AX5043 Use as Analog FM Transceiver

APPLICATION NOTE AND9313/D
AX5043
Use as Analog FM
Transceiver
Revision 2
2
Table of Contents
1. Introduction................................................................................................ 3
2. The AX5043_analog_FM_demo Project ....................................................... 4
3. Receive side ................................................................................................ 6
3.1. Hardware setup ............................................................................................. 6
3.2. Software configuration ................................................................................... 6
4. Transmit side .............................................................................................. 9
4.1. Hardware setup ............................................................................................. 9
4.2. Software configuration ................................................................................... 9
5. Conclusion ................................................................................................ 11
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AX5043ND9313/D
Introduction
1. Introduction
This application note describes how to use the AX5043 as an analog FM half-duplex
transceiver. A modified AX-RadioLAB project, ax5043_analog_FM_demo, is used to
demonstrate FM operation.
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The AX5043_analog_FM_demo Project
2. The AX5043_analog_FM_demo Project
The AX5043_analog_FM_demo_v1.1 project is a modified AX-RadioLAB_v2.3g project.
The SLAVE firmware demonstrates analog FM receive, whereas MASTER firmware
demonstrates analog FM transmit. AX-RadioLAB and the AXRadioV2API provide a basic
setup, allowing to check for the AX5043 IC, to set some general parameters (e.g. carrier
frequency) and to range the PLL.
The default configuration is for 200kHz wideband (broadcast) FM. For narrow band FM
the following changes should be made: The RX bandwidth should be adjusted using AXRadioLAB. The TX deviation should be adjusted by changing AX5043_FSKDEV0 in
COMMON/fm.c according to the formula in 4.2.
The following tables list those input parameters of AX-RadioLAB which are relevant here
and the derived AX5043 registers relevant for FM operation.
Parameter
Panel
Comment
Ref Osc Config
Pin Configuration
A TCXO should be used for narrow band FM (25kHz channels). An
XTAL is sufficient for common FM radio reception.
PA Config
Pin Configuration
Single ended or differential
VCO Config
Pin Configuration
Internal or external L, depending of carrier frequency.
Carrier frequency
PHY
RFDIV
PHY
Depending of carrier frequency
Symbol Rate
PHY
AX5043_RXDATARATE is computed from this parameter. In FM RX
mode this determines the rate at which the baseband signal is
sampled. Typically 100kS/s.
Channel Spacing
PHY
Fxtal
PHY
XTALCAP
PHY
Transmit Power
PHY
RX Bandwidth
PHY
Register
AX5043_FREQA3
AX5043_FREQA2
AX5043_FREQA1
AX5043_FREQA0
AX5043_DECIMATION
AX5043_RXDATARATE2
AX5043_RXDATARATE1
AX5043_RXDATARATE0
AX5043_AGCGAIN0
AX5043_AGCTARGET0
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The AX5043_analog_FM_demo Project
AX5043_PHASEGAIN0
AX5043_AMPLITUDEGAIN0
AX5043_FREQDEV10
AX5043_FREQDEV00
AX5043_BBOFFSRES0
AX5043_MODCFGF
AX5043_MODCFGA
AX5043_TXPWRCOEFFB1
AX5043_TXPWRCOEFFB0
AX5043_PLLRNGCLK
AX5043_BBTUNE
AX5043_BBOFFSCAP
AX5043_TMGRXBOOST
AX5043_TMGRXSETTLE
AX5043_TMGRXOFFSACQ
AX5043_RSSIREFERENCE
AX5043_0xF10
AX5043_0xF11
AX5043_0xF1C
AX5043_0xF21
AX5043_0xF22
AX5043_0xF23
AX5043_0xF26
AX5043_0xF34
AX5043_0xF35
AX5043_0xF44
AX5043_PLLLOOP
AX5043_PLLCPI
AX5043_PLLVCODIV
AX5043_PLLVCOI
AX5043_XTALCAP
AX5043_0xF00
AX5043_REF
AX5043_0xF18
Further AX5043 register settings relevant for analog FM operation are hard coded in the
COMMON/set_fm_tx() and COMMON/set_fm_rx() routines. They are described in the
further sections of this document.
AX5043 registers related to preamble matching and the packet engine are irrelevant for
analog FM operation.
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Receive side
3.
Receive side
3.1.
Hardware setup
Pin: PWRAMP or ANTSEL
Low pass filter
AX5043
Amplifier
The AX5043 features a bitstream DAC, which can output a demodulated baseband signal.
The output can be configured to be on pin ANTSEL or PWRAMP. On the AXSEM DVK2b
modules PWRAMP is used. To convert the bitstream into an audio signal a low pass filter
and an amplifier are required. The low pass filter on the DVK2b modules is first order
with a cutoff frequency of 1.8 kHz. For better audio quality a higher order filter is
required.
3.2.
Software configuration
Analog FM mode is selected by setting AX5043_MODULATION = 0x0B.
The following points have to be considered:
1. RX bandwidth and IF configuration function as in other receive modes.
2. The formula for AX5043_RXDATARATE is the same as for digital receive modes,
except that BITRATE is replaced by fSR, the sampling rate of the baseband signal.
3. AX5043_RXPARAMSETS = 0x00 ensures that the RX parameter set 0 is used at all
times. Therefore receiver parameter sets 1, 2 and 3 are not used. Registers related
to preamble matching and the packet engine are not used.
4. Bit timing recovery should be disabled by
AX5043_DRGAIN0 and AX5043_MAXDROFFSET to 0.
setting
AX5043_TIMEGAIN0,
5. The AFC loop controlled by AX5043_FREQGAINA and AX5043_FREQGAINB
(TRKFREQ) us used as an FM demodulator. The AFC loop controlled by
AX5043_FREQGAINC and AX5043_FREQGAIND (TRKRFFREQ) is used for tracking
static frequency offsets. AX5043_FREQUENCYLEAK > 0 prevents the demodulator
loop from tracking static frequency offsets.
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Receive side
6. In
the
formulas
for
AX5043_IFFREQ,
AX5043_DECIMATION
and
AX5043_RXDATARATE listed in the AX5043 Programming Manual, the quantity fxtal
should be replaced by fxtal/2^(ADCCLKMUX), where ADCLKMUX = 0xF35[1:0].
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Receive side
The following table shows register settings for FM radio reception with a 16 MHz
reference.
Register
Value
Calculation / Comment
AX5043_MODULATION
0x0B
Analog FM
AX5043_IFFREQ
0x0666
25kHz (f_xtal = 16MHz)
AX5043_RXPARAMSETS
0x00
only use receiver parameter set 0
AX5043_TIMEGAIN0
0x00
disable bit timing recovery, which would only add jitter
AX5043_DRGAIN0
0x00
off
AX5043_MAXDROFFSET
0x000000
off
AX5043_MAXRFOFFSET2
0x80CCCC track at LO1, max 50kHz @ f_xtal = 16MHz
AX5043_FREQUENCYGAINA0
0x0F
off
AX5043_FREQUENCYGAINB0
0x02
bandwidth of “inner” AFC loop used for FM demodulation.
f_3dB = 0.115*BR. This is the fastest setting available
AX5043_FREQUENCYGAINC0
0x1F
off
AX5043_FREQUENCYGAIND0
0x08
bandwidth of “outer” AFC loop (tracking frequency
mismatch), 78Hz @ BR=100kbps, f_xtal=16MHz
AX5043_FREQUENCYLEAK
0x04
FREQUENCYGAINB0 + 2, prevents the demodulator AFC loop
from tracking static frequency offsets
AX5043_DACCONFIG
0x03
output TRKFREQUENCY (=demodulated signal) on DAC
AX5043_DACVALUE1
0x00
AX5043_DACVALUE0
0x0c
DACSHIFT = 12 bit. This gives maximum volume,
downshifting further gives smaller volume
AX5043_PINFUNCPWRAMP
0x05
Use PWRAMP pin as DAC output
AX5043_PWRMODE
0x09
FULL RX
With this settings the AX5043 will output a FM radio channel at the set frequency. It is
possible to connect a standard high impedance ear piece directly to the pins on the
DVK2b module to listen to the radio.
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Transmit side
4. Transmit side
4.1.
Hardware setup
Low pass
filter
Microphone
Amplifier
Pins GPADC1 & GPADC2
AX5043
The baseband (audio) signal should be applied differentially to pins GPADC1 and GPADC2
(Pins 25 and 26) when using the standalone AX5043. The baseband (audio) signal should
be applied differentially to pins TST2 and TST1 (Pins 27 and 28) when using the SoC
AX8052F143.
The two single ended GPADC input signals should swing around the common mode
voltage of 800mV. The maximum common mode range is +/- 100mV. The maximum
differential input signal is 1Vpp. Thus the maximum negative input signal is ((800mV –
250mV) – (800mV + 250mV)) = -500mV, the maximum positive input signal is ((800mV
+ 250mV) – (800mV - 250mV)) = 500mV. The single ended input impedance of each
GPADC pin is 50kOhm.
A microphone signal should be amplified to +/- 500mVpp differential signal.
4.2.
Software configuration
Analog FM mode is selected by setting AX5043_MODULATION = 0x0B.
The register FSKDEV has a special meaning when FM modulation is selected. It defines
the conditioning of the ADC signal prior modulation.
AX5043_GPADCPERIOD controls the rate at which the analog baseband signal is
sampled. AX5043_TXRATE has no meaning in FM mode.
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Transmit side
Register
Value
Calculation / Comment
AX5043_MODULATION
0x0B
Analog FM
AX5043_FSKDEV2
0x00
AX5043_FSKDEV1
0xC0
GPADC13, enable sign extension and offset (=midcode)
subtraction.
AX5043_FSKDEV0
0x07
fdeviation = +/- 65kHz [max / min ADC value gives fdeviation = +/fxtal / 2^(AX5043_FSKDEV0[2:0]+1), allowed values are 0..7
AX5043_GPADCPERIOD
0x07
Fs = fxtal/32/GPADCPERIOD  5 gives 100kHz @ fxtal=16MHz.
This determines the sampling rate, AX5043_TXRATE has no
meaning in FM mode.
AX5043_GPADCCTRL
0x06
continuous sampling of GPADC13
AX5043_PINFUNCDATA
0x04
Enables continuous TX operation, rather than powering up the
PA only if there is committed FIFO data. This is similar to wire
mode, except that no data is read from the pin in FM mode.
AX5043_PWRMODE
0x0D
FULL TX
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Conclusion
5. Conclusion
It is possible to implement FM audio and data transceivers with the AX5043.
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