Comtech EF Data CDM-600/600L - Manual (Page 156)

CDM-600/600L Open Network Satellite Modem Revision 3
Clocking Modes and Drop and Insert (D&I) MN/CDM600L.IOM
8–2
8.2.2 Tx Terrestrial Clock
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 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.
8.2.3 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 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 (SD), synchronous with this clock. It is optional whether the DTE also returns the
clock (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.
8.2.4 Rx Loop-Timed, Rx<>Tx (Asymmetric Loop Timing)
The CDM-600/600L 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.
8.2.5 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.