SPIRIT1 datasheet(40/94 Pages) STMICROELECTRONICS

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Block description

SPIRIT1

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Doc ID 022758 Rev 3

By default, the calibration is disabled at reset to avoid using an out-of-range reference

frequency (for instance, when the XTAL is 26 MHz and the digital divider is active, in fact, by

default). After the internal clock divider is correctly configured, the user can enable the RCO

calibration in the PROTOCOL[2] register.

8.6.1

RC oscillator calibration

RC oscillator calibration is enabled when bit RCO_CALIBRATION is set in the

PROTOCOL[2] register (by default the calibration is disabled). The calibration words found

by the calibration algorithm are accessible in the RCO_VCO_CALIBR_OUT[1:0] registers

(fields RWT_OUT[3:0] and RFB_OUT[4:0],).

When the calibration is disabled, the frequency of the RC oscillator is set by a couple of

configuration words, namely RWT_IN[3:0] and RFB_IN[4:0], in the

RCO_VCO_CALIBR_IN[2:0] registers (fields RWT_IN[3:0] and RFB_IN[4:0]). RWT_IN[3:0]

can range from 0 up to 13 (decimal value) affecting the raw value of the frequency, while the

more accurate and fine control is up to RFB_IN[4:0] (ranging from 1 up to 31).

8.7

AFC

The SPIRIT1 implements an automatic frequency compensation algorithm to balance

TX/RX crystal frequency inaccuracies. The receiver demodulator estimates the centre of the

received data and compensates the offset between nominal and receiver frequency.

The tracking range of the algorithm is programmable and is a fraction of the receive channel

bandwidth. Frequency offset compensation is supported for 2-FSK, GFSK, and MSK

modulation.

When the relative frequency error between transmitter and receiver is less than half the

modulation bandwidth, the AFC corrects the frequency error without needing extra

bandwidth. When the frequency error exceeds BWmod/2, some extra bandwidth is needed

to assure proper AFC operation under worst-case conditions. The AFC can be disabled if

the TX/RX frequency misalignment is negligible with respect to the receiver bandwidth, for

example, when using a TCXO.

8.8

Receiver

The SPIRIT1 contains a low-power low-IF receiver which is able to amplify the input signal

and provide it to the ADC with a proper signal to noise ratio. The RF antenna signal is

converted to a differential one by an external balun, which performs an impedance

transformation also. The receiver gain can be programmed to accommodate the ADC input

Table 23.

RC calibrated speed

Digital domain clock

24_26MHz_SELECT

RC calibrated speed

24 MHz

34.7 kHz

26 MHz

34.7 kHz

25 MHz

36.1 kHz

25 MHz

33.3 kHz

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