SX1231
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
00b / 16
10b / 24
01b / 20
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2
1
1
1
0
0
0
125.0
166.7
200.0
250.0
333.3
400.0
500.0
62.5
83.3
100.0
125.0
166.7
200.0
250.0
3.5.6. DC Cancellation
DC cancellation is required in zero-IF architecture transceivers to remove any DC offset generated through self-reception.
It is built-in the SX1231 and its adjustable cutoff frequency fc is controlled in RegRxBw:
4 × RxBw
-----------------------------------------
fc =
2π × 2DccFreq + 2
The default value of DccFreq cutoff frequency is typically 4% of the RxBw (channel filter BW). The cutoff frequency of the
DCC can however be increased to slightly improve the sensitivity, under wider modulation conditions. It is advised to adjust
the DCC setting while monitoring the receiver sensitivity.
3.5.7. Complex Filter - OOK
In OOK mode the SX1231 is modified to a low-IF architecture. The IF frequency is automatically set to half the single side
bandwidth of the channel filter (F = 0.5 x RxBw). The Local Oscillator is automatically offset by the IF in the OOK receiver.
IF
A complex filter is implemented on the chip to attenuate the resulting image frequency by typically 30 dB.
Note this filter is automatically bypassed when receiving FSK signals (ModulationType = 00 in RegDataModul).
3.5.8. RSSI
The RSSI block evaluates the amount of energy available within the receiver channel bandwidth. Its resolution is 0.5 dB,
and it has a wide dynamic range to accommodate both small and large signal levels that may be present. Its acquisition
time is very short, taking only 2 bit periods. The RSSI sampling must occur during the reception of preamble in FSK, and
constant “1” reception in OOK.
Note The receiver is capable of automatic gain calibration, in order to improve the precision of its RSSI measurements.
This function injects a known RF signal at the LNA input, and calibrates the receiver gain accordingly. This
calibration is automatically performed during the PLL start-up, making it a transparent process to the end-user.
3.5.9. Cordic
The Cordic task is to extract the phase and the amplitude of the modulation vector (I+j.Q). This information, still in the
digital domain is used:
Phase output: used by the FSK demodulator and the AFC blocks.
Amplitude output: used by the RSSI block, for FSK demodulation, AGC and automatic gain calibration purposes.
Rev 2 - Nov 2009
Page 26
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