SX1276IMLTRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
Manufacturer
SX1276IMLTRT Datasheet PDF : 132 Pages
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SX1276/77/78/79
WIRELESS, SENSING & TIMING
DATASHEET
4.1.4. Frequency Settings
Recalling that the frequency step is given by:
FSTEP = F----X-2---O1--9-S---C--
In order to set LO frequency values following registers are available.
Frf is a 24-bit register which defines carrier frequency. The carrier frequency relates to the register contents by following
formula:
FRF = FSTEP × Frf(23,0)
4.1.5. Frequency Error Indication
The SX1276/77/78/79 derives its RF centre frequency from a crystal reference oscillator which has a finite frequency
precision. Errors in reference frequency will manifest themselves as errors of the same proportion from the RF centre
frequency.
In LoRa receive mode the SX1276/77/78/79 is capable of measuring the frequency offset between the receiver centre
frequency and that of an incoming LoRa signal. The modem is intolerant of frequency offsets in the region of +/- 25% of the
bandwidth and will accurately report the error over this same range.
The error is read by reading the three RegFei registers. The contents of which are a signed 20 bit two's compliment word,
FreqError. The frequency error is determined from the register contents by:
Where Fxtal is the crystal frequency.
FError = F-----r--e---q----E--F--r--xr--t-o-a--rl----×-----2---2---4 × B-----W---5--[-0-k--0-H-----z---]
To correct the measured frequency error there are two steps to be taken. First the frequency error is subtracted from the
RF centre frequency. This calculation must be performed locally (or in a look-up-table), no provision is made in the circuit to
apply the correction automatically.
Secondly, assuming that the frequency error is due to reference oscillator drift, the data rate of the LoRa modem must also
be compensated accordingly. This is done by
PpmOffset = 0.95 * measured Offset [PPM]
Where PpmOffset is the value to be programmed into register 0x27 and the measured Offset is the PPM drift equivalent to
the frequency error reported by the LoRa frequency error indicator. The PpmOffset value is a signed two’s compliment
value.
Rev. 4 - March 2015
©2015 Semtech Corporation
Page 37
www.semtech.com
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