S2-LP datasheet(34/90 Pages) STMICROELECTRONICS

34 / 90 page

Block description

S2-LP

34/90

DocID029944 Rev 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 40: PA Bessel filter words

PA_FC bits

Cutoff Frequency (kHz)

Max Data rate (kbit/s)

12.5

62.5

100

125

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.

5.4.3

Direct polar mode

The S2-LP 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:

Equation 8

– Frequency deviation in polar mode

�� = ��_�� ∗

��_��_��

128

Where

��_�� is the frequency deviation programmed in the chip by the

registers MOD[1:0] (see Section 5.4.1: "Frequency modulation"),

��_��_�� is the

first byte of the bytes couple sampled from the TX_FIFO.

The

��_��_�� is interpreted as a 2-complement 8-bit number, thus it can be either

a positive or a negative value.

The instantaneous frequency is given by the formula:

Equation 9

– Instantaneous frequency in polar mode

�� = ��_�� + ��

The second byte of the TX_FIFO couple drives the PA giving an instantaneous output

power.

The output power will be generated according to this value following the same code as the

PA_POWER registers (see Section 5.6.1: "PA configuration").

Figure 6: "Direct polar mode" shows how the byte couples are sampled from the TX FIFO

and sent to the SYNTH and PA blocks.

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