Data Sheet

21
Version 1.1
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Figure 9 - SX1257 Digital I/Q Power Spectral Density
The SX1257 RX PLLs can be configured to any
frequency inside the 868/900 MHz ISM band with
a 61 Hz step. The SX1257 streams I/Q samples
through a 2 wire digital interface. The bits stream
corresponds directly to the I/Q sigma delta ADCs
outputs sampled at 32 MSps. This delta sigma
stream must be low-passed and decimated to
recover the available 80dB dynamic of the ADCs.
After decimation the usable spectrum bandwidth
is ±400 kHz centered on the RX PLL carrier
frequency.
The following plot gives the spectral power
content of the I/Q bit stream.
The quantization noise raises sharply outside the
-400 to+400 kHz range. For more details on the
SX1257 radio specifications please consult the
specific product datasheet.
The following plot represents a possible use case
11.5
For words of more than 1 byte, MSBs are sent first.
Bytes 9 to 15 vary depending whether the FSK or
the LoRa TX modem is being used.
The user payload starts at byte 16. This is the first
byte that will be received by the end point. Bytes
0 to 15 are not transmitted and are just used to
dynamically configure the gateway prior to
emission.
Receiver IF Frequencies Configuration
Each IF path intermediate frequency can be
programmed independently from -2 to +2 MHz.
The following sections give a few programming
examples for various use cases.
11.5.1 PyGate Using 2 x SX1257 Radios
where:
- Radio A PLL is set to 867.0 MHz
- Radio B PLL is set to 868.4 MHz
- The system uses 8 separate 125 kHz LoRa
channels for star connection to sensors
- One high speed 250 kHz LoRa channel for
connection to a relay
- One high speed 200 kHz GFSK channel for
meshing