AK2401 Datasheet, PDF (40/103 Pages) Asahi Kasei Microsystems – Direct Conversion Transceiver

[AK2401]
 OFST1
 When OFST1[7:0] bits <Address 0x11> are written while OFST1 is valid.
(<Address 0x0F, 0x10> become valid when <Address 0x11> is written.)
 When the offset frequency setting is changed to OFST1 from OFST2 by changing RDOC_FM bit
or the TX_PDN pin.
 OFST2
 When OFST[7:0] bits <Address 0x11> are written while OFST2 is valid. (Note that OFST2 is not
valid when writing to the register <Address0x2B> although frequency offset setting register of
OFST2 are <Address 0x29~2B>.)
 When the offset frequency setting is changed to OFST2 from OFST1 by changing RDOC_FM bit
or the TX_PDN pin.
PLL synthesizer frequency that considers offset frequency is calculated as shown below. Setting values
of OFST1 and OFST2 are in 2’s complement fromat and the MSB will be the sign bit. On the other hand,
FRAC and MOD are in straight binary code. In the expression below, OFST means either OFST1 or
OFST2.
FRAC+OFST
Frequency Setting + Offset Frequency = PFD Frequency × (INT+ _MOD )
When (FRAC+OFST)/MOD ≥ 1, (FRAC+OFST-MOD)/MOD will be a fraction since the integer number of
frequency dividing INT is added and becomes (INT+1) by internal processing. When
(FRAC+OFST)/MOD < 0, (FRAC+OFST+MOD)/MOD will be a fraction since the integer number of
frequency dividing INT is subtracted and becomes (INT-1) by internal processing.
The expression to calculate the OFST register setting value from desired offset frequency is as below.
Offset Frequency
OFST = _{PFD Frequency} × MOD
It will be closer to the desired offset frequency when the OFST value is near to an integer and the MOD
value is higher. Example 1 and Example 2 are expressing the same setting frequency but it is able to
obtain closer value of desired offset frequency with Example 2.
○ Calculation Examples
Example 1) When offset frequency is positive value (1):
If Frequency Setting= 490.0375MHz, PFD Frequency = 2.4MHz
INT =204, FRAC=8015 and MOD= 43968,
Frequency Setting = 2.4 × (204 + 8015 / 43968) = 490.037500000… [MHz]
To obtain 100Hz Offset Frequency,
OFST = 100 / 2400000 × 43968 = 1.832
The setting value will be 2(dec) by rounding off OFST.
Therefore, the actual Offset Frequency is as follows
Offset Frequency = 2400000 × 2 / 43968 = 109.2 [Hz]
In this case, offset error is 9.2Hz.
017003093-E-00
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