RFM22B/23B
When AFC is enabled, the preamble length needs to be long enough to settle the AFC. In general, one byte of
preamble is sufficient to settle the AFC. Disabling the AFC allows the preamble to be shortened from 40 bits to 32
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 "6.7. 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 for the
remainder of the packet. 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. The AFC loop includes a bandwidth limiting mechanism improving the
rejection of out of band signals. When the AFC loop is enabled, its pull-in-range is determined by the bandwidth
limiter value (AFCLimiter) which is located in register 2Ah.
AFC_pull_in_range = ±AFCLimiter[7:0] x (hbsel+1) x 625 Hz
The AFC Limiter register is an unsigned register and its value can be obtained from the HOPERF Register
Calculator spreadsheet.
The amount of error correction feedback to the Fractional-N PLL before the preamble is detected is controlled from
afcgearh[2:0]. The default value 000 relates to a feedback of 100% from the measured frequency error and is
advised for most applications. Every bit added will half the feedback but will require a longer preamble to settle.
The AFC operates as follows. The frequency error of the incoming signal is measured over a period of two bit
times, after which it corrects the local oscillator via the Fractional-N PLL. After this correction, some time is allowed
to settle the Fractional-N PLL to the new frequency before the next frequency error is measured. The duration of
the AFC cycle before the preamble is detected can be programmed with shwait[2:0]. It is advised to use the default
value 001, which sets the AFC cycle to 4 bit times (2 for measurement and 2 for settling). If shwait[2:0] is
programmed to 3'b000, there is no AFC correction output. It is advised to use the default value 001, which sets the
AFC cycle to 4 bit times (2 for measurement and 2 for settling).
The AFC correction value may be read from register 2Bh. The value read can be converted to kHz with the
following formula:
AFC Correction = 156.25Hz x (hbsel +1) x afc_corr[7: 0]
Frequency Correction
RX
Freq Offset Register Freq Offset Register
AFC Freq Offset Register
TX
AFC disabled
AFC enabled
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HOPERF [ HOPERF ]
