RF69 datasheet(17/81 Pages) ETC2

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RF69

Page 17

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3.2.2. 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 RF69, please ensure that the CLKOUT signal is disabled when not

required.

3.2.3. 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.

3.2.3.1. 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 RF69 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.

3.2.3.2. PLL Bandwidth

The bandwidth of the RF69 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

3.2.3.3. Carrier Frequency and Resolution

The RF69 PLL embeds a 19-bit sigma-delta modulator and its frequency resolution, constant over the whole frequency

range, and is given by:

X O S C

S T E P

----------------

The carrier frequency is programmed through RegFrf, split across addresses 0x07 to 0x09:

R F = FS T E P  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.

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