Comtech EF Data SNM-1001 - Manual (Page 34)

SNM-1001 Network Control Modem Theory of Operation
Rev. 1 4–3
4.1.1 Specifications
Modulation Type QPSK
Digital Data Rate:
QPSK, 1/2 Rate 19.2 kbit/s
Test Modes Carrier null and quadrature (dual and offset) CW
Frequency Range 50 to 180 MHz
Frequency Select Method Synthesized
Frequency Step Size 100 Hz
Frequency Stability (RF)
±
10 PPM internal oscillator
Frequency Stability (SCT) ±
10 PPM internal oscillator
Phase Error 2.5° maximum
Filtering Type Nyquist, pre-equalized
Spectral Occupancy Spectral density is -30 dB,
±
0.75 symbol rate
Spurious and Harmonics -55 dBc, 0 to 500 MHz
Output Power Level Range -5 to -30 dBm,
±
0.5 dB
Output Stability
±
0.5 dB
Output Power Adjustment 0.5 dB step size
Output Impedance 75
Ω
Output Return Loss 20 dB minimum
Scrambling CCITT V.35
FEC Encoding Convolutional K=7 1/2 Rate Viterbi
Decoding Soft-decision Viterbi
Rate 1/2
Reported Faults AGC level fault
Synthesizer fault
I channel filter activity
Q channel filter activity
Clock activity fault
4.1.2 Theory of Operation
The modulator is composed of two basic sections: the baseband processing section and
the RF section. The modem M&C controls all programmable functions in both sections.
Data to be transmitted will come from the interface card via the demodulator. The format
is EIA-422, and includes a clock that is synchronous with the data. The data signal at this
point is clean and free of jitter. The data signal goes to the scrambler, which provides
energy dispersal. It then goes to a differential encoder. The data signal passes to the
Viterbi K = 7 convolutional encoder.
The output of the encoder generates two separate data streams to drive the I&Q channels
of the modulator. If selected from the front panel menu, one channel can be inverted,
causing a spectral inversion.