RFM12B_15 Datasheet, PDF (22/41 Pages) List of Unclassifed Manufacturers – Universal ISM Band FSK Transceiver

RFM12B
Bit 5-4 (rl1 to rl0): Range limit. Limits the value of the frequency offset register to the next values:
rl1 rl0
00
01
10
11
Max deviation
No restriction
+15 f _res to -16 f _res
+7 f _res to -8 f _res
+3 f _res to -4 f _res
f res :
433 MHz bands: 2.5 kHz
868 MHz band: 5 kHz
915 MHz band: 7.5 kHz
Bit 3 (st):
Bit 2 (fi):
Bit 1 (oe):
Bit 0 (en):
Strobe edge, when st goes to high, the actual latest calculated frequency error is stored into the offset register of
the AFC block.
Switches the circuit to high accuracy (fine) mode. In this case, the processing time is about twice as long, but the
measurement uncertainty is about half.
Enables the frequency offset register. It allows the addition of the offset register to the frequency control word of
the PLL.
Enables the calculation of the offset frequency by the AFC circuit.
In manual mode, the strobe signal is provided by the microcontroller. One measurement cycle (and strobe) signal can compensate
about 50-60% of the actual frequency offset. Two measurement cycles can compensate 80%, and three measurement cycles can
compensate 92%. The ATGL bit in the status register can be used to determine when the actual measurement cycle is finished.
In automatic operation mode (no strobe signal is needed from the microcontroller to update the output offset register) the AFC circuit
is automatically enabled when the VDI indicates potential incoming signal during the whole measurement cycle and the circuit
measures the same result in two subsequent cycles.
Without AFC the transmitter and the receiver needs to be tuned precisely to the same frequency. RX/TX frequency offset can lower
the range. The units must be adjusted carefully during production, stable, expensive crystal must be used to avoid drift or the output
power needs to be increased to compensate yield loss.
The AFC block will calculate the TX-RX offset. This value will be used to pull the RX synthesizer close to the frequency of the
transmitter. The main benefits of the automatic frequency control: cheap crystal can be used, the temperature or aging drift will not
cause range loss and no production alignment needed.
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