Radyne DMD2050, Operation Manual

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Theory of Operation DMD2050 Universal Satellite Modem
3-12 TM110 - Rev. 1.3
3.6.8 EXT IF REF: External IF Reference
This is not actually a clock, but does have some clocking implications. When the external
reference is used, the master oscillator within the DMD2050 is locked to the external reference,
and the internal accuracy and stability of the DMD2050 assumes that of the External Reference.
Therefore, not only are the transmit frequencies of the DMD2050 locked to the external reference,
but the modem’s internal SCT Oscillator is locked to the external reference as well.
3.7 Transmit Timing
As shown in Figure 3-7, Transmit Terrestrial Data enters the modem and is clocked into a dejitter
FIFO. Data is clocked out of the FIFO by the Modulator Clock. The Modulator Clock and Phase-
Locked Loop (PLL), in conjunction with the Dejitter FIFO, reduces the input jitter. Jitter reduction
exceeds the jitter transfer specified in CCITT G.821.
3.7.1 EXT CLK as TX Clock Source (MIL-188-114A, RS-422)
Data must be clocked into the modem by either the SCTE or SCT Source. If EXT CLK is
selected as the Tx Clock Source, then SCTE must be supplied to the modem. The output of the
dejitter buffer will be clocked with EXT CLK. This case should only be used if SCTE has
excessive jitter and will degrade link performance.
3.7.2 SCT or SCTE
If SCT is selected, then only data that is synchronous to the SCT Clock is required to be supplied
to the modem. It is intended for the terminal equipment to use the SCT as its clock source. The
Autophase Circuit will automatically ensure that the data is clocked correctly into the modem.
Therefore, a return clock is not necessary. The Clock Polarity should be set to Auto.
If SCTE is selected, then SCTE must be supplied to the modem. The Clock Polarity should be
set to AUTO.
3.7.3 G.703 Interface
If the G.703 Interface is selected, then the Tx Clock Source will default to SCTE and the Clock
Polarity will default to Auto.
3.7.4 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.
3.8 Receive Timing
Any of the clocking selections, SCTE, SCT, EXT CLK, or RxSat (SCR) may be selected as the
Buffer Clock. Data will be clocked out of the buffer at the data rate synchronous to the selected
clock source.
3.9 Loop Timing
If loop timing is desired (i.e.; the modem timing is slaved to the far end master station), the
modem clocks can be configured as follows: