EM346 Datasheet, PDF (24/142 Pages) Silicon Laboratories – High-Performance, Integrated Dual ZigBee PRO

EM346
4. Radio Module
The radio module consists of an analog front end and digital baseband as shown in Figure 3.1 on page 22.
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 EM346’s co-existence with other 2.4 GHz transceivers such as Zigbee/ 802.15.4-2003, IEEE 802.11-2007, 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 EM346 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 802.15.4-2003-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 Ember software includes calibration algorithms that use
this interface to reduce the effects of silicon process and temperature variation.
4.1.2. RSSI and CCA
The EM346 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 EM346 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 EM346 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 EM346.
4.2.1. TX Baseband
The EM346 TX baseband in the digital domain spreads the 4-bit symbol into its IEEE 802.15.4-2003-defined 32-
chip sequence. It also provides the interface for the Ember software to calibrate the TX module to reduce silicon
process, temperature, and voltage variations.
4.2.2. TX_ACTIVE and nTX_ACTIVE Signals
For applications requiring an external PA, two signals are provided called TX_ACTIVE and nTX_ACTIVE. These
signals are the inverse of each other. They can be used for external PA power management and RF switching
logic. In transmit mode the TX baseband drives TX_ACTIVE high, as described in Table 7.4 on page 55. In receive
mode the TX_ACTIVE signal is low. TX_ACTIVE is the alternate function of PC5, and nTX_ACTIVE is the alternate
function of PC6. See "7. GPIO (General Purpose Input/Output)" on page 48 for details of the alternate GPIO
functions. The digital I/O that provide these signals have a 4 mA output sink and source capability.
4.3. Calibration
The Ember software calibrates the radio using dedicated hardware resources.
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