SX1276 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SX1276' PDF : 132 Pages View PDF
SX1276/77/78/79
WIRELESS, SENSING & TIMING
DATASHEET
4.2.3.6. Preamble Detector
The Preamble Detector indicates the reception of a carrier modulated with a 0101...sequence. It is insensitive to the
frequency offset, as long as the receiver bandwidth is large enough. The size of detection can be programmed from 1 to 3
bytes with PreambleDetectorSize in RegPreambleDetect as defined in the next table.
Table 20 Preamble Detector Settings
PreambleDetectorSize
00
01
10
11
# of Bytes
1
2 (recommended)
3
reserved
For normal operation, PreambleDetectTol should be set to be set to 10 (0x0A), with a qualifying preamble size of 2 bytes.
The PreambleDetect interrupt (either in RegIrqFlags1 or mapped to a specific DIO) then goes high every time a valid
preamble is detected, assuming PreambleDetectorOn=1.
The preamble detector can also be used as a gate to ensure that AFC and AGC are performed on valid preamble. See
section 4.2.6. for details.
4.2.3.7. Image Rejection Mixer
The SX1276/77/78/79 employs an image rejection mixer (IRM) which, uncalibrated, 35 dB image rejection. A low phase
noise PLL is used to perform calibration of the receiver chain. This increases the typical image rejection to 48 dB.
4.2.3.8. Image and RSSI Calibration
An automated process is implemented to calibrate the phase and gain imbalances of I and Q receive paths. This calibration
enhances image rejection and improves RSSI precision. It is launched under the following circumstances:
Automatically at Power On Reset or after a Manual Reset of the chip (refer to section 7.2), only for the Low Frequency
front-end, and is performed at 434MHz
Automatically when a pre-defined temperature change is observed, if the option is enabled. A selectable temperature
change, set with TempThreshold (5, 10, 15 or 20°C), is detected and reported in TempChange, if the temperature
monitoring is turned On with TempMonitorOff=0.This interrupt flag can be used by the application to launch a new
image calibration at a convenient time if AutoImageCalOn=0, or immediately when this temperature variation is
detected, if AutoImageCalOn=1
Upon user request, by setting bit ImageCalStart in RegImageCal, when the device is in Standby mode
Notes - The calibration procedure takes approximately 10ms. It is recommended to disable the fully automated
(temperature-dependent) calibration, to better control when it is triggered (and avoid unexpected packet losses)
- To perform the calibration, the radio must be temporarily returned to FSK/OOK mode
- The automatic IQ and RSSI calibration done at POR and Reset is only valid at 434 MHz (the value of RegFrf at
POR). To improve accuracy of RSSI and image rejection, this calibration should be replicated at the frequency (ies)
Rev. 4 - March 2015
©2015 Semtech Corporation
Page 53
www.semtech.com
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