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SDM-300A Satellite Modem
4.3.1
Demodulator Specifications
Refer to Appendix C for demodulation specifications.
4.3.2
Theory of Operation
The demodulator functions as an advanced, digital, coherent-phase-lock receiver and
decoder. Demodulator faults are also reported to the front panel. The demodulator
consists of the following basic subsections.
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The modulated IF signal at 50 to 180 MHz enters the RF module for conversion to an IF
frequency. The IF is then sampled by an A to D converter and digitally demodulated. The
I and Q data is then sent to the digital Nyquist filters, resulting in a filtered, digital
representation of the received signal. The digital data is then sent to four separate
circuits:
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The AGC provides a gain feedback signal to the RF section. This closed loop control
ensures that the digital representation of the I and Q channels is optimized for the Costas
and Clock loops, as well as the soft-decision mapping circuitry.
When the active decoder determines that the modem is locked, the M&C stops the sweep
and begins the de-stress process. This involves fine tuning the DDS based on the phase
error in the Costas loop. The de-stress process continues as long as the modem is locked.
If the carrier is interrupted, the M&C resumes the sweep process.
The digital Costas loop, in conjunction with a Direct Digital Synthesizer (DDS),
performs the carrier recovery function. The Costas loop consists of a Costas phase
detector, loop filter, and DDS, all implemented digitally. The DDS performs the function
of a Voltage-Controlled Oscillator (VCO) in an analog implementation, but can be easily
Rev. 4
Digital Costas Loop
RF Section
Automatic Gain Control
Analog-to-Digital (A/D) Converter
Soft Decision Mapping
Programmable Vector Rotation
Digital Nyquist Filters
FEC Decoder
Digital Clock Recovery Loop
Decoder
Automatic Gain Control
Carrier Recovery (Costas) Loop
Clock Recovery Loop
Soft Decision Mapping
Theory of Operation
4–9
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