RFM31 Datasheet, PDF (24/137 Pages) –

RFM31
The AFC function shares registers 73h and 74h with the Frequency Offset setting. If AFC is enabled (D6 in
―Register 1Dh. AFC Loop Gearshift Override,‖), the Frequency Offset shows the results of the AFC algorithm for the
current receive slot. When selecting the preamble length, the length needs to be long enough to settle the AFC. In
general two bytes of preamble is sufficient to settle the AFC. Disabling the AFC allows the preamble to be shortened
by about 8 bits. Note that with the AFC disabled, the preamble length must still be long enough to settle the receiver
and to detect the preamble (see "6.6. Preamble Length"). The AFC corrects the detected frequency offset by
changing the frequency of the Fractional-N PLL. When the preamble is detected, the AFC will freeze. In multi-packet
mode the AFC is reset at the end of every packet and will re-acquire the frequency offset for the next packet. An
automatic reset circuit prevents excessive drift by resetting the AFC loop when the tuning exceeds 2 times the
frequency deviation (as set by fd[8:0] in register 71h and 72h) in high band or 1 times the frequency deviation in low
band. This range can be halved by the ―afcbd‖ bit in register 1Dh.
AFC disabled
AFC enabled
Frequency Correction
Freq Offset Register
AFC
Add
1D
R/W
R/W
Function/Descrip
tion
AFC Loop Gearshift
Override
D7
afcbd
D6
enafc
D5
afcgearh
[2]
D4
D3
afcgear afcgear
h[1]
h[0]
D2
afcgearl[2]
POR
D1
D0
Def.
afcgearl[1] afcgearl[0] 40h
24
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