SI4438 Datasheet, PDF (22/44 Pages) Silicon Laboratories – Low active power consumption

Si4438
5. Internal Functional Blocks
The following sections provide an overview to the key internal blocks and features.
5.1. RX Chain
The internal low-noise amplifier (LNA) is designed to be a wide-band LNA that can be matched with three external
discrete components to cover any common range of frequencies in the sub-GHz band. The LNA has extremely low
noise to suppress the noise of the following stages and achieve optimal sensitivity; so, no external gain or front-end
modules are necessary. The LNA has gain control, which is controlled by the internal automatic gain control (AGC)
algorithm. The LNA is followed by an I-Q mixer, filter, programmable gain amplifier (PGA), and ADC. The I-Q
mixers downconvert the signal to an intermediate frequency. The PGA then boosts the gain to be within dynamic
range of the ADC. The ADC rejects out-of-band blockers and converts the signal to the digital domain where
filtering, demodulation, and processing is performed. Peak detectors are integrated at the output of the LNA and
PGA for use in the AGC algorithm.
5.2. RX Modem
Using high-performance ADCs allows channel filtering, image rejection, and demodulation to be performed in the
digital domain, which allows for flexibility in optimizing the device for particular applications. The digital modem
performs the following functions:
Channel selection filter
TX modulation
RX demodulation
Automatic Gain Control (AGC)
Preamble detection
Invalid preamble detection
Radio signal strength indicator (RSSI)
Automatic frequency compensation (AFC)
Cyclic redundancy check (CRC)
The digital channel filter and demodulator are optimized for ultra-low-power consumption and are highly
configurable. Supported modulation types are GFSK, FSK, GMSK, and OOK. The channel filter can be configured
to support bandwidths ranging from 850 down to 1.1 kHz. A large variety of data rates are supported ranging from
100 bps up to 500 kbps. The configurable preamble detector is used with the synchronous demodulator to improve
the reliability of the sync-word detection. Preamble detection can be skipped using only sync detection, which is a
valuable feature of the asynchronous demodulator when very short preambles are used in protocols, such as
MBus. The received signal strength indicator (RSSI) provides a measure of the signal strength received on the
tuned channel. The resolution of the RSSI is 0.5 dB. This high-resolution RSSI enables accurate channel power
measurements for clear channel assessment (CCA), carrier sense (CS), and listen before talk (LBT) functionality.
A comprehensive programmable packet handler including key features of Silicon Labs’ EZMAC is integrated to
create a variety of communication topologies ranging from peer-to-peer networks to mesh networks. The extensive
programmability of the packet header allows for advanced packet filtering, which, in turn enables a mix of
broadcast, group, and point-to-point communication. A wireless communication channel can be corrupted by noise
and interference, so it is important to know if the received data is free of errors. A cyclic redundancy check (CRC)
is used to detect the presence of erroneous bits in each packet. A CRC is computed and appended at the end of
each transmitted packet and verified by the receiver to confirm that no errors have occurred. The packet handler
and CRC can significantly reduce the load on the system microcontroller allowing for a simpler and cheaper
microcontroller. The digital modem includes the TX modulator, which converts the TX data bits into the
corresponding stream of digital modulation values to be summed with the fractional input to the sigma-delta
modulator. This modulation approach results in highly accurate resolution of the frequency deviation. A Gaussian
filter is implemented to support GFSK, considerably reducing the energy in adjacent channels.
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