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Theory of Operation
4.3
Demodulator
A block diagram of the demodulator is shown in Figure 4-3.
The demodulator converts PSK modulated carriers within the 50 to 180 MHz range to a
demodulated baseband data stream. The converted modulation types are BPSK, QPSK,
OQPSK, and 8PSK (refer to Section 4.2.3 for a description of modulation types). The
demodulator then performs FEC on the data stream using Viterbi or Sequential decoding
algorithms.
LO O P B AC K
R F IN P U T
SW IT C H
R F IN PU T
5 0 TO 18 0 M H z
RE F
V C XO
O C XO
O P T
H IG H
S TAB ILITY
EX T R EFE RE N C E
4–8
A G C
LP F
LO # 1
R E FE R EN C E
RE F
SE Q U EN T IAL
P LL
D E C O DE R
O P TIO N AL
RE E D -SO L O M O N
C O D E C
O P TIO NA L
O V ER H E AD DE F RA M IN G
IB S /ID R , A SY N C /A U PC
D R O P & IN SE R T
Figure 4-3. Demodulator Block Diagram
BP F
L O # 2
V ITE R B I
D E CO D E R
D O P P LER /
M U X
P LE SIO C HR O N O U S
BU F FE R
SDM-300A Satellite Modem
B PF
A TO D
CO N V E RT E R
D IG ITA L
S IG N A L
P R O C E S SIN G
D IG ITA L
C LO C K
RE C O V E RY -
LO O P
IN TE R FA C E
Rev. 4
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