Idi: Insert Data In; Scr: Serial Clock Receive; Ext If Ref: External If Reference; Transmit Timing - Radyne DMD15 Installation And Operation Manual

Operation DMD15/DMD15L IBS/IDR Satellite Modem

3.11.7 IDI: Insert Data In

This clock source is only used as an external frame source selected in D&I Mode. If External
Frame Source is selected, then IDI must be selected for the buffer clock. For this case, a
Receive T1/E1 Trunk is input into J5 and a buffer clock is derived.

3.11.8 SCR: Serial Clock Receive

This Receive Clock is recovered from the satellite's receive signal from the satellite. SCR is
sometimes referred to as Receive Clock, Satellite Clock, or as RT (Receive Timing).

3.11.9 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 DMD15/DMD15L is locked to the external
reference, and the internal accuracy and stability of the DMD15/DMD15L assumes that of the
External Reference. Therefore, not only are the transmit and receive frequencies of the
DMD15/DMD15L locked to the external reference, but the modem's internal SCT Oscillator is
locked to the external reference as well.

3.12 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.

3.12.1 EXT CLK as TX Clock Source (RS-422 or V.35 Interface)

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.

3.12.2 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.
3.12.3 G.703 Interface
If the G.703 Interface is selected, then the Tx Clock Source must be set to SCTE and the Clock
Polarity should be set to AUTO.

3.13 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.
3-12 TM051 – Rev. 5.8