Reference And Clock Distribution - Comtech EF Data SLM-7650 Installation And Operation Manual

SLM-7650 Satellite Modem
Theory of Operation
4.4.3.3

Reference and Clock Distribution

4.4.3.3.1 Modem Frequency Reference
A modem frequency reference is distributed within the modem enclosure. This
frequency reference is the frequency standard for all frequencies generated within the
modem. This reference is a phase lock loop circuit that is driven by either an internal
high stability oven-controlled oscillator or an external modem reference signal. The
external modem reference input on the rear panel of the modem is user selectable as 1, 5,
10 or 20 MHz.
4.4.3.3.2 Modem Clocking
Clocking the data in and out of the modem is extremely important. Loss of
synchronization will occur if the selections are not correct. The operator must understand
the clock settings at the user equipment before any selections are made at the modem.
The modem clock selection is designed to accommodate most system timing
requirements that are encountered in a satellite communication link. Clock selections on
the transmit baseband section will define the timing standard for the carrier sent toward
the satellite. Clock selections on the receive baseband section will define the timing
standard for the data sent toward the terrestrial side of the satellite link.
The basic system timing modes discussed here fall variations of three basic timing
configurations Master/Slave, Master/Master, and Asymmetrical Master/Slave.
Transmit Clock choices:
1. Internal (ST)
2. TX Terrestrial (Terminal Timing (TT) TIA/EIA-449 pins 17 and 35)
3. Data Source SYNC (Recovers Clock from the Data)
Receive Buffer Clock Choices:
1. Internal (ST)
2. TX Terrestrial Clock (Terminal Timing (TT) TIA/EIA-449 pins 17 and 35)
3. External Master Clock, (MC) pins 16 and 34
4. RX Satellite (Buffer is OFF)
Note: The highlighted denotes Master Clock and the doted box denotes the clock
schematic.
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Revision 4
MN/SLM7650.IOM