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DMD1050 Satellite Modem Board
3.10.2 RX Buffer Clock Options
The DMD1050 supports a number of RX Buffer clock options that can be recovered from the
satellite, terrestrial links or internally. The various clocking options allow users to determine
which clock will best fit their applications. Figure 3-9 gives an overview on how the modem
processes the various clocks for the Tx Clock and the Rx Buffer Clock. Tx and Rx Clocks may
be independently locked. The following Buffer clock selections are available on the DMD1050:
�
SCTE (External Tx Terrestrial Clock)
�
SCT (Internal Oscillator)
�
Rx Satellite Clock
The DMD1050 handles RX Buffer clock selections based on source priority levels. Refer to
Appendix D "Web Browser Setup". The user assigns 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 with activity. When activity resumes on a higher priority
source, the modem resumes using the higher priority source
Clock Source
RX SAT
SCTE
SCT
3.10.3 RX SAT Clock
The RX Sat clock is recovered from the satellite that is received from the distant end. If selected
the Buffer Clock is lock to the RX sat clock.
3.10.4 SCTE: Serial Clock Transmit External
When SCTE is selected as the Rx Buffer clock, the modem receives the clock from the Transmit
Terrestrial interface.
3.10.5 SCT: Serial Clock Transmit
If SCT clock is selected as the RX Buffer clock source, then it should be configured for internal.
SCT is sometimes referred to as Internal Timing or Send Timing (ST).
3.10.5.1
External Reference: J8 SMA Female
This is not actually a clock, but does have some clocking implications. When the external
reference is used, the master oscillator within the DMD1050 is locked to the external reference,
and the internal accuracy and stability of the DMD1050 assumes that of the External Reference.
Therefore, not only are the transmit frequencies of the DMD1050 locked to the external
reference, but the modem's internal SCT Oscillator is locked to the external reference as well.
MN-DMD1050
Revision 9
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Theory of Operation
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