SI4463 datasheet(29/53 Pages) SILABS

29 / 53 page

Si4463/61/60-C

Rev 1.0

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



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 kHz 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 in some applications. 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 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

is centered in the passband 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 13 on page 27, an additional byte of preamble is typically required to

settle the AFC.

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