Demodulator - Comtech EF Data SLM-7650 Installation And Operation Manual

SLM-7650 Satellite Modem
Theory of Operation
The modulated carrier is applied to the IF section for conversion to the correct output
frequency. The spectral shape will be identical to that of the input data streams, but
double-sided about the carrier frequency.
The IF synthesizer provides the proper frequencies to convert the modulator IF to the
desired output frequency in the 70 or 140 MHz range. The synthesizer incorporates a
Direct Digital Synthesis (DDS) chip to accommodate 1 Hz steps over the full IF range.
The frequency stability of the IF signal will match the stability of the modem reference.
The signal is sent to the output amplifier. The amplifier takes the low level signal from
the modulator section and amplifies the signal to the proper level for output from the
module. The amplifier circuitry provides programmable control of the output level over a
range of +5 to -30 dB, in 0.1 dB steps. The amplifier has power leveling of
maintain the stability of the output level over time and temperature.
4.4.2

Demodulator

The demodulator will provide baseband data and clock to the customer by processing the
down-converted RF signal from the satellite. The frequency of the input IF signal for the
demodulator is 70 ± 20 MHz or 140 ± 40 MHz. The input level is between -15 dBm and
-55 dBm. The demodulator uses Viterbi or trellis decoding with or without concatenated
Reed Solomon decoding (optional Turbo Product Code). A large receive buffer is
available to compensate for any frequency changes caused by the satellite link.
The demodulator functions as an advanced digital coherent phase-lock receiver with
selectable FEC decoders. The demodulator is controlled by commands from the M&C
and fault information from the demodulator is sent to the M&C. The demodulator is
composed of eight basic subsections.
The major demodulator subsections are:
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Input Amplifier
Input AGC
IF Synthesizer
Demodulator
I/Q Nyquist Filters
FEC Decoding
Descrambler/Differential Decoder
Buffer
4–14
Revision 4
MN/SLM7650.IOM
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0.5 dB to