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SDM-309B Satellite Modem
5.3.3 Theory of Operation
As mentioned in Section 5.3.1, the Demodulator is composed of six basic subSections.
The first three make up the RF portions of the board while the last three handle the
baseband processing. A Costas Loop is used for carrier recovery, however the
Demodulator board provides only the I and Q channel signals and has a VCXO input. It
does not do the costas processing.
The incoming modulated carrier enter the Demodulator in the AGC Amplifier (1) where
it is filtered and amplified. The AGC circuit, which is controlled off card, provides
variable gain to maintain the signal level into the IF converter at a constant level over the
entire input dynamic range. The signal is converted to the fixed IF by the Frequency
Synthesizer (3). The Synthesizer is programmable externally, to allow for acquisitions of
carriers in the 50 to 90 MHz or 100 to 180 MHz range.
The IF is split into two channels in the Quadrature Demodulator (2). Using an in-phase
and a quadrature carrier from a VCXO controlled externally, the quadrature demodulator
produces the I and Q channel baseband signals. These two signals are amplified and
filtered in the Nyquist Filters (4). These filters are matched filters to those of the
transmitter resulting in optimal detection of the transmitted data. In addition, they
effectively remove adjacent channels which could corrupt the detection process.
The signal is amplified to the final level in the Baseband Amplifier (5). At this point the
signal looks like the classic "Eye Pattern". It is then quantized in the Soft Decision
Interface (6) and output to the card I/O connector.
5.4 Reacquisition, Fast Acquisition And Directed Sweep
5.4.1 General
The fast acquisition algorithm has been permanently installed in the 4.XX and 6.XX
versions of the modem software. The carrier should be acquired within 30 seconds
regardless of the symbol rate. The upper limit on the symbol rate for fast acquisition is
128 Kbps. The lower limit is 64 Kbps. Fast acquisition will work in conjunction with the
sweep reacquisition function. However, fast acquisition will not work concurrently with
the directed sweep function. The sweep range parameter must be set at 50000 Hz to
enable fast acquisition. If the sweep range is set at any frequency less than 50000 Hz, the
modem is in the directed sweep mode. Turning on the directed sweep function
automatically disables the fast acquisition function.
MN/U-SDM309B Rev. #
Theory of Operation
5-5–19
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