RFM92 Datasheet, PDF (49/126 Pages) List of Unclassifed Manufacturers – Low Power Long Range Transceiver Module

RFM92W/93W V3.0
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 19 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 4.2.7. for details.
4.2.3.7. Image Rejection Mixer
The RFM92W/93W employs an image rejection mixer (IRM) which, uncalibrated, gives 35 dB image rejection. The low
phase noise PLL is used to perform calibration of the receiver chain. Which increases the typical image rejection to 48
dB. This process is fully automated in FSK/OOK mode and radio power-up.
4.2.3.8. Image and RSSI Calibration
An automatic calibration process is used to calibrate the phase and gain of both I and Q receive paths. This calibration
allows enhanced image frequency rejection and improves the RSSI precision. This calibration process is launched under
the following circumstances:
Š Automatically at Power On Reset or after a Manual Reset of the chip (refer to section 7.2). For applications where the
temperature remains stable, or if the Image Rejection is not a major concern, this single calibration will suffice.
Š Automatically when a pre-defined temperature change is observed.
Š Upon User request, by setting bit ImageCalStart in RegImageCal, when the device is in Standby mode. Note that in
LoRa^TM mode the calibration command is inaccessible. To perform the calibration the radio must be returned temporarily
to FSK/OOK mode.
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.
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