STM32W108HB datasheet(30/179 Pages) STMICROELECTRONICS

30 / 179 page

Radio frequency module

STM32W108CB, STM32W108HB

30/179

Doc ID 16252 Rev 2

Radio frequency module

The radio module consists of an analog front end and digital baseband as shown in

Figure 1: STM32W108 block diagram.

4.1

Receive (Rx) path

The Rx path uses a low-IF, super-heterodyne receiver that rejects the image frequency

using complex mixing and polyphase filtering. In the analog domain, the input RF signal

from the antenna is first amplified and mixed down to a 4 MHz IF frequency. The mixers'

output is filtered, combined, and amplified before being sampled by a 12 Msps ADC. The

digitized signal is then demodulated in the digital baseband. The filtering within the Rx path

improves the STM32W108's co-existence with other 2.4 GHz transceivers such as IEEE

802.15.4, IEEE 802.11g, and Bluetooth radios. The digital baseband also provides gain

control of the Rx path, both to enable the reception of small and large wanted signals and to

tolerate large interferers.

4.1.1

Rx baseband

The STM32W108 Rx digital baseband implements a coherent demodulator for optimal

performance. The baseband demodulates the O-QPSK signal at the chip level and

synchronizes with the IEEE 802.15.4-defined preamble. An automatic gain control (AGC)

module adjusts the analog gain continuously every ¼ symbol until the preamble is detected.

Once detected, the gain is fixed for the remainder of the packet. The baseband despreads

the demodulated data into 4-bit symbols. These symbols are buffered and passed to the

hardware-based MAC module for packet assembly and filtering.

In addition, the Rx baseband provides the calibration and control interface to the analog Rx

modules, including the LNA, Rx baseband filter, and modulation modules. The ST RF

software driver includes calibration algorithms that use this interface to reduce the effects of

silicon process and temperature variation.

4.1.2

RSSI and CCA

The STM32W108 calculates the RSSI over every 8-symbol period as well as at the end of a

received packet. The linear range of RSSI is specified to be at least 40 dB over temperature.

At room temperature, the linear range is approximately 60 dB (-90 dBm to -30 dBm input

signal).

The STM32W108 Rx baseband provides support for the IEEE 802.15.4-2003 RSSI CCA

method, Clear channel reports busy medium if RSSI exceeds its threshold.

4.2

Transmit (Tx) path

The STM32W108 Tx path produces an O-QPSK-modulated signal using the analog front

end and digital baseband. The area- and power-efficient Tx architecture uses a two-point

modulation scheme to modulate the RF signal generated by the synthesizer. The modulated

RF signal is fed to the integrated PA and then out of the STM32W108.

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