Comtech EF Data DMDVR20 LBST Rev 1.1 - Manual (Page 33)

Theory of Operation DMDVR20 LBST Satellite Modem
3-12 TM136 – Rev. 1.1
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 4 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 Priority
RX SAT 1 of 4
SCTE 2 of 4
SCT 3 of 4
EXC BNC 4 of 4
3.5.2.1 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.
3.5.3 EXT REF: External Reference, Top BNC Port, J10
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.
External reference port input is specified at 0 to +6 dBm.
3.6 RS530/422/V.35 Interface (Standard)
Data must be clocked into the modem by either the SCTE or SCT Source. If SCTE is selected as
the Tx Clock Source, then SCTE must be supplied to the modem on the EIA-530 port. The output
of the dejitter buffer will be clocked with this source. SCT should be used if SCTE has excessive
jitter.