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OM20 Universal Outdoor Modem
The modem handles RX Buffer clock selections based on source priority levels. The user
assigns priorities to the clock sources based on source selections. Source 1 has the highest
priority and Source 5 being the last resort or 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
EXC BNC
EXT IDI
Refer to the Remote Protocol Manual TM117 for Web Browser manual TM117.
4.6.2.1 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.
4.6.2.2 SCTE: Serial Clock Transmit External
When SCTE is selected as the Rx Buffer clock, the modem receives the clock from the Transmit
Terrestrial interface.
4.6.2.3 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).
4.6.2.4 EXT CLK/EXT BNC:
The External Clock connector is not supported in the OM20.
4.6.2.5 EXT IDI: Insert Data In
External IDI is used only for E1/T1 Drop and Insert applications. The available T1/E1 Frame
Source selections are External, Internal, and IDI/DDO Loopback. The T1/E1 Frame Source
selections can be accessed in the INTERFACE/RX SETUP menus. If Ext IDI is selected as the
RX Buffer clock, then user must first specify T1/E1 Frame Source.
External (RX Buffer Clock recovered from the data)
Internal (RX Buffer Clock recovered from the internal clock)
IDI/DDO Loopback (RX Buffer Clock recovered from the data and looped back)
4.6.3 EXT REF: External Reference, Optional
The External Clock connector is optional and must be requested at time of order. This is not
actually a clock, but does have some clocking implications. When the external reference is used,
the master oscillator within the modem is locked to the external reference, and the internal
accuracy and stability of the unit assumes that of the External Reference. Therefore, not only are
the transmit frequencies locked to the external reference, but the modem's internal SCT Oscillator
is locked to the external reference as well.
TM116 – Rev. 1.2
Priority
1
of
5
2
of
5
3
of
5
4
of
5
5
of
5
Theory of Operation
Not Supported in the OM20
4-14
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