Comtech EF Data SNM-1001L - Manual (Page 322)

SNM-1001L Satellite Modem Revision 1
Burst Mode Modulator Operation MN/SNM1001L.OM
C–8
Using Digital Signal Processing (DSP) techniques, the demodulator looks for carrier power in
an 8 kHz bandwidth. When a carrier is detected, the DSP calculates the offset from the nominal
frequency. The DSP then zeros out the offset. This occurs during the CW portion of the
preamble sequence. During the second part of the preamble sequence, the clock phase is
recovered. When the unique word is detected, the Demod determines the ambiguity of the
received signal. It then corrects the ambiguity, if necessary, and starts feeding data to the Viterbi
decoder. A delay generator determines when the first bit of the data packet comes out of the
Viterbi decoder, and initiates the synchronous load of the 2
15 -1 synchronous descrambler. After
the descrambler starts the lock, the RR lines are set to true, denoting that valid data is being
received. The demodulator, when locked, continually monitors the incoming data for the end-
of-message marker. When the end-of-message marker is detected, a delay generator determines
when the remaining data has been flushed out of the modem, and the Lock and RR line is set to
false.
Note: The data packet must not be less than 48 bits of data. There is no maximum length for the
data packet.
C.4 Burst Mode Demodulator: Specifications
C.4.1 Digital Data Rate
The digital data rate is fixed at 19.2. The modem automatically calculates and sets the symbol
rate.
Modulation Type Encoding Type Data Rate
QPSK 1/2 Viterbi 19.2 kbps
QPSK 1/2 Viterbi 57.6 kbps
C.4.2 Demodulation and FEC Decoding Types
QPSK demodulation and 1/2 rate Viterbi forward error correction decoding is standard.
Encoder Code Rate Modulation
Viterbi, K7 1/2 QPSK
C.4.3 Descrambling Types
One of the following descrambling types can be selected by the operator.
• 2^15 -1 Synchronous
• None