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Si4455
3.1. Receiver Chain
The internal low-noise amplifier (LNA) is designed to be a wideband 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; therefore, 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.
The RX and TX pins can be directly tied externally.
3.2. Receiver 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
?? Preamble detection
?? Invalid preamble detection
?? TX modulation
?? RX demodulation
?? Automatic Gain Control (AGC)
?? Automatic frequency compensation (AFC)
?? Radio signal strength indicator (RSSI)
?? 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, and OOK. The channel filter can be configured to
support bandwidths ranging from 850 kHz down to 40 kHz. A large variety of data rates are supported ranging from
0.5 kbps 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. 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 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. The default bandwidth-time (BT) product is 0.5 for all programmed data rates.
3.2.1. Received Signal Strength Indicator
The received signal strength indicator (RSSI) is an estimate of the signal strength in the channel to which the
receiver is tuned. The RSSI measurement is done after the channel filter, so it is only a measurement of the
desired or undesired in-band signal power. The Si4455 uses a fast response register to read RSSI and so can
complete the read in 16 SPI clock cycles with no requirement to wait for CTS. The RSSI value reported by this API
command can be converted to dBm using the following equation:
14
Rev 1.1
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