AX5051
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CIRCUIT DESCRIPTION
The AX5051 is a true single chip low−power CMOS
transceiver primarily for use in SRD bands. The on−chip
transceiver consists of a fully integrated RF front−end with
modulator, and demodulator. Base band data processing is
implemented in an advanced and flexible communication
controller that enables user−friendly communication via the
SPI interface.
AX5051 can be operated from 2.2 V to 3.6 V power
supply over a temperature range from −40°C to 85°C, it
consumes 11 − 45 mA for transmitting depending on the
output power, 19 mA for receiving in high sensitivity mode
and 18 mA for receiving in low power mode.
The AX5051 features make it an ideal interface for
integration into various battery powered SRD solutions such
as ticketing or as transceiver for telemetric applications e.g.
in sensors. As primary application, the transceiver is
intended for UHF radio equipment in accordance with the
European Telecommunication Standard Institute (ETSI)
specification EN 300 220−1 and the US Federal
Communications Commission (FCC) standard CFR47, part
15. The use of AX5051 in accordance to FCC Par 15.247,
allows for improved range in the 915 MHz band.
Additionally AX5051 is compatible with the low frequency
standards of 802.15.4 (ZigBee). It therefore incorporates a
DSSS engine, which spreads data on the transmitter and
despreads data for the receiver. Spreading and despreading
is possible on all data rates and modulation schemes. The net
transfer rate is reduced by a factor of 15 in this case. For
802.15.4 either 600 or 300 kbps modes have to be chosen.
The AX5051 sends and receives data via the SPI port in
frames. This standard operation mode is called Frame Mode.
Pre and post ambles as well as checksums can be generated
automatically. Interrupts control the data flow between a
controller and the AX5051.
The AX5051 behaves as a SPI slave interface.
Configuration of the AX5051 is also done via the SPI
interface.
AX5051 supports any data rate from 1 kbps to 350 kbps
for FSK and MSK and from 1 kbps for 600 kbps for ASK and
10 kbps to 600 kbps PSK. To achieve optimum performance
for specific data rates and modulation schemes several
register settings to configure the AX5051 are necessary, they
are outlined in the following, for details see the AX5051
Programming Manual.
The receiver supports multi−channel operation for all data
rates and modulation schemes.
Voltage Regulator
The AX5051 uses an on−chip voltage regulator to create
a stable supply voltage for the internal circuitry at pin VREG
from the primary supply VDD_IO. All VDD pins of the
device must be connected to VREG. The antenna pins
ANTP and ANTN must be DC biased to VREG. The I/O
level of the digital pins is VDD_IO.
The voltage regulator requires a 1 mF low ESR capacitor
at pin VREG.
In power−down mode the voltage regulator typically
outputs 1.7 V at VREG, if it is powered−up its output rises
to typically 2.5 V. At device power−up the regulator is in
power−down mode.
The voltage regulator must be powered−up before receive
or transmit operations can be initiated. This is handled
automatically when programming the device modes via the
PWRMODE register.
Register VREG contains status bits that can be read to
check if the regulated voltage is above 1.3 V or 2.3 V, sticky
versions of the bits are provided that can be used to detect
low power events (brown−out detection).
Crystal Oscillator
The on−chip crystal oscillator allows the use of an
inexpensive quartz crystal as the RF generation subsystem’s
timing reference. Although a wider range of crystal
frequencies can be handled by the crystal oscillator circuit,
it is recommended to use 16 MHz as reference frequency for
ASK and PSK modulations independent of the data rate. For
FSK it is recommended to use a 16 MHz crystal for data rates
below 200 kbps and 24 MHz for data rates above 200 kbps.
The oscillator circuit is enabled by programming the
PWRMODE register. At power−up it is not enabled.
To adjust the circuit’s characteristics to the quartz crystal
being used without using additional external components,
both the transconductance and the tuning capacitance of the
crystal oscillator can be programmed.
The transconductance is programmed via register bits
XTALOSCGM[3:0] in register XTALOSC.
The integrated programmable tuning capacitor bank
makes it possible to connect the oscillator directly to pins
CLK16N and CLK16P without the need for external
capacitors. It is programmed using bits XTALCAP[5:0] in
register XTALCAP.
To synchronize the receiver frequency to a carrier signal,
the oscillator frequency could be tuned using the capacitor
bank however, the recommended method to implement
frequency synchronization is to make use of the high
resolution RF frequency generation sub−system together
with the Automatic Frequency Control, both are described
further down.
Alternatively a single ended reference (TXCO, CXO)
may be used. The CMOS levels should be applied to
CLK16P via an AC coupling with the crystal oscillator
enabled.
SYSCLK Output
The SYSCLK pin outputs the reference clock signal
divided by a programmable integer. Divisions from 1 to
2048 are possible. For divider ratios > 1 the duty cycle is