Rainbow Electronics RF22 - Manual (Page 16)

RF22
Version: 0.1 Date: 12/23/2008
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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.
Add R/W Function/Description D7 D6 D5 D4 D3 D2 D1 D0 POR Def. Notes
73 R/W Frequency Offset fo[7] fo[6] fo[5] fo[4] fo[3] fo[2] fo[1] fo[0] 00h 73
74 R/W Frequency Offset fo[9] fo[8] 00h
3.2.9.6. 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 versus carrier
offset and the impact of AFC are illustrated in Figure 9.
Sensitivity vs Carrier Frequency Offset
(Rb = 100 kHz, Fd = 50 kHz)
-104
-102
-100
-98
-96
-94
-92
-90
-88
-86
-100-50050100
Frequency Offset (kHz)
RX Sensit
iv
ity
(dB
m)
AFC Disable
AFC Enable
Figure9. Sensitivity vs. Carrier Frequency Offset
The AFC function shares registers 73h and 74h with the Frequency Offset setting. If AFC is enabled, the Frequency
Offset shows the results of the AFC algorithm for the current receive slot. When selecting the preamble length, the
length needs to be long enough to settle the AFC. In general two bytes of preamble is sufficient to settle the AFC.
Disabling the AFC allows the preamble to be shortened. Note that with the AFC disabled, the preamble length must still
be long enough to settle the receiver and to detect the preamble (see "6.7. Preamble Length" on page 45). The AFC
corrects the detected frequency offset by changing the frequency of the Fractional-N PLL. At the end of the preamble
the AFC will freeze. In multi-packet mode the AFC is reset at the end of every packet and will re-acquire the frequency
offset for the next packet. An automatic reset circuit prevents excessive drift by resetting the AFC loop when the tuning
exceeds 2 times the frequency deviation (as set by fd[8:0] in register 71h and 72h) in high band or 1 times the
frequency deviation in low band. This range can be halved by the “afcbd” bit in register 1Dh.
In TX mode the "Register 73h. Frequency Offset 1" is used to provide an offset to the programmed transmit frequency.
This offset allows fine tuning of the transmit frequency to account for the variability of the TX reference frequency. Note
that reading this register shows the frequency offset calculated from the last AFC action not what was previously
written to the Frequency Offset register.
Frequency Correction
RX TX
AFC disabled Freq Offset Register Freq Offset Register
AFC enabled AFC Freq Offset Register