oscillator dies until I remove my finger, then it started right back up.

I have tried a variety of load cap sizes (26pF to 39pF). It seems to start up with expected shifts in frequency, but always has the susceptability to the finger test.

I am on a 3.3V supply and the amplitude of the oscillation is ~2V which implies sufficient -R. Is this correct?

How concerning is this? Right now I am really concerned. Any ideas how to fix this? Is it a case of insufficient loop gain?

Is it possible to have too big of a feedback resistor? I have 10M+ for other reasons.

BTW it is a CMOS oscillator in an Eric Vittoz type architecture.

Thanks in advance for any help,
stefannm
 

Can anybody help me for pierce oscillator layout. used as macro.

Actually i have done modifications in Pierce oscillator layout.
actually frequency is 20MHz. used wide metal routing for xtalin and xtalout nets, which are going to core pins, and done the sheilding for those nets.

subcircuits are op-amp,comparator pierce oscillation part, buffer etc.

as far as i know this output is going to PLL input.

Actually i need layout constraints to follow to my circuit.

What are the applications of Pierce oscillator.
 

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