Comtech EF Data SDM-309B - Manual (Page 138)

Theory of Operation SDM-309B Satellite Modem
5-5–14 MN/U-SDM309B Rev. #
5.2.3 Theory Of Operation
5.2.3.1 Demod Processor
The Demod Processor, in conjunction with the Demodulator, reconstructs the digital data
stream that was transmitted but corrupted by transmission channel impairments. The
Demod Processor accepts 11 bit quantized signals from the Demodulator for both the I
and Q channels.
Two of the levels are mainly used for Clock Recovery while the other 9 are used by the
Costas Calculator and in generation of the 2 bit soft decision symbols required by the
Viterbi Decoder.
The Costas Calculator generates a phase error term from the I and Q channel quantized
data. This error term is scaled by input from the M & C, then is output to the analog
portion of the loop. The sweep voltage, which is also controlled by the M & C, is
summed with the integrated error term and is output from the board to drive the VCXO
on the Demodulator.
The Clock Loop is contained on this card. It consists of a phase locked loop with a
VCXO for a reference. The phase locked loop generates a clock, four times the desired
data rate. From this clock Dividers generate the Data Rate Clock and the Symbol Rate
Clock. The Symbol Rate Clock is compared with the Quantized I and Q channel data to
generate a phase error term. The error term is scaled and integrated before it drives the
VCXO on the phase locked loop thus closing the outer loop and driving the Symbol Rate
Clock to synchronize to the incoming Symbols.
The I and Q channel data is sampled and converted to 3 bit quantized R0 and R1 parallel
code bits for use by the Viterbi Decoder.
5.2.3.2 Viterbi Decoder
The Viterbi decoder operates, in conjunction with the convolutional encoder in the
transmit modem, to correct errors in the received data stream from the demodulator due
to the transmission channel. The Viterbi decoder processes three bit quantized R0 and
R1 parallel code bits or symbols from the demodulator. The quantization is three bit soft
decision in sign/magnitude format. This data is a representation of the data transmitted,
corrupted by Additive White Gaussian Noise. The decoder’s task is to determine which
symbols have been corrupted by the transmission channel and correct as many as
possible. The code symbols produced by the encoder provide a means of doing just that.
The Viterbi decoder performs seven functions. The data passes through a ambiguity
resolver which compensates for the potential 90 degree phase ambiguity inherent in a
QPSK demodulator. The data is then "de-punctured" if the decoder is operating in 3/4 or