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3 clocking options, 1 g.703 interface clocking, 1 tx timing – Comtech EF Data SDM-9000 User Manual

Page 126: 2 rx timing, 1 rx timing (with buffer), 2 rx timing (without buffer)

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SDM-9000 Satellite Modem

Operation

Rev.4

4–53

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

Generated internal to the modem

TX TERRESTRIAL CLOCK

Recovered from the terrestrial send data

EXT. REF. CLOCK

External reference supplied by the terrestrial network

RX CLOCK

(See note)

Recovered from the RX satellite network

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.