RFM95 Datasheet, PDF (47/121 Pages) List of Unclassifed Manufacturers – Low Power Long Range Transceiver Module

WIRELESS & SENSING
RFM95/96/97/98(W)
PRELIMINARY
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:
Š The measurement must be launched during the reception of preamble.
Š 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):
B W _{20 d B}
=
2
×
⎛
⎝
F_DEV
+
--2-----_⎠⎞
The frequency error, in Hz, can be calculated with the following formula:
FEI = F_STEP × FeiValue
RFM96/7/8 in Rx mode
Preamble-modulated input signal
Signal level > Sensitivity
Set FeiStart
=1
FeiDone
No
=1
Yes
Read
FeiValue
Figure 16. FEI Process
Page 47
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