Comtech EF Data SDM-100A - Manual (Page 113)

SDM-100A Satellite Modem Operation
Rev. 0 3–79
3.3 Clocking Options
Methods of clocking the data from the terrestrial equipment to the satellite (and vice
versa) will depend upon the application. The two most common options and
recommended configurations are described in the following paragraphs.
3.3.1 Master/Master
This application is used when both earth stations have high stability clocks, and the
receive data will be clocked to the local network.
Refer to Figure 3-29 for the clocking block diagram, transmit clock options, and buffer
clock options.
The disadvantages of the Master/Master application is that the receive data will slip, since
the clocks will not be synchronized. Also, the transmit and receive data rates must be
equal. If “External Clock” is used, it must equal the TX and RX rates as well. However, if
the buffer is properly set up, the slips will be an exact frame length, causing minimum
loss of data. By using very high stability clocks, the expected time between slips can be
many days.
Loss of the buffer clock will mean the buffer will not be emptied, and data will not be
available. The buffer clock will normally revert to the low stability internal reference
automatically.
3.3.2 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 it is not required to synchronize with a local
clock. Refer to Figure 3-30 for the clocking block diagram, transmit clock options, and
buffer clock options. The disadvantage of the Master/Slave application is that the signal
received at the slave station is subject to Doppler shift.
Note: The length of the buffer at the master end will need to be twice the length that is
normally required, since it will be compensating for the Doppler shift on the outward and
return paths.