Comtech EF Data SDM-300L3 - Manual (Page 399)

B–1
Appendix B. Burst Mode
Modulator Operation
B.1 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.
B.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