Comtech EF Data SNM-1010L - Manual (Page 152)

SNM-1010L Data/Control Modem Revision 0
Clocking Options MN/SNM1010L.OM
Figure 5-1 provides the following information:
• Clocking block diagram
• Transmit clock options
• Buffer clock options
5.1.2 Master/Slave
This application is used where the far end earth station does not have local access to a
high stability reference clock, or where synchronization with a local clock is not required.
The disadvantage of the master/slave application is that the signal received at the slave
station is subject to Doppler shift.
Note: To properly compensate for the Doppler shift on the outbound and return paths, the
length of the buffer at the master end must be twice the normally required length.
5.1.3 EIA-449, V.35 or EIA-530 Master/Slave
Refer to
Figure 5-2 for:
• Clocking block diagram
• Transmit clock options
• Buffer clock options
• V.35 timing signals
• EIA-449 timing signals
• EIA-530 timing signals
The use of loop timing in the modem is an option for both EIA-449 and V.35 operation.
SCT (LOOP): SCT (INTERNAL) clock no longer applies when the modem has loop
timing on. The TX clock source is now recovered from the RX satellite data. This
recovered clock is put out on the ST line and is used to clock the terrestrial equipment.
The TX terrestrial clock is now essentially the same as the RX satellite clock, except that
it has been buffered by the terrestrial equipment.
Select TX TERRESTRIAL for the TX clock source when in loop timing, if the user
equipment is being slaved off of the modem. The SCT (LOOP) indication serves as a
reminder that the SCT internal clock is now the recovered clock, not the internal
oscillator.
Select SCT (LOOP) when the terrestrial equipment does not provide a transmit terrestrial
clock.
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