Rainbow Electronics RF22 - Manual (Page 17)

RF22
Version: 0.1 Date: 12/23/2008
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Add R/W Function/Description D7 D6 D5 D4 D3 D2 D1 D0 POR Def.
1D R/W AFC Loop Gearshift
Override
afcbd enafc afcgearh[2] afcgearh[1] afcgearh[0] afcgearl[2] afcgearl[1] afcgearl[0] 40h
3.2.10. TX Data Rate Generator
The data rate is configurable between 1–128 kbps. For data rates below 30 kbps the ”txdtrtscale” bit in register 70h
should be set to 1. When higher data rates are used this bit should be set to 0.
The TX date rate is determined by the following formula:
[ ]
e txdtrtscal
MHz txdr
TX DR
⋅ +
×
=
5 16 2
1 0 : 15
_
[ ]
MHz
TX DR
txdr
e txdtrtscal
1
2 _
0 : 15
5 16
⋅ + ×
=
The txdr register may be found in the following registers.
Add R/W Function/Description D7 D6 D5 D4 D3 D2 D1 D0 POR Def.
6E R/W TX Data Rate 1 txdr[15] txdr[14] txdr[13] txdr[12] txdr[11] txdr[10] txdr[9] txdr[8] 0Ah
6F R/W TX Data Rate 0 txdr[7] txdr[6] txdr[5] txdr[4] txdr[3] txdr[2] txdr[1] txdr[0] AAh
4. Modulation Options
4.1. Modulation Type
The RF22 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 10 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 Unmodulated Carrier
01 OOK
10 FSK
11
GFSK (enable TX Data CLK when direct mode is used)
S
igD
at
a
_F
S
K
[0
,:
:]
0
Mod
S
pec
trum_FSK
-80
-60
-40
-100
-20
time, usec
S
ig
D
a
ta
_G
F
S
K
[0
,::]
TX Modulation Spectrum -- FSK vs GFSK (Continuous PRBS)
-0.5
0.0
0.5
-1.0
1.0
-1.0
-0.5
0.0
0.5
1.0
-1.5
1.5
freq, KHz
ModS
pec
tru
m
_G
FSK
DataRate TxDev BT_Filter
0.5
ModIndex
1.0
-80
-60
-40
-100
-20
TX Modulation Time Domain Waveforms -- FSK vs. GFSK
64000.0 32000.0
50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 -50 -100 -150 -200 -250
Figure10. FSK vs GFSK Spectrums