SX1232
WIRELESS & SENSING
DATASHEET
The frequency error, in Hz, can be calculated with the following formula:
FEI = FSTEP × FeiValue
SX1232 in Rx mode
Preamble-modulated input signal
Signal level > Sensitivity
Set FeiStart
= 1
No
FeiDone
= 1
Yes
Read
FeiValue
Figure 13. FEI Process
3.5.9. 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 . The AFC is executed each time the receiver is enabled, if AfcAutoOn = 1.
RF
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 SX1232 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.
A detailed description of the receiver setup to enable the AFC is provided in section 4.3.
Rev 3 - August 2012
Page 34
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