RFM22
7.1.1. Advanced FSK and GFSK Settings
In nearly all cases, the information in Table 16, ―RX Modem Configurations for FSK and GFSK,‖ can be used to
determine the required FSK and GFSK modem parameters. The section includes a more detailed discussion of the
various modem parameters to allow for experienced designers to further configure the modem performance.
In FSK or GFSK mode the receiver can handle a wide range of modulation indices ranging from 0.5 up to 32. The
modulation index (h) is defined by the following:
2×Fd
Rb×(1+enmanch)
h =
When the modulation index is 1 or higher the modulation bandwidth can be approximated by the following equation:
Rb
2
BWmod
=(
×(1+enmanch)+2×Fd
)
When the modulation index is lower than 1 the modulation bandwidth can be approximated by the following:
BWmod
=(
Rb×(1+enmanch)+ Fd
)
Where BWmod is an approximation of the modulation bandwidth in kHz, Rb is the payload bit rate in kbps, Fd is the
frequency deviation of the received GFSK/FSK signal in kHz and enmanch is the Manchester Coding parameter
(see Reg. 70h, enmach is 1 when Manchester coding is enabled, enmanch is 0 when disabled).
The bandwidth of the channel select filter in the receiver might need some extra bandwidth to cope with tolerances
in transmit and receive frequencies which depends on the tolerances of the applied crystals. When the relative
frequency error (Ferror) between transmitter and receiver is less than half the modulation bandwidth (BWmod) then
the AFC will correct the frequency error without needing extra bandwidth. When the frequency error exceeds
BWmod/2 then some extra bandwidth will be needed to assure proper AFC operation under worst case conditions.
When the AFC is enabled it is recommended to set the bandwidth of the channel select filter (BWch-sel) according
to the formulas below:
BWmod
F
F
error
≤
=> BW ch-sel = BWmod
2
BWmod
error
>
=> BW ch-sel = 2×F error
2
When the AFC is disabled it is recommended to set the bandwidth of the channel select filter (BWch-sel) according
to the following:
BW ch-sel = BWmod+2×F error
When the required bandwidth (BW) is calculated then the three filter parameters, ndec_exp, dwn3_bypass and
filset, can be found from the table below. When the calculated bandwidth value is not exactly available then select
the higher available bandwidth closest to the calculated bandwidth.
45
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