AK2400 Datasheet, PDF (34/52 Pages) Asahi Kasei Microsystems – High integrated receiver for PMR-LMR

[AK2400]
Frequency Setting (PLL SYNTH)
Frequency Setup
AK2400 is a Fractional-N type synthesizer that takes 2¹⁸ as the denominator, which calculates the integer
and numerator to be set using the following formulas:
Frequency setting = F_PFD × (Integer + Numerator / 2¹⁸)
Integer = ROUND (Target Frequency / F_PFD)
Numerator = ROUND {(Target Frequency – Integer × F_PFD) / (F_PFD / 2¹⁸)}
Note)
ROUND: Rounded off to the nearest integer
F_PFD : Phase Frequency Detector comparative Frequency ([REFIN] pin input frequency / R divider ratio)
Calculation examples
Example 1) The numerator is positive when the target frequency is 950.0375MHz and the Phase
Frequency Detector comparative Frequency is 1MHz.
Integer = 950.0375MHz / 1MHz = 950.0375
It is rounded off to 950 (decimal) = 3B6 (hexadecimal) = 0011 1011 0110 (binary)
Numerator = (950.0375MHz - 950 × 1MHz) / (1MHz / 2¹⁸) = 9830.4
It is rounded off to 9830 (decimal) = 2666 (hexadecimal) = 10 0110 0110 0110 (binary)
Frequency setting =1MHz × (950 + 9830 / 2¹⁸) = 950.0374985MHz
(In this case the error between the calculated frequency and the target frequency is 1.5Hz.)
Example 2) The numerator is negative when the target frequency is 950.550MHz and the Phase
Frequency Detector comparative Frequency is 1MHz.
Integer = 950.550MHz / 1MHz = 950.550
It is rounded off to 951 (decimal) = 3B7 (hexadecimal) = 0011 1011 0111 (binary)
Numerator = (950.550MHz - 951 × 1MHz) / (1MHz / 2¹⁸) = -117964.8
It is rounded off to -117965 (decimal), which is deduced from 2¹⁸ to be converted into binary
for 2's complementary expression.
2¹⁸ - 117965 (decimal) = 144179 (decimal) = 23333 (hexadecimal) = 10 0011 0011 0011
0011 (binary)
Frequency setting =1MHz × (951 + (-117965/2¹⁸)) = 950.5499992MHz
(In this case the error between the calculated frequency and the target frequency is 0.8Hz.)
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