Transmit Timing Options; Mil-188-114A, Rs-422 (Tx Clock Source - Sct Or Scte); Interface; Hssi Interface - Comtech EF Data DMD-2050E Installation And Operation Manual

DMD2050E Universal Satellite Modem
3.6

Transmit Timing Options

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.
3.6.1

MIL-188-114A, RS-422 (TX Clock Source - SCT or SCTE)

Data must be clocked into the modem by either the SCTE or SCT Source. If EXT CLK is desired
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.
If SCTE is selected, then SCTE
set to AUTO.
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.
3.6.2 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.
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.
3.6.3

HSSI Interface

If the HSSI Interface is selected, then the Tx Clock Source will default to SCTE and the Clock
Polarity will default to Auto.
MN-DMD2050E Revision 2
must
be supplied to the modem. The Clock Polarity should be
Theory of Operation
3–18