S2-LPTXQTR datasheet(21/64 Pages) STMICROELECTRONICS

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4.4.1

Frequency modulation

For frequency modulation 2-(G)FSK and 4-(G)FSK the frequency deviation can be tuned in wide range that

depends on

fxo(XTALfrequency)accordingthefollowingformula:

Frequency deviation

fdev=

fXO219·roundD·FDEV_M·B8

D·B if FDEV_E=0

fXO219·roundD·256+FDEV_M·2FDEV_E−1·B8

D·B

if FDEV_E >0

(10)

Where

fxoistheXTALoscillationfrequency,DisthereferencedividerandBisthebandselector.

The frequency deviation programmed corresponds to the deviation of the outer constellation symbols. The

deviation of the inner symbols is 1/3 of such programmed values, as reported in the table below, where 4 options

are available.

Furthermore, since the payload is normally arranged in bytes, the arrangement can change the mapping for both

2-(G)FSK and 4-(G)FSK modulations, by using the CONST_MAP (register MOD1), in the following way:

Table 29. Constellation mapping 2-(G)FSK

Format

Symbol

CONST_MAP coding

2-(G)FSK

-FDEV

+FDEV

-FDEV

Table 30. Constellation mapping 4-(G)FSK

Format

Symbol

CONST_MAP coding

4-(G)FSK

-FDEV/3

-FDEV

+FDEV/3

+FDEV

-FDEV

-FDEV/3

+FDEV

+FDEV/3

+FDEV

-FDEV/3

-FDEV

+FDEV

+FDEV/3

-FDEV

-FDEV/3

Furthermore, in the 4-(G)FSK it is also possible to swap the symbols using the 4FSK_SYM_SWAP field (register

PCKTCTRL3) as follows:

(11)

When 4FSK_SYM_SWAP = 0: S0 = <b7b6>

S1 = <b5b4>

S2 = <b3b2>

S3 = <b1b0>

When 4FSK_SYM_SWAP = 1 S0 = <b6b7>

S1 = <b4b5>

S2 = <b2b3>

S3 = <b0b1>

4.4.1.1

Gaussian shaping

In 2-GFSK or 4-GFSK mode, the Gaussian filter BT product can be set by using the register BT_SEL to 1 or 0.5.

The Gaussian filtering is implemented by poly-phase filtering with eight taps per symbol time. In order to further

smooth the filter shape and improve spectral shaping, the output of the filter can be linearly interpolated by setting

the register MOD_INTERP_EN.

S2-LPTX

Digital modulator

DS13100 - Rev 1

page 21/64

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