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48
Preliminary Rev. 0.4
7.1.1. Advanced FSK and GFSK Settings
In nearly all cases, the information in Table 16, “RX Modem Configurations for FSK and GFSK,” on page 47 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:
When the modulation index is 1 or higher the modulation bandwidth can be approximated by the following
equation:
When the modulation index is lower than 1 the modulation bandwidth can be approximated by the following:
Where
BW
mod
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 (
F
error
) between transmitter and receiver is less than half the modulation bandwidth (
BW
mod
) then
the AFC will correct the frequency error without needing extra bandwidth. When the frequency error exceeds
BW
mod
/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 (
BW
ch-sel
) according
to the formulas below:
When the AFC is disabled it is recommended to set the bandwidth of the channel select filter (
BW
ch-sel
) according
to the following:
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.
enmanch
Rb
Fd
h
1
2
Fd
enmanch
Rb
BW
2
1
2
mod
Fd
enmanch
Rb
BW
1
mod
error
sel
ch
error
sel
ch
error
F
BW
BW
F
BW
BW
BW
F
2
2
2
mod
mod
mod
error
sel
ch
F
BW
BW
2
mod