Comtech EF Data SNM-1001 - Manual (Page 37)

Theory of Operation SNM-1001 Network Control Modem
4–6 Rev. 1
4.2.1 Specifications
Digital Data Rate:
QPSK, 1/2 Rate 19.2 kbit/s
Symbol Rate 19.2 ks/s
IF Frequency 50 to 180 MHz, in 100 Hz steps
Input Level -30 to -55 dBm
Decoding Type 1/2
Filter Mask Closed network
Scrambler Types 2 15 -1 Synchronous
Modulation Types QPSK
4.2.2 Theory of Operation
The demodulator card functions as an advanced, fully digital, coherent phase-lock
receiver, and a Viterbi or Sequential decoder.
The following subsections make up the demodulator:
•
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
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
± 1 x 10 -5 .
The two channels are then passed through identical anti-aliasing filters, D/A converters,
and digital Nyquist filters.