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Preliminary Rev. 0.4
4. Modulation Options
4.1. Modulation Type
The Si4430 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.
Figure 11. FSK vs GFSK Spectrums
modtyp[1:0]
Modulation Source
00
Unmodulated Carrier
01
OOK
10
FSK
11
GFSK (enable TX Data CLK when direct mode is used)
TX Modulation Time Domain Waveforms -- FSK vs. GFSK
-1.0
-0.5
0.0
0.5
1.0
-1.5
1.5
SigData_FSK[0,::]
50
100
150
200
250
300
350
400
450
0
500
-0.5
0.0
0.5
-1.0
1.0
time, usec
SigData_GFSK[0,::]
TX Modulation Spectrum -- FSK vs GFSK (Continuous PRBS)
-80
-60
-40
-100
-20
ModSpectrum_FSK
-200
-150
-100
-50
0
50
100
150
200
-250
250
-80
-60
-40
-100
-20
freq, KHz
ModSpectrum_GFSK
DataRate
64000.0
TxDev
32000.0
BT_Filter
0.5
ModIndex
1.0