Comtech EF Data SDM-300A, Installation And Operation Manual

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SDM-300A Satellite Modem Theory of Operation
Rev. 4 4–7
The two identical digital Nyquist filters are followed by Direct Digital Modulation.
Symbol rates up to 2.5 Mbit/s can be achieved automatically.
The modulated carrier is applied to the RF section for conversion to the correct output
frequency. 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 a single loop,
and incorporates a Direct Digital Synthesis (DDS) chip to accommodate 1 Hz steps over a
range of 130 MHz. The RF section has a frequency stability of
±
1 x 10
-5
. An optional
2 x 10 -7 Temperature Compensated Crystal Oscillator (TCXO) can be installed.
The signal is sent to the output amplifier. The amplifier takes the low level signal from
the modulator section and amplifies the signal to the proper level for output from the
module. The amplifier circuitry provides programmable control of the output level over a
range of -5 to -30 dB, in 0.1 dB steps. The amplifier has power leveling of
±
0.5 dB to
maintain the stability of the output level over time and temperature. The +5 dB output
option is capable of outp5 to -20 dBm.
4.2.3 Theory of Modulation Types
The modulation types for the modem include BPSK, QPSK, OQPSK, or 8PSK. Refer to
Appendix C for encoding information.
The PSK data transmission encoding method uses the phase modulation technique. This
method varies the phase angle of the carrier wave to represent a different bit value for the
receiver. The higher levels of modulation are required for an operating range that has a
limited bandwidth.
The order of modulation is represented by mPSK, where “m” relates to the number of
discrete phase angles. Refer to the following list for a brief description of the modulation
types.
•
BPSK: 2 discrete phase angles represent the 2 possible states of a symbol.
•
QPSK (OQPSK): 4 discrete phase angles represent the 4 possible states of a
symbol.
•
8PSK: 8 discrete phase angles represent the 8 possible states of a symbol.
Note: The code rate determines the number of symbols per bit.