4-Level FSK Modem Data Pump
FX919A
FEC Generator/Checker
In transmit mode, this circuit adds Forward Error Correction bits to the transmitted data, then converts the
resulting binary data to 4-level symbols. In receive mode, it translates received 4-level symbols to binary data,
using the FEC information to correct a large proportion of transmission errors.
Interleave/De-interleave Buffer
This circuit interleaves data symbols within a block before transmission and de-interleaves the received data so
that the FEC system is best able to handle short noise bursts or fades.
Frame Sync Detect
This circuit, which is only active in receive mode, is used to look for the 24-symbol Frame Synchronisation
pattern which is transmitted to mark the start of every frame.
Rx I/P Amp
This amplifier allows the received signal input to the modem to be set to the optimum level by suitable selection
of the external components R1 and R2. The value of R1 should be calculated to give 0.2 x V
DD
pk-pk at the
RXFB pin for a received '...+3 +3 -3 -3 ...' sequence.
A capacitor may be fitted in series with R1 if ac coupling of the received signal is desired (see Section 1.6.4),
otherwise the dc level of the received signal should be adjusted so that the signal at the modem's RXFB pin is
centred around V
BIAS
(�½ V
DD
).
RRC Low Pass Filter
This filter, which is used in both transmit and receive modes, is a linear-phase lowpass filter with a 'Root Raised
Cosine' frequency response defined by:
H(f)
=1
= square root of {0.5 [1 - sin(
π
T (f - 0.5/T)/b)]}
=0
where b = 0.2, T = 1/symbol rate
for
for
for
0
(1-b)/(2T)
<= f < (1-b)/(2T)
<= f <= (1+b)/(2T)
f
(1+b)/(2T) <
This frequency response is illustrated in Figure 5.
In transmit mode, the 4-level symbols are passed through this filter to eliminate the high frequency components
which would otherwise cause interference into adjacent radio channels.
Figure 4 Translation of Binary Data to Filtered 4-Level Symbols in Tx Mode
©
1996 Consumer Microcircuits Limited
8
D/919A/4
CMLMICRO [ CML MICROCIRCUITS ]
