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C.1
Burst Mode Modulator: Theory of Operation
The modulator is composed of two basic sections: the baseband processing section and the RF
section. The modem M&C controls all programmable functions in both sections.
C.1.1
Burst Mode
From the front panel, TX Terrestrial or SCT can be selected for the modulator clock input. An
activity detector monitors the selected clock, to automatically switch the clock to SCT if there is
a fault.
When a transmission is to be initiated, the RTS input must be activated. When detected, the
modulator will put out a pure carrier for 96 clock cycles at 19.2 kbps or 288 clock symbols at
57.6 kbps, followed by a clock training sequence for 352 clock cycles. A 31-bit unique word is
then transmitted.
At this time, the CTS line will go false. The next bit of data into the modem will be the first bit
transmitted.
When the last bit of the data packet has been sent to the modem, the user deactivates the RTS
line.
When the modulator detects RTS going false, the modulator will flush the convolutional
encoder (6 bits), then put out the unique word prime twice (62 bits). At this point, the packet is
complete, and the modulator will return the CTS to true. The modulator is now ready for the
next packet.
Data to be transmitted will come from the interface card via the demodulator. The format is RS-
422, and includes a clock that is synchronous with the data. The data signal at this point is clean
and free of jitter. The data signal goes to the scrambler, which provides energy dispersal. There
is no need for a differential encoder in burst mode, as the ambiguities are resolved using the
unique word. The data signal passes to the 1/2 rate Viterbi K=7 convolutional encoder.
Appendix C. BURST MODE
MODULATOR OPERATION
C–1
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