SX1276 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SX1276' PDF : 132 Pages View PDF
SX1276/77/78/79
WIRELESS, SENSING & TIMING
DATASHEET
4.2.3.4. Frequency Error Indicator
This frequency error indicator measures the frequency error between the programmed RF centre frequency and the carrier
frequency of the modulated input signal to the receiver. When the FEI is performed, the frequency error is measured and
the signed result is loaded in FeiValue in RegFei, in 2’s complement format. The time required for an FEI evaluation is 4 bit
periods.
To ensure correct operation of the FEI:
The measurement must be launched during the reception of preamble.
The sum of the frequency offset and the 20 dB signal bandwidth must be lower than the base band filter bandwidth. i.e.
The whole modulated spectrum must be received.
The 20 dB bandwidth of the signal can be evaluated as follows (double-side bandwidth):
BW20dB = 2 × FDEV + B---2--R--
The frequency error, in Hz, can be calculated with the following formula:
FEI = FSTEP × FeiValue
The FEI is enabled automatically upon the transition to receive mode and automatically updated every 4 bits.
4.2.3.5. AFC
The AFC is based on the FEI measurement, therefore the same input signal and receiver setting conditions apply. When
the AFC procedure is performed the AfcValue is directly subtracted from the register that defines the frequency of
operation of the chip, FRF. 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. Allowing the next frequency correction to be performed
from the initial centre frequency.
Start the AFC evaluation from the previously corrected frequency. This may be useful in systems in which the centre
frequency experiences cumulative drift - such as the ageing of a crystal reference.
The SX1276/77/78/79 offers an alternate receiver bandwidth setting during the AFC phase allowing the accommodation of
larger frequency errors. The setting RegAfcBw sets the receive bandwidth during the AFC process. In a typical receiver
application the, once the AFC is performed, the radio will revert to the receiver communication or channel bandwidth
(RegRxBw) for the ensuing communication phase.
Note that the FEI measurement is valid only during the reception of preamble. The provision of the PreambleDetect flag
can hence be used to detect this condition and allow a reliable AFC or FEI operation to be triggered. This process can be
performed automatically by using the appropriate options in StartDemodOnPreamble found in the RegRxConfig register.
A detailed description of the receiver setup to enable the AFC is provided in section 4.2.6.
Rev. 4 - March 2015
©2015 Semtech Corporation
Page 52
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