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December 14, 2010


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Carl Schwab

John, My experience with spurious responses with frequency controlling xtals is mainly with 3rd and 5th overtone, and an occasional fundamental. Two points from my experience. 1) Don’t overdrive the xtal—can cause heating and brings up the spurious if they are going to be a problem. 2)Any circuits I designed always had a gain setter (usually a strategic resistor) that was adjusted to optimize starting and stability.

FWIW the frequency pull ability varies as the square of the overtone ratio i.e. a third overtone is 9 times as hard to pull and a 5th overtone is 25 times as hard to pull.------Carl Schwab

John D.

Hi, Carl.

The oscillator as shown is typical of what is often rcommended and whatever drive level the gate devices provide, that's it and you really have no control.

You can trim the crystal's drive level of course, but I don't know by what criteria would one judge the optimization of that trimming given that the output is logic level stuff.

That's why I say, buy an oscillator from a compamy that is in-the-biz rather than let an FPGA or µP/µC or ASIC do its own oscillation.


Eddy van Keulen

Quartz crystals can have a substantial amount of unwanted modes that can bite you one way or another. Theoretically you can model all these resonances, spurious or overtones, with separate series branches.
These spurious modes are the result of a specific crystal design. I believe they are the result of reflections between edge of electrode and edge of quartz disc. The crystal manufacturer can change the design to move the spurious further from the main mode and/or increase the spurious resistance so your circuit can not accidentally get stuck on a spurious anymore.
I would place a resistor between the oscillator inverter output and the 22pF on the right. This improves (lowers) the drive and also improves (increases) the Q. You can try values in the range 100~1000 ohms.


Tom Christian

I want to thank everyone for their comments
and John for his work.
This is the sort of useful stuff they should tell you
in school. I've run into this problem before
but just assumed it was a "bad" crystal.

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