RFM31B Datasheet, PDF (19/63 Pages) List of Unclassifed Manufacturers – ISM RECEIVER

RFM31B
3.5. Frequency Control
For calculating the necessary frequency register settings it is recommended that customers use
HOPERF Register Calculator worksheet (in Microsoft Excel) available on the product website.
These methods offer a simple method to quickly determine 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 an RF signal,the desired channel frequency,f_carrier, must be programmed into the RFM31B.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 (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 Offset Adjustment". 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 _carrier
 10MHz*(hbsel 1)*( fb[4 : 0]  24 
fc[15 : 0])
64000
Add R/W Function/Description D7
D6
D5
D4
D3
D2 D1 D0 POR 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 Offset 2 Reserved Reserved Reserved Reserved Reserved Reserved fo[9] fo[8] 00h
75 R/W Frequency Band Select Reserved sbsel
hbsel
fb[4]
fb[3]
fb[2] fb[1] fb[0] 35h
76 R/W
77 R/W
Nominal Carrier
Frequency 1
Nominal Carrier
Frequency 0
fc[15] fc[14] fc[13] fc[12] fc[11] fc[10] fc[9] fc[8] BBh
fc[7]
fc[6]
fc[5]
fc[4]
fc[3]
fc[2] fc[1] fc[0] 80h
The integer part (N) is determined by fb[4:0]. Additionally, the 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 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:
fc[15 : 0]


10
f _carrier
MHz * (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
19