Radyne OM20 Specifications download pdf (Page 40)

DMD1050 Satellite Modem Board Theory of Operation

The following paragraphs define the types of clocking options available to the user at the Front

Panel.

� SCT (Internal Oscillator)

� SCTE (External Tx Terrestrial Clock)

� Rx Satellite Clock

3.10.1.1 SCTE: Serial Clock Transmit External

The SCTE clock is the Transmit Terrestrial Clock associated with the data interface. SCTE is an

external clock received from the terrestrial equipment and the modem utilizes the terrestrial clock

to lock the internal clock.

In Figure 3-9, the Transmit Terrestrial Data enters the modem and is clocked into a deter FIFO.

Data is clocked out of the FIFO by the Modulator Clock. The Modulator Clock and Phase-

Locked Loop (PLL), in conjunction with the Dejitter FIFO, which reduces the input jitter. Jitter

reduction exceeds the jitter transfer specified in CCITT G.821.

SCTE is sometimes referred to as Tx Terrestrial Timing or Terminal Timing. Terminal Timing is

reference to the RS422 or MIL 188-114A synchronous interfaces.

3.10.1.2 SCT: Serial Clock Transmit

The SCT clock can be generated internally or recovered from the satellite. The SCT clock source

can be used as the TX clock source, RX Buffer Clock source and the Terrestrial Terminal

equipment for clocking the transmit data. If the SCT clock is recovered from the satellite, then it

is referred to as SCR. SCR is also referred to as Receive Clock, Satellite Clock, or Receive

Timing (RT).

When SCT clock is configured as Internal, the frequency of the clock is set the same as the

Transmit Terrestrial Clock rate. If SCT clock is configured as SCR, the internal clock is set to the

same rate as the incoming receive satellite clock. SCT is sometimes referred to as Internal

Timing or Send Timing (ST). In the event that the satellite clock is lost, the modem will

automatically switch over to the Internal Clock and revert back to SCR when activity is detected.

If SCT is selected, then Terrestrial data that is synchronous to the SCT Clock is required to be

supplied by the modem. It is intended for the terminal equipment to use the SCT as its clock

source. The Autophase Circuit will automatically ensure that the data is clocked correctly into

the modem. Therefore, a return clock is not necessary. The Clock Polarity should be set to Auto.

MN-DMD1050 3–18

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Radyne OM20 Specifications Page 40