Application Note
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Application Note
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AN069
SWRA237 Page 1 of 16
AN069
Low Cost, Long Range One Way Audio
Communications at 900 MHz
By Michael Burns
Keywords
• CC1101
• CC1150
• Audio
• MSP430
• Frequency Diversity
• Forward Error Correction (FEC)
• FCC 15.247 Compliance
Introduction
This design implements a one way audio
link, based on the Texas Instruments
CC1150 (transmit only), CC1101
(transmit/receive), and the MSP430F2232
microcontroller. The system consists of a
‘transmitter’ card and a ‘receiver’ card, and
operates in the 900 MHz band (868 – 928
MHz). A power amplifier is included on the
transmitter card, to increase the power
output to +20 dBm while maintaining
acceptable harmonic suppression. The
receiver card contains a Low Noise
Amplifier to increase the receiver
sensitivity. Reliable communications is
maintained over a range in excess of 600
feet (180 meters) in ‘line of sight’
environments (non frequency diversity
applications).
The goals for this design were low cost,
and long range. To this end, an external
CODEC (COder-DECoder) is not used.
The MSP430F2232s 10 bit ADC is used to
sample audio data and Timer A is used in
PWM (Pulse Width Modulation) mode to
recover the audio from the digital data.
Audio data is sent in ‘packets’; that is, 36
(‘single Tx’ mode) or 44 (‘double Tx’
mode) ADC samples are collected and
sent in a single packet every 4.5 (‘single
Tx’ mode) or 5.5 (‘double Tx’ mode)
msecs.
Two transmission techniques will be
described. In ‘single Tx’ mode, each
packet is sent only once. In ‘double Tx’
mode, each packet is sent twice, at two
different frequencies. ‘Single Tx’ mode
allows for the use of a lower data rate (150
kbps) or for the use of Forward Error
Correction (FEC) at a higher data rate
(300 kbps). It is also possible to increase
the audio sample rate from 8 kHz to 12
kHz in ‘single Tx’ mode. ‘Double Tx’ offers
increased robustness, but requires the use
of a higher data rate (300 kbps) and
reduced output power due to FCC 15.247
restrictions.
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