RFM22
7. RX Modem Configuration
7.1. Modem Settings for FSK and GFSK
The modem performs channel selection and demodulation in the digital domain. The channel filter bandwidth is
configurable from 620 to 2.6 kHz. The data-rate, modulation index, and bandwidth are set via registers 1C–25. The
modulation index is equal to 2 times the peak deviation divided by the data rate (Rb).
Table 16 gives the modem register settings for various common data-rates. Select the desired data-rate (Rb), and
Deviation (Fd) to determine the proper register settings. For data-rates and modulation types not listed in the table
a calculator tool within EXCEL can be used.
When Manchester coding is disabled, the required channel filter bandwidth is calculated as BW = 2 x (Fd + 0.25Rb)
where Fd is the frequency deviation and Rb is the data rate. For modulation indices below 1 the required channel
filter bandwidth is calculated as BW = Fd + Rb. The channel filter needs to be increased when the frequency offset
between transmitter and receiver is more than half the channel filter bandwidth. In this case it is recommended to
enable the AFC and choose the IF bandwidth equal to 2 x frequency offset.
Table 16. RX Modem Configurations for FSK and GFSK
RX Modem setting examples for GFSK and FSK
Application parameters
Register values (hex)
Rb
kbps
2
Fd
kHz
5
mod index
BW -3dB
kHz
dwn3_bypass
ndec_exp[2:0]
filset[3:0]
rxosr[10:0] ncoff[19:0] crgain[10:0]
1Ch
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1Ch
3
3
0
3
0
2
0
2
0
1
1
0
0
0
0
0
0
0
0
0
1Ch
3
20,21h
0FA
0D0
683
068
341
068
1A1
064
190
068
064
0C8
068
064
064
050
045
078
078
060
21,22,23h
08312
09D49
013A9
13A93
02752
13A93
04EA5
147AE
051EC
13A93
147AE
0A3D7
13A93
147AE
147AE
1999A
1D7DC
11111
11111
15555
24,25h
06B
0A0
005
278
00A
4EE
024
521
02B
4EE
521
0A6
4D5
521
291
668
76E
446
0B8
2AD
5.00
4.00
30.00
2.00
18.75
1.00
9.38
1.00
8.00
1.00
1.00
4.00
1.02
1.00
2.00
1.00
1.00
1.00
6.00
2.00
11.5
11.5
75.2
12.1
95.3
18.9
95.3
18.9
90
2.4
2.4
4.8
4.8
9.6
9.6
10
4.8
36
3
1
4.8
45
4
4
4.8
45
1
4
5
1
10
40
3
19.2
20
9.6
10
37.7
37.7
95.3
75.2
75.2
112.1
75.2
90
1
1
20
40
4
38.4
40
19.6
20
1
1
40
40
5
50
25
1
57.6
100
100
125
28.8
50
3
191.5
620.7
335.5
F
E
8
300
125
44
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