RFM23
3.6. Frequency Control
3.6.1. Frequency Programming
In order to receive or transmit an RF signal, the desired channel frequency, fcarrier, must be programmed into the
RFM23. Note that this frequency is the center frequency of the desired channel and not an LO frequency. The carrier
frequency is generated by a Fractional-N Synthesizer, using 10 MHz both as the reference frequency and the clock
of the (3rd order) ΔΣmodulator. This modulator uses modulo 64000 accumulators. This design was made to obtain
the desired frequency resolution of the synthesizer. The overall division ratio of the feedback loop consist of an
integer part (N) and a fractional part (F).In a generic sense, the output frequency of the synthesizer is:
fout = 10MHz x (N + F)
The fractional part (F) is determined by three different values, Carrier Frequency (fc[15:0]), Frequency Offset (fo[8:0]),
and Frequency Modulation (fd[7:0]). Due to the fine resolution and high loop bandwidth of the synthesizer, FSK
modulation is applied inside the loop and is done by varying F according to the incoming data; this is discussed
further in "3.6.4. Frequency Deviation". Also, a fixed offset can be added to fine-tune the carrier frequency and
counteract crystal tolerance errors. For simplicity assume that only the fc[15:0] register will determine the fractional
component. The equation for selection of the carrier frequency is shown below:
f
carrier = 10MHz x (hbsel + 1) x (N + F)
fc[15: 0]
64000
f
TX =10MHz *(hbsel+ 1)*( fb[4 : 0] +24+
)
POR
Def.
00h
Function/Descr
iption
D7
D6
D5
D4
D3
D2
D1
D0
Add R/W
R/W
Frequency Offset 1
Frequency Offset2
Frequency Band
Select
fo[7]
fo[6]
fo[5]
fo[4]
fo[3]
fo[2]
fo[1] fo[0]
fo[9] fo[8]
73
R/W
00h
74
R/W
35h
BBh
80h
sbsel
fc[14]
fc[6]
hbsel
fc[13]
fc[5]
fb[4]
fc[12]
fc[4]
fb[3]
fc[11]
fc[3]
fb[2]
fc[10]
fc[2]
fb[1] fb[0]
fc[9] fc[8]
fc[1] fc[0]
75
Nominal Carrier
Frequency 1
R/W
fc[15]
fc[7]
76
Nominal Carrier
Frequency 0
R/W
77
The integer part (N) is determined by fb[4:0]. Additionally, the output frequency can be halved by connecting a ÷2
divider to the output. This divider is not inside the loop and is controlled by the hbsel bit in "Register 75h. Frequency
Band Select". This effectively partitions the entire 240–930 MHz frequency range into two separate bands: High
Band (HB) for hbsel = 1, and Low Band (LB) for hbsel = 0. The valid range of fb[4:0] is from 0 to 23. If a higher value
is written into the register, it will default to a value of 23. The integer part has a fixed offset of 24 added to it as shown
in the formula above. Table 12 demonstrates the selection of fb[4:0] for the corresponding frequency band.
After selection of the fb (N) the fractional component may be solved with the following equation:
f
TX
fc[15:0]=
(
- fb[4:0]-24
)
* 64000
10MHz * (hbsel + 1)
fb and fc are the actual numbers stored in the corresponding registers.
22
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