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Theory of Operation
4.2.1
Modulator Specifications
Refer to Appendix C for modulator specifications.
4.2.2
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:
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If the modem is so equipped, the data is first processed by the optional overhead or
Reed-Solomon PCB. 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 OQPSK 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 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.
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 OQPSK) mode, the data is split into two separate data
streams to drive the I and Q channels of the modulator. Refer to Section 4.2.3 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.
4–6
Scrambler/Differential Encoder
Convolutional Encoder
Programmable Vector Rotation
I/Q Nyquist Filters
Modulator
RF Synthesizer
Output Amplifier
Output Level Control
SDM-300A Satellite Modem
Rev. 4
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