RFM31 Datasheet, PDF (35/137 Pages) –

RFM31
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 15 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 15. RX Modem Configurations for FSK and GFSK
Rb
kbps
2
2.4
2.4
4.8
4.8
9.6
9.6
10
10
19.2
20
20
38.4
40
40
50
57.6
100
100
125
RX Modem setting examples for GFSK and FSK
Application parameters
Register values (hex)
Fd
mod index BW -3dB dwn3_bypass ndec_exp[2:0] filset[3:0] rxosr[10:0] ncoff[19:0] crgain[10:0]
kHz
kHz
1Ch
1Ch
1Ch
20,21h
21,22,23h
24,25h
5
5.00
11.5
0
3
3
0FA
08312
06B
4.8
4.00
11.5
0
3
3
0D0
09D49
0A0
36
30.00
75.2
0
0
1
683
013A9
005
4.8
2.00
12.1
0
3
4
068
13A93
278
45
18.75
95.3
0
0
4
341
02752
00A
4.8
1.00
18.9
0
2
1
068
13A93
4EE
45
9.38
95.3
0
0
4
1A1
04EA5
024
5
1.00
18.9
0
2
1
064
147AE
521
40
8.00
90
0
0
3
190
051EC
02B
9.6
1.00
37.7
0
1
1
068
13A93
4EE
10
1.00
37.7
0
1
1
064
147AE
521
40
4.00
95.3
0
0
4
0C8
0A3D7
0A6
19.6
1.02
75.2
0
0
1
068
13A93
4D5
20
1.00
75.2
0
0
1
064
147AE
521
40
2.00
112.1
0
0
5
064
147AE
291
25
1.00
75.2
0
0
1
050
1999A
668
28.8
1.00
90
0
0
3
045
1D7DC
76E
50
1.00
191.5
1
0
F
078
11111
446
300
6.00
620.7
1
0
E
078
11111
0B8
125
2.00
335.5
1
0
8
060
15555
2AD
35
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