AK2400 Datasheet, PDF (35/52 Pages) Asahi Kasei Microsystems – High integrated receiver for PMR-LMR

[AK2400]
Frequency switching adjustment (PLL SYNTH)
AK2400 has an offset adjustable register which can tune the carrier frequency set by {NUM[17:0]} in
<Address1>
and {INT[14:0]} in <Address2>. When the offset register: {OFST[17:0]} in <Address6> is accessed,
{NUM[17:0]} and {INT[14:0]} are recalculated automatically and their recalculated data are used in
delta-sigma and N-divider. This operation is suitable for AFC and DFM applications.
When frequency offset is not used, the offset register must be written 00000 (hexadecimal).
When OFFSET register is used, set DITH=0(OFF).
Setting examples
Example 1) The frequency offset is positive when the frequency offset is 100Hz and the Phase
Frequency Detector comparative frequency is 1MHz.
Frequency offset = 100Hz / (1MHz/2¹⁸) = 26.2
It is round off to 26 (decimal) = 1A (hexadecimal) = 11010 (binary)
Example 2) The frequency offset is negative when the frequency offset is -100Hz and the Phase
Frequency Detector comparative frequency is 1MHz.
Frequency offset = -100Hz / (1MHz/2¹⁸) = -26.2
It is round off to -26 (decimal), which is deduced from 2¹⁸ to be converted
into binary for 2’s complementary expression.
2¹⁸ – 26 = 262118 (decimal) = 3FFE6 (hexadecimal) = 11 1111 1111 1110 0110 (binary)
Algorithm of recalculation
Sum = NUM + OFST
No
Sum  0.5
Yes
NUM_recal = (NUM + OFST) -1
INT_recal = INT +1
Sum < -0.5
I
N
T
Yes
NUM_recal = (NUM + OFST) +1
INT_recal = INT -1
No
NUM_recal = NUM + OFST
INT_recal = INT
written data in integer register {INT[14:0]}
NUM
written data in numerator register {NUM[17:0]}
OFST
written data in offset register {OFST[17:0]}
INT_recal
recalculated integer data
NUM_recal recalculated numerator data
Figure 13 Frequency switching adjustment
014008989-E-01
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2015/4