Comtech EF Data SDM-300A - Manual (Page 46)

SDM-300A Satellite Modem Revision 6
Functional Description MN/SDM300A.IOM
2–6
2.2.1 Theory of Operation
The modulator is composed of eight basic subsections. These subsections are divided into the
baseband processing section and the RF section of the modulator. The modulator controls all
programmable functions on this module. Fault information from the modulator is sent to the
M&C.
The major modulator subsections are:
• Scrambler/Differential
Encoder
• Convolutional Encoder
• Programmable Vector Rotation
• I/Q Nyquist Filters
• Modulator
• RF Synthesizer
• Output Amplifier
• Output Level Control
If the modem is so equipped, the optional overhead or Reed-Solomon PCB first processes the
data. The data is then sent to the scrambler for energy dispersal, and then to the differential
encoder. The differential encoder is a 2-bit encoder, which allows for resolution of two of the
four ambiguity states of the QPSK or Offset QPSK demodulator.
The data is sent to the convolutional encoder for encoding the baseband data. The code rates
1/2, 3/4, 7/8, and 2/3 are based on the symbol rate range of 2.4 kbps to 2.5 Mbps.
• For Viterbi codes, the convolutional encoder encodes the data at 1/2 rate. If the selected
code rate is 3/4, then 2 of every 6 symbols are punctured. For 3 bits in, there are 4 symbols
out.
• For Sequential codes, the convolutional encoder generates the parity bits from the input data
stream, which allows for error correction at the far end of the link. The rate of the encoder
may be 1/2, 3/4, 7/8, and 2/3.
For example, the 7/8 rate puts out 8 symbols for every 7 bits in. In QPSK (or Offset
QPSK) mode, the data is split into two separate data streams to drive the I and Q channels
of the modulator. Refer to Section 2.2.2 for the theory of modulation types.
After the convolutional encoder, the data is sent to a programmable vector rotation circuit. This
feature provides the user with data communications compatibility for spectrum reversal of the I
and Q channels before and after satellite transmission.
The I and Q channel data then pass through a set of variable rate digital Nyquist filters.