RFM23
4. Modulation Options
4.1. Modulation Type
The RFM23 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 11 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)
Figure 11. FSK vs GFSK Spectrums
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