SX1231
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
3.3.2. CLKOUT Output
The reference frequency, or a fraction of it, can be provided on DIO5 (pin 12) by modifying bits ClkOut in RegDioMapping2.
Two typical applications of the CLKOUT output include:
To provide a clock output for a companion processor, thus saving the cost of an additional oscillator. CLKOUT can be
made available in any operation mode except Sleep mode and is automatically enabled at power on reset.
To provide an oscillator reference output. Measurement of the CLKOUT signal enables simple software trimming of the
initial crystal tolerance.
Note to minimize the current consumption of the SX1231, please ensure that the CLKOUT signal is disabled when not
required.
3.3.3. PLL Architecture
The frequency synthesizer generating the LO frequency for both the receiver and the transmitter is a fractional-N sigma-
delta PLL. The PLL incorporates a third order loop capable of fast auto-calibration, and it has a fast switching-time. The
VCO and the loop filter are both fully integrated, removing the need for an external tight-tolerance, high-Q inductor in the
VCO tank circuit.
3.3.3.1. VCO
The VCO runs at 2, 4 or 6 times the RF frequency (respectively in the 915, 434 and 315 MHz bands) to reduce any LO
leakage in receiver mode, to improve the quadrature precision of the receiver, and to reduce the pulling effects on the VCO
during transmission.
The VCO calibration is fully automated. A coarse adjustment is carried out at power on reset, and a fine tuning is
performed each time the SX1231 PLL is activated. Automatic calibration times are fully transparent to the end-user, as their
processing time is included in the TS_TE and TS_RE specifications.
3.3.3.2. PLL Bandwidth
The bandwidth of the SX1231 Fractional-N PLL is wide enough to allow for:
High speed FSK modulation, up to 300 kb/s, inside the PLL bandwidth
Very fast PLL lock times, enabling both short startup and fast hop times required for frequency agile applications
3.3.3.3. Carrier Frequency and Resolution
The SX1231 PLL embeds a 19-bit sigma-delta modulator and its frequency resolution, constant over the whole frequency
range, and is given by:
FXOSC
----------------
219
FSTEP
=
The carrier frequency is programmed through RegFrf, split across addresses 0x07 to 0x09:
FRF = FSTEP × Frf(23,0)
Note The Frf setting is split across 3 bytes. A change in the center frequency will only be taken into account when the
least significant byte FrfLsb in RegFrfLsb is written. This allows for more complex modulation schemes such as m-
ary FSK, where frequency modulation is achieved by changing the programmed RF frequency.
Rev 2 - Nov 2009
Page 17
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