Burst Mode Demodulator: Theory Of Operation; Burst Mode - Comtech EF Data SNM-1001L Installation And Operation Manual

SNM-1001L Satellite Modem
Burst Mode Modulator Operation
C.3

Burst Mode Demodulator: Theory of Operation

The demodulator card functions as an advanced, fully digital, coherent phase-lock receiver, and
a Viterbi or Sequential decoder.
C.3.1

Burst Mode

The following subsections make up the demodulator:
The modulated signal enters the RF module, where it is converted from an IF signal at 50 to 180
MHz to I&Q baseband channels. The synthesizer has multiple loops, and incorporates a DDS
chip to accommodate 100 Hz steps over a range of 130 MHz. The RF section has a frequency
stability of
The two channels are then passed through identical anti-aliasing filters, D/A converters, and
digital Nyquist filters.
The result is a filtered, digital representation of the received signal. A Costas loop maintains the
phase lock during the message. A phase-lock loop maintains the data clock. The soft-decision
mapper converts the I&Q samples to soft-decision values. The soft-decision values are then fed
to the Viterbi decoder, where error detection and correction are performed.
The I&Q channels are also used to calculate the AGC and AOC voltages. The AGC and AOC
are fed back to the RF module.
Finally, the data from the output of the Viterbi decoder is descrambled with a 2
descrambler, and routed to the interface card. There also is a summary fault relay that provides a
FORM C output located on the demodulator board.
The data clock phase can be selected from the Interface Utility menu.
•
RF synthesizer
•
IF amplifier
•
Quadrature demodulator
•
Identical anti-aliasing filters
•
D/A converters
•
Digital Nyquist filters
•
Costas loop
•
Clock loop
•
Automatic Gain Control (AGC)
•
Automatic Offset Control (AOC)
•
Unique word detector
•
Ambiguity resolver
•
Soft-decision decoder
•
Synchronous descrambler
•
End of message detector
1 x 10 .
± -5
C–7
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