EM357-RTR datasheet(25/241 Pages) SILABS

25 / 241 page

Rev 1.3

4. Radio Module

The radio module consists of an analog front end and digital baseband as shown in Figure 3.1 on page 23.

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 EM35x’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 EM35x 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 EM35x 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 EM35x 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 EM35x 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 EM35x.

4.2.1. TX Baseband

The EM35x 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 7.5 on page 57. 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 50 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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