SI4464 Datasheet, PDF (30/56 Pages) Silicon Laboratories – HIGH-PERFORMANCE

Si4464/63/61/60
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)
Image Rejection Calibration
Packet handling including EZMAC^® features
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, 4GFSK, 4FSK, 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 1 Mbps. 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 and 4GFSK, considerably reducing the energy in adjacent
channels. The default bandwidth-time product (BT) is 0.5 for all programmed data rates, but it may be adjusted to
other values.
5.2.1. Automatic Gain Control (AGC)
The AGC algorithm is implemented digitally using an advanced control loop optimized for fast response time. The
AGC occurs within a single bit or in less than 2 µs. Peak detectors at the output of the LNA and PGA allow for
optimal adjustment of the LNA gain and PGA gain to optimize IM3, selectivity, and sensitivity performance.
5.2.2. Auto Frequency Correction (AFC)
Frequency mistuning caused by crystal inaccuracies can be compensated for by enabling the digital automatic
frequency control (AFC) in receive mode. There are two types of integrated frequency compensation: modem
frequency compensation, and AFC by adjusting the PLL frequency. With AFC disabled, the modem compensation
can correct for frequency offsets up to ±0.25 times the IF bandwidth. When the AFC is enabled, the received signal
will be centered in the pass-band of the IF filter, providing optimal sensitivity and selectivity over a wider range of
frequency offsets up to ±0.35 times the IF bandwidth. When AFC is enabled, the preamble length needs to be long
enough to settle the AFC. As shown in Table 12 on page 28, an additional byte of preamble is typically required to
settle the AFC.
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