Comtech EF Data SDM-650B Installation And Operation Manual page 130

Theory of Operation
For example, the rate of the encoder at 7/8 rate means 8 bits are output for every 7 bits
input. If the modulator is in the QPSK mode, the data is split into two separate data
streams to drive the in-phase and quadrature channels of the modulator.
From the encoder, the data passes to the Nyquist filters. There are two identical Nyquist
filters, one for the in-phase channel and one for the quadrature channel. Each filter is
implemented as an FIR digital filter, and provides spectral shaping and equalization.
The I and Q filtered data is applied to the modulator, which converts the data to a QPSK
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 90 MHz, or 100 to 180 MHz range. The frequencies
generated are locked to a single, high stability, crystal oscillator which results in an
output frequency of high stability.
The final subsection of the modulator is the output amplifier. The output amplifier takes
the low level signal from the modulator and amplifies the signal to the proper level for
output from the module.
The output amplifier contains circuitry which provides programmable control of the
output level over a 25 dB range, in 0.5 dB steps. The output amplifier also provides
power leveling to maintain the stability of the output level over time and temperature.
5–6
SDM-650B Satellite Modem
Rev. 6