Comtech EF Data SNM-1001L - Manual (Page 316)

SNM-1001L Satellite Modem Revision 1
Burst Mode Modulator Operation MN/SNM1001L.OM
C–2
The output of the encoder generates two separate data streams to drive the in-phase and
quadrature channels of the modulator. The data signal passes through a set of variable-rate
digital Nyquist filters. There are activity detectors on both the In-phase and Quadrature (I&Q)
channel Nyquist filters.
The digital Nyquist filters are followed by Digital to Analog (D/A) converters and
reconstruction filters. These filters provide proper spectral shaping and equalization. The filters
are under control of the M&C.
The I&Q filtered data signals are applied to the RF modulator, which converts them to a
modulated carrier. The spectral shape will be identical to that of the input data streams, but
double-sided about the carrier frequency.
The RF synthesizer provides the proper frequencies to convert the modulator IF to the desired
output frequency in the 50 to 180 MHz range. 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 signal from the power combiner is sent to the output amplifier, which amplifies the low-
level signal from the modulator section to the proper level for output from the module. The
amplifier contains circuitry which provides programmable control of the output level over a
range of -5.0 to -30.0 dBm, in 0.1 dB steps. Power leveling is provided at
± 1.0 dB to maintain
the stability of the output level over time and temperature.
Note: The data packet must not be less than 48 bits of data. There is no maximum length for the
data packet.
C.2 Burst Mode Modulator: Specifications
C.2.1 Digital Data Rate
The digital data rate is selectable at 19.2 or 57.6 kbps. The modem automatically calculates and
sets the symbol rate.
Modulation Type Encoding Type Data Rate
QPSK 1/2 Viterbi 19.2 kbps
QPSK ½ Viterbi 57.6 kbps
C.2.2 Modulation and Encoding Types
QPSK modulation and 1/2 rate Viterbi forward error correction encoding is standard.
Encoder Code Rate Modulation
Viterbi, K7 1/2 QPSK