RFM66W Datasheet, PDF (35/87 Pages) List of Unclassifed Manufacturers – 868 & 915MHz Ultra Low Power High Link Budget Integrated UHF Transceiver

WIRELESS & SENSING
The frequency error, in Hz, can be calculated with the following formula:
FEI = F_STEP × FeiValue
RFM66W in Rx mode Preamble-
modulated input signal Signal
level > Sensitivity
RFM66W
DATASHEET
Set FeiStart
=1
FeiDone
No
=1
Yes
Read
FeiValue
Figure 13. FEI Process
3.5.10. AFC
The AFC is based on the FEI block, and therefore the same input signal and receiver setting conditions apply. When the
AFC procedure is done, AfcValue is directly subtracted to the register that defines the frequency of operation of the chip,
F_RF. The AFC is executed each time the receiver is enabled, if AfcAutoOn = 1.
When the AFC is enabled (AfcAutoOn = 1), the user has the option to:
 Clear the former AFC correction value, if AfcAutoClearOn = 1
 Start the AFC evaluation from the previously corrected frequency. This may be useful in systems in which the LO
keeps on drifting in the “same direction”. Ageing compensation is a good example.
The RFM66W offers an alternate receiver bandwidth setting during the AFC phase, to accommodate large LO drifts. If
the user considers that the received signal may be out of the receiver bandwidth, a higher channel filter bandwidth can
be programmed in RegAfcBw, at the expense of the receiver noise floor, which will impact upon sensitivity.
The FEI is valid only during preamble, and therefore the PreambleDetect flag can be used to validate the current FEI result
and add it to the AFC register. The link between PreambleDetect interrupt and the AFC is controlled by
StartDemodOnPreamble in RegRxConfig.
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