Comtech EF Data SNM-1002 - Manual (Page 102)

SNM-1002 LinkSync
 Modem
Revision 2
Theory of Operation MN/SNM1002.OM
4–8
4.3.2 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. Refer to Chapter 5 for a list of reported faults.
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
The data is 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 demodulator.
The data is sent to the convolutional encoder for encoding the baseband data. The code
rates 1/2, 3/4, and 7/8 are based on the symbol rate range of 2.4 kbit/s to 2.5 Mbit/s. 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.
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.