Comtech EF Data SDM-300A, Installation And Operation Manual

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Theory of Operation SDM-300A Satellite Modem
4–10 Rev. 4
programmed to the desired center frequency via the M&C. The output of the DDS is sent
to the RF module and provides the reference to which the local oscillator is locked. The
M&C sweeps the local oscillator (via DDS programming) through the user-specified
sweep range.
The digital clock loop, in conjunction with another DDS, performs the clock recovery
function. The clock loop consists of a phase detector, loop filter, and DDS, all
implemented digitally. The DDS performs the function of a VCO in an analog
implementation. The recovered data and symbol clocks are then used throughout the
demodulator.
The soft decision mapper converts the digital I and Q data to 3-bit soft decision values.
These values are then fed to the programmable vector rotation circuit, providing
compatibility with spectrum reversal of the I and Q channels.
The output of the vector rotation circuit is then sent to the Viterbi decoder and optional
Sequential decoder. The output is then sent to the optional Reed-Solomon or Overhead
PCB.