Comtech EF Data SNM-1010L - Manual (Page 172)

SNM-1010L Data/Control Modem Revision 0
Theory of Operation MN/SNM1010L.OM
From the encoder, the data signal passes through a set of variable-rate digital Nyquist
filters. There are four filter sets that can normally be selected: INTELSAT Open
Network, Comtech EF Data Closed, Comstream Compatible Closed, and Fairchild
Compatible Closed.
There are activity detectors on both the I&Q channel Nyquist filters. The digital Nyquist
filters are followed by Digital-to-Analog converters and reconstruction filters. These
filters provide proper spectral shaping and equalization. The filters are under control of
the M&C. Symbol rates from 19.2 to 128 kbit/s can be achieved without requiring the
filter to be changed.
The I&Q filtered data signals are applied to the RF modulator, which converts them to a
modulated carrier. The spectral shape is identical to that of the input data streams, but is
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 950 to 1750 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 graduated steps. Power leveling
is provided at
± 1.0 dB to maintain the stability of the output level over time and
temperature.
Fault information from the modulator is sent to the M&C, including:
• Synthesizers out-of-lock • I channel digital filter activity
• RF output leveled • Q channel digital filter activity
• Input data clock activity
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