SX1231
ADVANCED COMMUNICATIONS & SENSING
3.5. Receiver Description
DATASHEET
The SX1231 features a digital receiver with the analog to digital conversion process being performed directly following the
LNA-Mixers block. The zero-IF receiver is able to handle (G)FSK and (G)MSK modulation. ASK and OOK modulation is,
however, demodulated by a low-IF architecture. All the filtering, demodulation, gain control, synchronization and packet
handling is performed digitally, which allows a very wide range of bit rates and frequency deviations to be selected. The
receiver is also capable of automatic gain calibration in order to improve precision on RSSI measurements.
3.5.1. Block Diagram
Rx Calibration
Reference
Σ/Δ
LNA
CORDIC
Mixers Modulators
DC
Single to
Differential
Channel
Filter
Complex
Filter
Cancellation
Phase
Output
FSK
Demodulator
RFIO
Module
Output
OOK
Demodulator
From
PA1
RSSI
Bypassed
in FSK
Local
Oscillator
AFC
AGC
Figure 6. Receiver Block Diagram
The following sections give a brief description of each of the receiver blocks.
3.5.2. LNA - Single to Differential Buffer
The LNA uses a common-gate topology, which allows for a flat characteristic over the whole frequency range. It is
designed to have an input impedance of 50 Ohms or 200 Ohms (as selected with bit LnaZin in RegLna), and the parasitic
capacitance at the LNA input port is cancelled with the external RF choke. A single to differential buffer is implemented to
improve the second order linearity of the receiver.
The LNA gain, including the single-to-differential buffer, is programmable over a 48 dB dynamic range, and control is either
manual or automatic with the embedded AGC function.
Note In the specific case where the LNA gain is manually set by the user, the receiver will not be able to properly handle
FSK signals with a modulation index smaller than 2 at an input power greater than the 1dB compression point,
tabulated in section 3.5.3.
Table 11 LNA Gain Settings
LnaGainSelect
LNA Gain
Any of the below, set by the AGC loop
Max gain
Gain Setting
000
001
010
011
100
101
110
111
-
G1
G2
G3
G4
G5
G6
-
Max gain - 6 dB
Max gain - 12 dB
Max gain - 24 dB
Max gain - 36 dB
Max gain - 48 dB
Reserved
Rev 2 - Nov 2009
Page 22
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