RFM23
3.6.5. Frequency Offset Adjustment
When the AFC is disabled the frequency offset can be adjusted manually by fo[9:0] in registers 73h and 74h. The
frequency offset adjustment and the AFC both are implemented by shifting the Synthesizer Local Oscillator
frequency. This register is a signed register so in order to get a negative offset you will need to take the twos
complement of the positive offset number. The offset can be calculated by the following:
DesiredOffset = 156.25Hz x (hbsel + 1) x fo[9 : 0]
DesiredOffset
156.25Hz x (hbsel + 1)
fo[9 : 0] =
The adjustment range in high band is: ±160 kHz, and adjustment range in low band is: ±80 kHz. For example to
compute an offset of +50 kHz in high band mode fo[9:0] should be set to 0A0h. For an offset of –50 kHz in high
band mode the fo[9:0] register should be set to 360h.
When AFC is enabled the same registers can be used to read the offset value as automatically obtained by the
AFC. A stable offset value can read after preamble detection using the preamble detection or sync word detection
interrupt.
POR
Def.
00h
Not
es
Function/Descri
ption
D7
D6
D5
D4
D3
D2
D1
D0
Add R/W
R/W
73
Frequency Offset
Frequency Offset
fo[7]
fo[6]
fo[5]
fo[4]
fo[3]
fo[2]
fo[1] fo[1]
fo[9] fo[8]
73
R/W
00h
74
3.6.6. Auto Frequency Control (AFC)
The receiver supports automatic frequency control (AFC) to compensate for frequency differences between the
transmitter and receiver reference frequencies. These differences can be caused by the absolute accuracy and
temperature dependencies of the reference crystals. Due to frequency offset compensation in the modem, the
receiver is tolerant to frequency offsets up to 0.25 times the IF bandwidth when the AFC is disabled. When the AFC
is enabled, the received signal will be centered in the pass-band of the IF filter, providing optimal sensitivity and
selectivity over a wider range of frequency offsets up to 0.35 times the IF bandwidth. The trade-off of receiver
sensitivity (at 1% PER) versus carrier offset and the impact of AFC are illustrated in Figure 10.
26
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