S2-LPS datasheet(24/73 Pages) STMICROELECTRONICS

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In case PA_POWER[0] = 0 then the modulation will be OOK, otherwise when PA_POWER[0] is not set to zero the

modulation will be ASK. The 0/1 mapping can be reversed by setting the CONST_MAP register to any value other

than zero.

When ASK is selected, a bit '1' is transmitted with a power ramp increasing from the minimum value specified

by register PA_POWER[0] to specified PA maximum level in register PA_POWER[PA_LEVEL_MAX_INDEX], vice

versa for a bit '0'. The duration of each power step is a multiple of 1/8 of the symbol time, configurable with the

power at the programmed value. If more '0's are transmitted consecutively, the PA power remains at minimum

power for all '0's following the first one.

In order to improve the spectral emission mask is ASK a digital interpolation optional features have been

implemented. When this feature is enabled, thought the register PA_INTERP_EN, the modulator linearly

interpolates the power values specified in the PA_POWER registers before being applied to the PA.

The interpolation factor of each ramp step is 64 time the data rate corresponding to DATA_RATE_E < 5 it is

automatically scaled as 64/2(DATA_RATE_E-5) for 5 ≤ DATA_RATE_E < 11 and it is automatically disabled for

DATA_RATE_E=11.

Note that the number of clock cycles between successive PA ticks, for DATA_RATE_E≥5, is always between 8

and 4 (8 for DATA_RATE_M=0; 4 for DATA_RATE_M=65535).

OOK/ASK demodulation is controlled by the OOK_PEAK_DECAY parameter ( recommended value is 3) in the

RSSI_FLT register.

4.4.2.1

OOK smoothing

The OOK can be smoothed using a FIR filter added in the data path. This feature is activated by setting the

FIR_EN bit at 1 inside register PA_CONFIG1.

The FIR filter is not fully customizable but it can be set in 3 different configurations that change the spectrum

shape (and thus the bandwidth):

•

filter: it is the proper FIR filtering function of the stream of bits 8 times oversampled;

•

ramp: the FIR filter is optimized to perform a ramping between PA_POWER_MAX and PA_POWER_0 (for

OOK should be set to 0).

•

switch: logic 1s and 0s are associated with a single value of power and no transition between the 2 is

envisaged.

When the FIR_EN bit is 1, the DIG_SMOOTH_EN (PA_POWER_0 register) must be set to 1.

Finally, a 2nd order Bessel analog filter can be used to smooth the output signal. The bandwidth of this filter

should be set according to the data rate used by setting the PA_FC field of the register PA_CONFIG0 according

to the following table:

Table 36. PA Bessel filter words

PA_FC bits

Cut-off frequency (kHz)

Max. data rate (kbit/s)

12.5

62.5

100

125

Note:

The FIR ramping modes are used in a mutually exclusive way with the digital ramping. When the digital ramping

is used, the FIR ramping should be disabled. Vice versa, if the FIR ramping is used, the digital one is not used.

4.4.3

Direct polar mode

The S2-LPS allows the user to drive the SYNTH and the PA at a very low level. The byte couples written in the

TX_FIFO are sampled with a rate related to the DATARATE chip setting (sampling rate = 8*DATARATE).

The first byte of the couple drives the frequency synthesizer to obtain an instantaneous output frequency

deviation given by the formula below:

Frequency deviation in polar mode

fdev=fdev_programmed*fdev_fifo_sample

128

(11)

S2-LPS

Digital modulator

DS13831 - Rev 1

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