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CDM-600 Satellite Modem Revision 7
Clock Modes and Drop and Insert (D&I) MN/CDM600.IOM
10–2
10.1.2 Tx Terrestrial
In this mode, the modem expects to see the DTE provide the clock, so that it can phase-
lock its internal circuits. In this case, the modem does not provide any signal on ST, but
instead requires a clock signal on Terminal Timing (TT), synchronous with the data. If no
clock is present, an alarm will be generated and the modem will substitute its internal
clock.
G.703: This is the ‘natural’ clock mode.
10.1.3 Rx Loop-Timed, RX=TX
In certain circumstances, a terminal at the distant-end of a satellite link may be required
to provide a clock to the DTE equipment which is locked to the receive satellite signal.
This is similar to Internal Clock mode, in that the modem will source Send Timing (ST)
to the DTE, but now the timing is derived from the demodulator. The DTE then clocks
from this source, and gives the modem transmit data (Send Data, or SD), synchronous
with this clock. It is optional whether the DTE also returns the clock (Terminal Timing,
or TT) - the modem can accept it, if it is present, but uses ST if it is not. If the
demodulator loses lock, the modem’s internal clock will be substituted, so an accurate
and stable clock is present on ST, rather than a clock that may jitter and wander in a
random fashion.
G.703: Does not apply.
10.1.4 Rx Loop-Timed, RX<>TX (Asymmetric Loop Timing)
The CDM-600 incorporates circuitry which permits loop timing when the TX and RX
data rates are not the same. In this case the clock frequency appearing at ST will be
whatever the TX data rate is programmed to, but phase-locked to the demodulator’s
receive symbol clock. In all other respects the operation is the same as for ‘standard’ loop
timing.
G.703: Does not apply.
10.1.5 External Clock
The modem will accept a station clock at the rear of the modem to serve as its ST clock
directly. In this case, the station clock must be equal to the transmit data rate being used.
G.703: Does not apply.