RFM22B/23B
3.5.5. Frequency Offset Adjustment
When the AFC is disabled the frequency offset can be adjusted manually by fo[9:0] in registers 73h and 74h. It is
not possible to have both AFC and offset as internally they share the same register. 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 it is necessary to take the twos complement of the positive
offset number. The offset can be calculated by the following:
DesiredOffset 156.25Hz(hbsel 1) fo[9:0]
DesiredOffset
fo[9:0]
156.25Hz(hbsel 1)
The adjustment range in high band is ±160 kHz and in low band it 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.
D7
D6
D5
D4
D3
D2
D1
D0
POR
Def.
Add R/W Function/Description
73 R/W
74 R/W
Frequency Offset
Frequency Offset
fo[7]
fo[6]
fo[5]
fo[4]
fo[3]
fo[2]
fo[1] fo[0]
fo[9] fo[8]
00h
00h
3.5.6. Automatic Frequency Control (AFC)
All AFC settings can be easily obtained from the settings calculator. This is the recommended method to program
all AFC settings. This section is intended to describe the operation of the AFC in more detail to help understand the
trade-offs of using 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 11.
Figure 11. Sensitivity at 1% PER vs. Carrier Frequency Offset
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