RFM22B Datasheet, PDF (21/74 Pages) List of Unclassifed Manufacturers – ISM TRANSCEIVER MODULE

RFM22B/23B
3.5. Frequency Control
For calculating the necessary frequency register settings it is recommended that customers use
the HOPERF Register Calculator worksheet (in Microsoft Excel) available on the product website.
These methods offer a simple method to quickly determi ne the correct settings based on the
application requirements. The following information can be used to calculated these values manually.
3.5.1. Frequency Programming
In order to receive or transmit an RF signal, the desired channel frequency, f_carrier, must be programmed into the
RFM22B/23B. The carrier frequency is generated by a Fractional-N Synthesizer, using 10 MHz both as the
reference frequency and the clock of the (3^rd 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 as follows:
f_OUT  10MHz  (N  F )
The fractional part (F) is determined by three different values, Carrier Frequency (fc[15:0]), Frequency Offset
(fo[8:0]), and Frequency Deviation (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 "3.5.4. 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  (hbsel 1)  (N  F )
f_TX
 10MHz *(hbsel 1) *( fb[4 : 0]  24 
fc[15 : 0])
64000
Add R/W Function/Description D7
73 R/W Frequency Offset 1
fo[7]
74 R/W Frequency Offset 2
75 R/W Frequency Band Select
76 R/W
77 R/W
Nominal Carrier
Frequency 1
Nominal Carrier
Frequency 0
fc[15]
fc[7]
D6
fo[6]
sbsel
fc[14]
fc[6]
D5
fo[5]
hbsel
fc[13]
fc[5]
D4
fo[4]
fb[4]
fc[12]
fc[4]
D3
fo[3]
fb[3]
fc[11]
fc[3]
D2
D1 D0 POR Def.
fo[2] fo[1] fo[0] 00h
fo[9] fo[8] 00h
fb[2] fb[1] fb[0] 35h
fc[10] fc[9] fc[8] BBh
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 240–960 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:
fc[15 : 0]


10MHz
f_TX
* (hbsel
 1)

fb[4 : 0] 
24_{} *64000
fb and fc are the actual numbers stored in the corresponding registers.
Tel: [phone] Fax: [phone] E-mail: [e-mail] http://www.hoperf.com
21