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Old 16th May 2021, 9:44 pm   #3
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: 20.431 Mhz Crystal wanted

When you go to talk to a crystal manufacturer, one of two things will happen:

1) They will ask you what function it has and in which radio. Then, if you're lucky, they will have done crystals for this job (maybe different channels) before and know the recipe.

2) If they haven't, or it's something new they will need some data:

Case?
Series or parallel mode?
Load capacitance, pF?
Fundamental, or what overtone number?
Max temperature drift and over what temperature range.
Adjustment tolerance?

20.431MHz could be a fundamental or 3rd overtone. Pullability diminishes as the SQUARE of the overtone number, so a 3rd overtone crystal is 1/9 as pullable as a fundamental one. Series mode oscillators are somewhat more common. 10ppm adjustment and +/-10ppm over 0-70C would be a high quality crystal. 100ppm is microprocessor clock territory. To allow adjustment either way, spec'ing a 30pF or 20pF nominal series C is pretty usual.

David
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