Crystal oscillator frequency vs. supply voltage

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This experiment appeared on a local enthusiasts web. This was just an experiment with a few ATTiny2313 and one AT90S2313 and of course is not as exhaustive as the manufacturer's characterisation process... but the findings are quite interesting...

 

The original article is in Czech, but the automatic translation is quite legible.

 

JW

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Mildly interesting, but not at all surprising.  His tests show that the variation over the spec'd voltage for the part (1.8V-5V) is from -13ppm to +45ppm.  That's a total span of 60 ppm, which is quite reasonable.  He makes no mention of what crystal he's using, and that's an important factor in any bench test.  Not all crystals are created equally.  A '+/-30ppm' spec'd crystal may only be referring accuracy of any given unit coming out of the factory, not over any particular range of temperatures or voltage swings.  Crystals are generally characterised for accuracy over time (ageing), temperature, and sometimes operating voltage, and those figures can be larger than the quoted 'inherent' accuracy.

 

What is the CL and ESR of the chosen crystal?  What capacitors has he used?  Has he characterised the stray capacitance of the PCB?  What does the layout of the crystal and caps look like on the PCB?  All of these things need to be characterised, accounted for, and matched in the test circuit before you can generate any meaningful data from a test like this.

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