Clocking Options; Interface Clocking; Tx Timing; Rx Timing - Comtech EF Data SDM-9000 Installation And Operation Manual

SDM-9000 Satellite Modem

4.3 Clocking Options

Clocking of data between the terrestrial network and the satellite network depends on the
application. Sections 4.3.1 and 4.3.2 define the G.703 and ECL/MIL-STD-188 clocking
configurations.
4.3.1 G.703 Interface Clocking
Refer to Figure 4-21 for the clocking diagram.

4.3.1.1 TX Timing

The TX timing for the G.703 interface is always recovered from the send data supplied
by the terrestrial network. The recovered TX terrestrial clock is used as the reference to
send information over the satellite network.

4.3.1.2 RX Timing

The RX (satellite) clock is recovered from information demodulated and decoded from
the satellite network.

4.3.1.2.1 RX Timing (with Buffer)

If a plesiochronous/Doppler buffer is used, data received from the satellite network is
clocked into the buffer using the RX (satellite) clock. Data is clocked out of the buffer
and to the terrestrial network referenced to one of the four selectable clock sources listed
in the table below:
INTERNAL CLOCK
TX TERRESTRIAL CLOCK
EXT. REF. CLOCK
RX CLOCK
Note: When the RX clock is selected as the buffer clock source, the input and output
of the buffer are clocked by the same source. In this mode, the buffer is virtually
bypassed with a delay defined by the selected buffer size.

4.3.1.2.2 RX Timing (without Buffer)

If a plesiochronous/Doppler buffer is not used, RX data from the satellite network is
clocked to the terrestrial network using the RX (satellite) clock.
Rev.4
Generated internal to the modem
Recovered from the terrestrial send data
External reference supplied by the terrestrial network
Recovered from the RX satellite network
(See note)
Operation
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