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Choosing oscillator stabalizing cap values

Kelv

Hi Kevin,

 

I was just wondering if you know of any shorthand methods for picking the values of the stabalizing capacitors that are associated with a crystal oscillator, I only ask as I have encountered problems

with the startup of certain devices when I've used an arbitary value cap during prototyping.

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  • 0

    Good question Kelvin.

     

    I haven't come across such a tool as yet. I will do a bit of research and see what I can find (time permitting). I might have a mess about with a spreadsheet that does the equation but in the mean while if anybody knows of any online tools that do this, please let us know.

  • 0 in reply to hargreaves

    From what I can work out, there is not a simple solution to this. I am not an expert on crystals so I had to do a bit of research on this. It appears that the selection of the cap value has different criteria that cause the designer to pick a certain cap. The amplifier gain has to be high enough to drive the oscillator but if the gain is too high then the power consumption becomes too high. The other reason is that stray inductance and capacitance in the board layout and the component leads also play a part in the final values chosen.

     

    What I can add is advise on choosing the cap. You need a cap that is stable over temperature and stable over time. Low ESR is also beneficial. NP0/C0G are good as are silver mica and film will do. Avoid aluminium electrolytic and tantalum.

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  • 0 in reply to hargreaves

    From what I can work out, there is not a simple solution to this. I am not an expert on crystals so I had to do a bit of research on this. It appears that the selection of the cap value has different criteria that cause the designer to pick a certain cap. The amplifier gain has to be high enough to drive the oscillator but if the gain is too high then the power consumption becomes too high. The other reason is that stray inductance and capacitance in the board layout and the component leads also play a part in the final values chosen.

     

    What I can add is advise on choosing the cap. You need a cap that is stable over temperature and stable over time. Low ESR is also beneficial. NP0/C0G are good as are silver mica and film will do. Avoid aluminium electrolytic and tantalum.

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