RFM31 Datasheet, PDF (19/137 Pages) –

RFM31
3.6. Frequency Control
3.6.1. Frequency Programming
In order to transmit an RF signal, the desired channel frequency, fcarrier, must be programmed into the RFM31.Note that
this frequency is the center frequency of the desired channel and not an LO frequency. The carrier frequency is generated
rd
by a Fractional-N Synthesizer, using 10 MHz both as the reference frequency and the clock of the (3 order) ΔΣ modulator.
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)
fTX =10MHz *(hbsel+ 1)*( fb[4 : 0] +24+
fc[15: 0]
64000
)
Function/Descr
Add R/W
D7
D6
D5
iption
POR
D4
D3
D2
D1 D0
Def.
73 R/W Frequency Offset 1
fo[7]
fo[6]
fo[5]
fo[4]
fo[3]
fo[2] fo[1] fo[0] 00h
74 R/W Frequency Offset2 Reserved Reserved Reserved Reserved Reserved Reserved fo[9] fo[8] 00h
Frequency Band
75 R/W
Reserved sbsel
Select
hbsel
fb[4]
fb[3]
fb[2] fb[1] fb[0] 35h
Nominal Carrier
76 R/W
Frequency 1
fc[15]
fc[14]
fc[13]
fc[12]
fc[11]
fc[10] fc[9] fc[8] BBh
Nominal Carrier
77 R/W
fc[7]
fc[6]
fc[5]
Frequency 0
fc[4]
fc[3]
fc[2] fc[1] fc[0] 80h
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 11 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:
( ) ^fTX
fc[15:0]= _{10MHz * (hbsel + 1)}
- fb[4:0]-24
* 64000
fb and fc are the actual numbers stored in the corresponding registers.
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