AX5042 Datasheet, PDF (26/42 Pages) ON Semiconductor – Advanced multi-channel single chip UHF transceiver

26 Circuit Description
802.15.4 (ZigBee) DSSS
802.15.4 uses binary phase shift keying (PSK) with 300 kbit/s (868 MHz band) or 600 kbit/s (915
MHz band) on the radio. The usable bit rate is only a 15^th of the radio bit rate, however. A
spreading function in the transmitter expands the user bit rate by a factor of 15, to make the
transmission more robust. The despreader function of the receiver undoes that.
In 802.15.4 mode, the AX5042 framing unit performs the spreading and despreading function
according to the 802.15.4 specification. In receive mode, the framing unit will also
automatically search for the 802.15.4 preamble, meaning that no interrupts will have to be
serviced by the micro-controller until a packet start is detected.
The 802.15.4 is a universal DSSS mode, which can be used with any modulation or datarate as
long as it does not violate the maximum data rate of the modulation being used. Therefore
the maximum DSSS data rate is 16 kbps for FSK and 40 kbps for ASK and PSK.
5.12. RX AGC and RSSI
AX5042 features two receiver signal strength indicators (RSSI):
1. RSSI before the digital IF channel filter.
The gain of the receiver is adjusted in order to keep the analog IF filter output level
inside the working range of the ADC and demodulator. The register AGCCOUNTER
contains the current value of the AGC and can be used as an RSSI. The step size of
this RSSI is 0.625 dB. The value can be used as soon as the RF frequency generation
sub-system has been programmed.
2. RSSI behind the digital IF channel filter.
The demodulator also provides amplitude information in the TRKAMPL register. By
combining both the AGCCOUNTER and the TRKAMPL registers, a high resolution
(better than 0.1 dB) RSSI value can be computed at the expense of a few arithmetic
operations on the micro-controller. Formulas for this computation can be found in the
AX5042 Programming Manual.
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AX5042