Premier Mounts CDM-600 specifications Tx Terrestrial, Rx Loop-Timed, RX=TX, External Clock

CDM-600 Satellite Modem	Revision 7
Clock Modes and Drop and Insert (D&I)	MN/CDM600.IOM

10.1.2Tx 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.

10.1.3Rx 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.

10.1.4Rx 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.

10.1.5External 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.

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