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

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SDM-300A Satellite Modem Options
Rev. 4 A–25
The Doppler buffer receives data clocked by the overhead clock from the Demod and an
enable line from the DEMUX. The data is stored in RAM. Four options are allowed to
clock the data out of the buffer:
•
TX
•
RX
•
Internal
•
External
Based on this selection, terrestrial data is clocked out of the buffer to the selected drivers
and on to the end user.
A.1.4.3.6 Loop Timing Operation
The appropriate RX buffer clock choice for loop timing is RX Satellite. This will clock
out the data from the modem using the recovered clock from the demodulator IF input.
The SCT clock output will be phase-locked to the RX Satellite clock when loop timing is
selected. The operator can use the clock developed from the RX Satellite IF for clocking
data into the user device. This would be using RX timing. The operator also can use the
ST clock, which is now phased-locked to the receiver, to clock the user data (Send Data)
to the modem.
A.1.4.3.7 Baseband Loopback Operation
A baseband loopback option is provided. When selected, the input terrestrial data and
clock from the operator are looped back to the user as the output terrestrial data and
clock.
The terrestrial data and clock output from the DEMUX are also looped to the terrestrial
data and clock input at the MUX.