Comtech EF Data SNM-1000 - Manual (Page 124)

SNM-1000 Node Control Modem Revision 3
Theory of Operation MN/SNM1000.IOM
4–6
4.2 Modulator
The SNM-1000 modulator performs filtered Quadrature Phase Shift Keying (QPSK)
modulation onto a variable frequency/amplitude carrier. The modulator also provides the
following functions:
• Encodes data for the appropriate decoder
• Scrambles data
• Monitors and displays the modulator status without interrupting service
• Provides source control timing
• Appends a burst preamble for detection by the demodulator (burst mode)
The SNM-1000 creates a QPSK modulated carrier within the 50 through 180 MHz range
from the digital data stream provided by the interface section.
The following subsections make up the modulator:
• Scrambler
• Convolutional encoder
• Preamble and postamble generator (burst mode only)
• Modulator
• Output amplifier
• RF synthesizer
• SCT synthesizer
A block diagram of the modulator is shown in Figure 4-2.