Theory Of Operation - Comtech EF Data SDM-309B Installation And Operation Manual

Theory of Operation

5.1.3 Theory of Operation
As mentioned in Section 5.1.1, the modulator is composed of eight basic subSections.
SubSections 1-4 and 8 make up the baseband processing portion of the modulator while
subSections 5-7 form the RF portion of the modulator.
Data that is to be transmitted is input to the Digital Interface (1) of the modulator. The
format is RS-422 and includes a clock synchronous with the data. The data at this point
is clean and dejittered. A data rate clock provided by the Clock Synthesizer (8) and
buffered by the Digital Interface is output from the card. The frequency of this clock is
programmable. The use of this clock as the source timing signal for the link is optional.
In addition to these functions the Digital Interface provides buffering of M & C signals
to the microcomputer data bus. The data for all programmable functions pass across this
interface as well as module fault information from the Modulator back to the M & C.
Faults reported include, synthesizers out of lock, RF output leveled, input data clock
activity, and digital filter activity.
The Data is delivered from the Data Interface to the Differential Encoder then the
Scrambler (2). The Differential Encoder is a two bit encoder which allows for resolution
of two of the four ambiguity states of the QPSK demodulator or of both states of a BPSK
demodulator. It is programmable on or off. The Scrambler is according to CCITT V.35.
It provides a pseudo-random characteristic to the data stream for dispersal of the
transmitted energy, independent of the data pattern. It is programmable on or off.
The Data passes to the Convolutional Encoder (3). The Convolutional Encoder generates
the parity bits from the input data stream that allows for error correction at the far end of
the link. The rate of the encoder may be 7/8, 3/4, or 1/2. This means, for instance that for
7/8 rate, 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 (4). There are two identical
Nyquist filters, one for the in-phase channel and one for the quadrature channel. They are
each implemented as an FIR digital filter and provide the proper spectral shaping as well
as proper equalization.
The I and Q filtered data is applied to the Modulator (5) which converts them 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 (7) provides the proper frequencies to convert the Modulator IF to
the desired output frequency in the 50 to 90 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 (6). It take the low level
signal from the Modulator Section and amplifies it to the proper level for output from the
5-5–6
SDM-309B Satellite Modem
MN/U-SDM309B Rev. #