Rainbow Electronics RF22 - Manual (Page 30)

RF22
Version: 0.1 Date: 12/23/2008
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10 40 8.00 90 0 0 3 190 051EC 02B
19.2 9.6 1.00 37.7 0 1 1 068 13A93 4EE
20 10 1.00 37.7 0 1 1 064 147AE 521
20 40 4.00 95.3 0 0 4 0C8 0A3D7 0A6
38.4 19.6 1.02 75.2 0 0 1 068 13A93 4D5
40 20 1.00 75.2 0 0 1 064 147AE 521
40 40 2.00 112.1 0 0 5 064 147AE 291
50 25 1.00 75.2 0 0 1 050 1999A 668
57.6 28.8 1.00 90 0 0 3 045 1D7DC 76E
100 50 1.00 191.5 1 0 F 078 11111 446
100 300 6.00 620.7 1 0 E 078 11111 0B8
125 125 2.00 335.5 1 0 8 060 15555 2AD
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:
( )
enmanch Rb
Fd
h
+ Ч
Ч
=
1
2
When the modulation index is 1 or higher the modulation bandwidth can be approximated by the following equation:
( )
⎟
⎠
⎞
⎜
⎝
⎛
Ч + + Ч =
Fd enmanch
Rb
BW 2 1
2
mod
When the modulation index is lower than 1 the modulation bandwidth can be approximated by the following:
( ) ( )
Fd enmanch Rb BW
+ + × =
1
mod
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:
mod
mod
2
BW BW
BW
F
sel ch error
= ⇒ ≤
−
error sel ch
F BW
BW
Ferror
× = ⇒ >
−
2
2
mod
When the AFC is disabled it is recommended to set the bandwidth of the channel select filter (BW
ch-sel
) according to
the following:
error sel ch
F BW BW
× + =
−
2
mod
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.