Comtech EF Data DMD1050TS, Installation And Operation Manual

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DMD1050TS Satellite Modem Board
Revision 1
Theory of Operation 3–21 MN-DMD1050TS
3.10.1.1 Serial Clock Transmit External (SCTE)
The SCTE clock is the Transmit Terrestrial Clock associated with the data interface. SCTE is an
external clock received from the terrestrial equipment. The modem uses the terrestrial clock to
lock the internal clock.
Figure 3-10 shows how 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), along with the Dejitter FIFO, reduce 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 a
reference to the RS422 or MIL 188-114A synchronous interfaces.
3.10.1.2 Serial Clock Transmit (SCT)
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). If the satellite clock is lost, the modem will switch over to the Internal
Clock automatically, and revert to SCR when activity is detected.
If SCT is selected, then Terrestrial data that is synchronous to the SCT Clock must be supplied
by the modem. The terminal equipment uses the SCT as its clock source. The Autophase Circuit
makes sure that the data is clocked correctly into the modem automatically. A return clock is not
necessary. The Clock Polarity must be set to Auto.
3.10.2 Rx Buffer Clock Options
The DMD1050TS supports several Rx Buffer clock options that can be recovered from the
satellite, from terrestrial links or internally. The clocking options allow you to decide which clock
best fits the application. Figure 3-10 shows how the modem processes the clocks for the Tx Clock
and the Rx Buffer Clock. Tx and Rx Clocks can be locked independently. These Buffer clock
selections are available on the DMD1050TS:
•
SCTE (External Tx Terrestrial Clock)
•
SCT (Internal Oscillator)
•
Rx Satellite Clock
The DMD1050TS handles Rx Buffer clock selections based on source priority levels. See
Appendix D.
You assign priorities to the clock sources based on source selections. Source 1 has the highest
priority, and Source 3 has the lowest priority. If a fallback clock is selected, and activity is lost at
the highest priority source, the modem will fall back to the next highest priority clock that has
activity. When activity resumes on a higher priority source, the modem resumes using the higher
priority source.
Clock Source Priority
Rx SAT 1 3
SCTE 2 3
SCT 3 3