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CC112X/CC1175
SWRU295C
Page 24 of 108
4
Common Receive and Transmit Configurations
4.1
Modulation Formats
CC112X
supports amplitude and frequency shift modulation formats. The desired modulation format is
set in the
register.
Optionally, the data stream can be Manchester coded by the modulator and decoded by the
demodulator. This option is enabled by setting
. Note that
Manchester encoding/decoding is only performed on the payload (including optional length and
address field) and the CRC and that all packet handling features are still available. In applications
where preamble and sync word also need to be Manchester encoded, this can be achieved by
selecting
PREAMBLE_CFG1.PREAMBLE_WORD = 10b
or
11b
and manually encoding a two byte
long sync word and write it to
4.1.1
Frequency Shift Keying
CC112X
supports both 2-FSK and 4-FSK modulation. Both can optionally be shaped by a Gaussian
filter with BT = 0.5, producing a GFSK modulated signal. This spectrum-shaping feature improves
adjacent channel power (ACP) and occupied bandwidth. When selecting 4-(G)FSK, the preamble and
sync word is sent using 2-(G)FSK (see Figure 9).
+1
+1/3
-1/3
-1
1
0
1
0
1
0
1
0
1
1
0
1
0
0
1
1
00 01 01 11 10 00 11 01
Preamble
0xAA
Sync
0xD3
Data
0x17 0x8D
1/Symbol Rate
1/Symbol Rate
1/Symbol Rate
Figure 9: Data Sent Over the Air (
SOFT_TX_DATA_CFG.SYMBOL_MAP_CFG = 00b
)
In „true‟ 2-FSK systems with abrupt frequency shifting, the spectrum is inherently broad. By making
the frequency shift „softer‟, the spectrum can be made significantly narrower. Thus, higher data rates
can be transmitted in the same bandwidth using GFSK.
When 2-(G)FSK/4-(G)FSK modulation is used, the
register specifies the expected frequency deviation of incoming signals in RX and should be the same
as the TX deviation for demodulation to be performed reliably and robustly.
The frequency deviation is programmed with the
values in the
and
register. The value has an exponent/mantissa form, and the resultant
deviation is given by Equation 1 and Equation 2.
E
DEV
xosc
DEV
M
DEV
f
f
_
24
2
)
_
256
(
2
Equation 1: f
DEV
(
DEVIATION_E
> 0)
M
DEV
f
f
xosc
DEV
_
2
23
Equation 2: f
DEV
(
DEVIATION_E
= 0)