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I agree - in that I cannot find anything definitive in Altera's documentation.
So, to speculate - any single internal oscillator is not going to drift wildly across the specified frequency range with voltage or temperature. I'd suggest this is true of any type of oscillator circuit used internally, be it crystal, ceramic or capacitive based. The specified frequency range is far more likely to be as a result of process variation.
If you intend to characterise each device as you suggest, I expect you're likely to end up with a solution far more accurate than the specification suggests.
I look forward to a definitive answer - should there be one:)
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I logged a support request with Altera Support. They could not help either. All they would say is that the frequency will stay within the specification. When I get some boards working I'll run some tests and measure the frequencies (with some supply / temperature variation). I suspect that the wide frequency range implies the oscillator is capacitor based.
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