Comtech EF Data C5 - Manual (Page 217)

C5/K1/K3 Integrated Satellite Terminal System Theory of Operation
Rev. 0 5-7
5.3.1 Modulator Specifications
Refer to Chapter 2, Specifications.
5.3.2 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.
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 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.
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, or 7/8. For example, the 7/8 rate puts out 8 symbols for every
7 bits in. In QPSK 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.