RFM22B/23B
4. Modulation Options
4.1. Modulation Type
The RFM22B/23B supports three different modulation options: Gaussian Frequency Shift Keying (GFSK),
Frequency Shift Keying (FSK), and On-Off Keying (OOK). GFSK is the recommended modulation type as it
provides the best performance and cleanest modulation spectrum. Figure 12 demonstrates the difference between
FSK and GFSK for a Data Rate of 64 kbps. The time domain plots demonstrate the effects of the Gaussian filtering.
The frequency domain plots demonstrate the spectral benefit of GFSK over FSK. The type of modulation is
selected with the modtyp[1:0] bits in "Register 71h. Modulation Mode Control 2". Note that it is also possible to
obtain an unmodulated carrier signal by setting modtyp[1:0] = 00.
modtyp[1:0]
Modulation Source
00
01
10
11
Unmodulated Carrier
OOK
FSK
GFSK (enable TX Data CLK when direct mode is used)
TX Modulation Time Domain Waveforms -- FSK vs. GFSK
1.5
TX Modulation Spectrum -- FSK vs GFSK (Continuous PRBS)
-20
1.0
0.5
-40
-60
-80
0.0
-0.5
-1.0
-100
-20
-1.5
1.0
-40
-60
0.5
0.0
-80
-0.5
-1.0
-100
-250 -200 -150 -100
-50
0
50
100
150
200
250
0
50
100 150 200 250 300 350 400 450 500
time, usec
freq, KHz
DataRate
64000.0
TxDev
BT_Filter
0.5
ModIndex
1.0
32000.0
Figure 12. FSK vs GFSK Spectrums
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