SX1239 View Datasheet(PDF) - Semtech Corporation

Part Name

Description

Manufacturer

SX1239

Low Power Integrated UHF Receiver

Semtech Corporation 

SX1239

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:

Š 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.

Š 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 SX1239, please ensure that the CLKOUT signal is disabled when not

required.

3.3.3. PLL Architecture

The frequency synthesizer generating the LO frequency for the receiver 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.

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 SX1239 PLL is activated. Automatic calibration times are fully transparent to the end-user, as their

processing time is included in the TS_RE specifications.

3.3.3.2. PLL Bandwidth

The bandwidth of the SX1239 Fractional-N PLL is wide enough to allow for 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 SX1239 PLL embeds a 19-bit sigma-delta modulator and its frequency resolution, constant over the whole frequency

range, and is given by:

FSTEP

=

F----X----O---S---C--

219

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.

Rev 3 - Jan 2011

Page 16

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