User's Manual

8
Atmel AVR2080
8479A-AVR-01/12
applied to TRX register XOSC_CTRL (0x12), is stored in the onboard EEPROM (see
Section 4.2).
The reference frequ
ency is also available at pin CLKM of the radio transceiver and,
depending on the related register setting; it is divided by an internal prescaler. CLKM
clock frequencies of 16MHz, 8MHz, 4MHz, 2MHz, 1MHz, 250kHz, or 62.5kHz are
programmable (refer to [1]). The
CLKM signal is filtered by a low-pass filter to reduce
harmonic emissions within the 2.4GHz ISM band. The filter is designed to provide a
stable 1MHz clock signal with correct logic level to a microcontroller pin with
sufficiently suppressed harmonics. CLKM frequencies above 1MHz require a
redesign of R8 and C36. In case of RC cut-off frequency adjustments, depending on
the specific load and signal routing conditions, one may observe performance
degradation of channel 26.
NOTE Channel 26 (2480MHz) is affected by the following harmonics: 155 × 16MHz or
310 × 8MHz.
By default, CLKM is routed to a microcontroller timer input; check the individual
configuration resistors in the schematic drawing. To connect CLKM to the
microcontroller main clock input, assemble R3 with a 0 resistor.
4.5 RF section
The Atmel AT86RF231 radio transceiver incorporates all RF and BB critical
components necessary to transmit and receive signals according to IEEE 802.15.4 or
proprietary ISM data rates.
To further improve system TX output power and RX sensitivity a FEM is connected to
the radio transceiver.
The Skyworks SE2431L FEM [9] is a
high performance, fully integrated module in a
3 × 4 × 0.9mm³ 24 pin QFN package. It incorporates a transmit power amplifier (PA)
with harmonic filtering, a receive low noise amplifier (LNA) with optional bypass
switch, transmit/receive (TR) switching and an antenna diversity switch. A block
diagram of the SE2431L is shown in Figure 4-5.
Figure 4-5. SE2431L bl
ock diagram.
PA
LNA
CPS
CSD
CTX
Logic
control
SE2431L
ANT_SEL
TR
ANT1
ANT2