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SX126X IC datasheet page 88, I see two different formulas for the frequency deviation:

SX126x IC

Closest clue I can find is here:

param[5] = 0x00; // TX frequency deviation 25kHz (deviation * 2^25 / fxtal = 25000 * 2^25 / 32000000 = 0x006666)
  1. Which one is the correct one?
  2. A more 'sophisticated' question would be, where does this formula derive from? But I would be happy just knowing which is the correct of these two.
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1 Answer 1

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  1. They are the same equation.

  2. Without even looking at what the chip does, it looks like there is some phase accumulator implementing a DDS.

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  • \$\begingroup\$ Same equation how? The denominator changes. \$\endgroup\$ Commented Oct 1, 2023 at 9:54
  • \$\begingroup\$ @ChristianidisVasilis They are the same equation, with terms rearranged to either solve for Fdev register value based on deviation or deviation based on Fdev register value. Can you say why you think they aren't the same equation? \$\endgroup\$
    – Justme
    Commented Oct 1, 2023 at 9:59
  • \$\begingroup\$ Oh I didn't think they rearranged the terms, as the variables/names are similar! I get it now, thanks! They could use something like "FreqDevRegister" name/variable instead of Fdev and FrequencyDeviation. \$\endgroup\$ Commented Oct 1, 2023 at 10:01
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    \$\begingroup\$ @ChristianidisVasilis That is confusing indeed. \$\endgroup\$
    – Justme
    Commented Oct 1, 2023 at 10:02

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