Demodulator; Figure 5-7. Demodulator Block Diagram - Comtech EF Data SDM-650B Installation And Operation Manual

Theory of Operation

5.3 Demodulator

The modem demodulator card is a 10.25" x 14" card that fits in the lower-left slot of the
modem chassis.
The demodulator's function is to accept a desired QPSK or BPSK modulated signal in the
50 to 90 MHz, or 100 to 180 MHz range. The card converts the signal to filtered
baseband in-phase and quadrature signals, which are then quantized and output from the
card.
Several subsections make up the card:
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Refer to Figure 5-7 for a block diagram of the demodulator. Refer to Section 5.3.2 for a
detailed description of the subsections.
IF INPUT
50-90 MHZ
OR
100-180 MHZ
AGC INPUT
AGC CONTROLER
MICRO-COMPUTER
PROCESSOR
BUS INTERFACE
BUS
VCXO CONTROL
5–16
AGC amplifier
Quadrature demodulator
IF synthesizer
Nyquist filter
Baseband amplifier
Soft decision interface
LOWPASS FILTER
QUADRATURE
MULTIPLE LOOP
SYNTHESIZER VCXO

Figure 5-7. Demodulator Block Diagram

IF FILTER
I CHANNEL
NYQUIST FILTERS
Q CHANNEL
NYQUIST FILTERS
SDM-650B Satellite Modem
QUANTIZER
I CHANNEL
QUANTIZER
Q CHANNEL
Rev. 6