AK1590 Datasheet, PDF (12/36 Pages) Asahi Kasei Microsystems – 1GHz Delta-Sigma Fractional-N Frequency Synthesizer

[AK1590]
2. Frequency Offset Adjustment
AK1590 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 internally 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).
 Setting examples
Example 1) The frequency offset is positive; when the frequency offset is 100Hz and FPFD 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 FPFD is 1MHz.
Frequency offset = -100Hz / (1MHz/2¹⁸) = -26.2
It is round off to -26 (decimal), which is reduced 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
Yes
NUM_recal = (NUM + OFST) +1
INT_recal = INT -1
No
NUM_recal = NUM + OFST
INT_recal = INT
INT
NUM
OFST
INT_recal
NUM_recal
written data in integer register {INT[14:0]}
written data in numerator register {NUM[17:0]}
written data in offset register {OFST[17:0]}
recalculated integer data
recalculated numerator data
MS1478-E-01
12
2013/2