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SX1231
WIRELESS & SENSING PRODUCTS
When the AFC is automatically triggered ( AfcAutoOn = 1), the user has the option to:
DATASHEET
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Clear the former AFC correction value, if AfcAutoClearOn = 1
Start the AFC evaluation from the previously corrected frequency. This may be useful in systems in which the LO keeps
on drifting in the “same direction”. Ageing compensation is a good example.
The SX1231 offers an alternate receiver bandwidth setting during the AFC phase, to accommodate large LO drifts. If the
user considers that the received signal may be out of the receiver bandwidth, a higher channel filter bandwidth can be
programmed in RegAfcBw , at the expense of the receiver noise floor, which will impact upon sensitivity.
3.5.16. Optimized Setup for Low Modulation Index Systems
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For wide band systems, where AFC is usually not required (XTAL inaccuracies do not typically impact the sensitivity), it
is recommended to offset the LO frequency of the receiver to avoid desensitization. This can be simply done by
modifying Frf in RegFrfLsb . A good rule of thumb is to offset the receiver ’s LO by 10% of the expected transmitter
frequency deviation.
For narrow band systems, it is recommended to perform AFC. The SX1231 has a dedicated AFC, enabled when
AfcLowBetaOn in RegAfcCtrl is set to 1. A frequency offset, programmable through LowBetaAfcOffset in RegTestAfc , is
added and is calculated as follows:
Offset = LowBetaAfcOffset x 488 Hz
The user should ensure that the programmed offset exceeds the DC canceller ’s cutoff frequency, set through DccFreqAfc
in RegAfcBw.
RX
TX
RX & TX
FeiValue
f
Standard AFC
AfcLowBetaOn = 0
AfcValue
f
RX
TX
TX RX
FeiValue
Optimized AFC
AfcValue
LowBetaAfcOffset
AfcLowBetaOn = 1
Before AFC
f
After AFC
f
Figure 15. Optimized AFC (AfcLowBetaOn=1)
As shown on Figure 15, a standard AFC sequence uses the result of the FEI to correct the LO frequency and align both
local oscillators. When the optimized AFC is enabled ( AfcLowBetaOn=1 ), the receiver ’s LO is corrected by “ FeiValue +
LowBetaAfcOffset ”.
When the optimized AFC routine is enabled, the receiver startup time can be computed as follows (refer to section 4.2.3):
TS_RE_AGC&AFC (optimized AFC) = Tana + 4.Tcf + 4.Tdcc + 3.Trssi + 2.Tafc + 2.Tpllafc
Rev. 7 - June 2013
Page 35
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