SX1231 SEMTECH [Semtech Corporation], SX1231 Datasheet - Page 22

• Manufacturer Part Number: SX1231
• Description: Low Power Integrated UHF Transceiver
• Manufacturer: SEMTECH [Semtech Corporation]

3.5. Receiver Description
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
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
Table 11 LNA Gain Settings
Rev 2 - Nov 2009
RFIO
ADVANCED COMMUNICATIONS & SENSING
Rx Calibration
Reference
From
PA1
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.
LNA
Differential
Single to
Oscillator
Local
LnaGainSelect
Mixers
000
001
010
100
101
011
110
111
Modulators
Σ/Δ
Figure 6. Receiver Block Diagram
Any of the below, set by the AGC loop
Channel
Filter
Max gain - 12 dB
Max gain - 24 dB
Max gain - 36 dB
Max gain - 48 dB
Max gain - 6 dB
Page 22
AFC
AGC
LNA Gain
Reserved
Max gain
Cancellation
DC
Complex
Bypassed
Filter
in FSK
Gain Setting
CORDIC
Module
Output
Phase
Output
G1
G2
G3
G4
G5
G6
-
-
RSSI
Demodulator
DATASHEET
www.semtech.com
SX1231
FSK
Demodulator
OOK