Operating instructions

DMDVB20 LBST Satellite Modem Theory of Operation
TM138 – Rev. 1.0 3-11
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
3.5.1.1 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 3-7, the Transmit Terrestrial Data enters the modem and is clocked into a dejitter 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 synchronous interfaces.
3.5.1.2 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.
3.5.2 RX Buffer Clock Options
The modem supports a number of RX Buffer clock options that can be recovered from the
satellite, terrestrial links, internally or externally. The various clocking options allow users to
determine which clock will best fit their applications. Figure 3-7 gives an overview on how the
modem processes the various clocks for the Tx Clock and the Rx Buffer Clock. The modem
allows users to select clock polarity Tx and Rx Clocks may be independently locked. The
following RX Buffer clock selections are available:
 Rx Satellite Clock (Recovered from Satellite)