Comtech EF Data SDM-309B - Manual (Page 131)

SDM-309B Satellite Modem Theory of Operation
MN/U-SDM309B Rev. # 5-5–7
module. It contains circuitry which provides programmable control of the output level
over a 10 dB range in .5 dB Steps and power leveling to maintain the stability of the
output level over time and temperature.
5.2 Viterbi Decoder / Demodulator Processor
5.2.1 General Description
The modem Viterbi Decoder/Demod Processor is a 10.25" x 14" card (Figure 5-4
through Figure 5-6) that fits in the middle left slot of the modem chassis. The card
performs four separate functions. It contains the Digital Costas Processor which provides
signals to the Demodulator board for carrier recovery and automatic gain control, it
performs clock recovery of both the symbol clock and the data clock, it provides the
forward error correction function utilizing a Viterbi decoder, and it provides differential
decoding. There are three Viterbi decoder options available in the field.
The 0701 board is the oldest model used in the old SDM-309 modems. The original IBS
modems were used with the external channel units and could operate at data rates of 64
kbs to 2048 kbs. The M&C firmware used with the 0701 board is version 2.XX. If a
customer needs to replace a 0701 decoder board, they will receive the 2133 Viterbi board
and must upgrade the M&C firmware.
The 0949 board and the 2133 board can operate from 9.6 kbs (BPSK) to 8 Mbs.
Firmware is backward compatible between the 0949 and the 2133 boards, but older
firmware will not operate in the newer boards. An example would be a 2133 board that
has 6.XX firmware in the M&C could be replaced with an 0949 board. But an 0949
board that has 4.XX firmware in the M&C could not be replaced with a 2133 board
without upgrading the M&C firmware. All new SDM-309B modems will be shipped
with the new 2133 Viterbi decoder boards.
A block diagram of the board is found in Figure 5-7. A detailed description of the
module appears in Section 5.2.3
5.2.2 Viterbi Decoder/Demod Processor Specification
5.2.2.1 Demod Processor Specification
Operating Symbol Rate Range 19.2 Kb/s to 2048 Kb/s
Operating Code Rate Range 7/8, 3/4, 1/2
Input Signal 11 Level Quantized I and Q
Output Signals VCXO Drive, AGC Drive, I sign,
I mag, Q sign, Q mag, Symbol Clock,
Data Clock
Carrier Phase Error ± 3 ° Max
Carrier Loop Bandwidth Software Controlled