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DMD50 Universal Satellite Modem
3.7.3 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 0.1Vpp to 5.0Vpp (Sine or Square wave)
3.8
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.
3.8.1 G.703 Interface (Optional)
If the G.703 Interface is selected, then the Tx Clock Source will default to SCTE and the Clock
Polarity will default to Auto.
Loop timing with a G.703 Interface or Asymmetrical Data Rates requires external equipment at
the remote end that is capable of using the recovered RD Clock as source timing for (SCTE) SD.
The modem will not manipulate the clock frequency. Therefore, the transmit and receive clock
rates must be equal in order for the modem to perform loop timing.
3.8.2 HSSI Interface (Optional)
If the HSSI Interface is selected, then the Tx Clock Source will default to SCTE and the Clock
Polarity will default to Auto.
3.8.3 Ethernet Data Interface (Optional)
If the Ethernet Data Interface is selected, then the Tx Clock Source will default to SCTE and the
Clock Polarity will default to Normal.
If the Ethernet Data Interface is selected, then the Buffer Clock will default to RxSat and the
Buffer Clock Polarity will default to Normal.
MN-DMD50 Revision 4
Theory of Operation
3–14
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