Comtech EF Data RCS10 - Manual (Page 222)

Principles of Operation RCS10 Redundant Communication System
Page 5-6 TM058 - Rev. 2.3
The External Clock can then be connected to the External Reference Card and is then made
available to all modems for selection. Should another clock be desired other than the system
clock, it may be connected to the modem’s associated UIM and then selected for use through the
Interface Configuration Menu.
BNC EXC: BNC External Clock
Unbalanced external clock input into BNC connector J5.
Clock specification:
Frequency: 256 KHz - 10 MHz in 8 KHz steps
Level: 0.5 V pp to 5 V pp
NOTE: When using the external clock port, the External Clock Frequency MUST match the
data rate.
IDI: Insert Data In
This clock source is only used for 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.
SCR: Serial Clock Receive
This is the Receive Clock that is recovered from the receive signal from the satellite.
EXT IF REF: External IF Reference
This is not actually a clock, but does have some clocking implications. When External Reference is
used, the master oscillator within the DMD10 Modem is locked to the External Reference and the
internal accuracy and stability of the DMD10 assumes that of the External Reference. Therefore, not
only are the transmit and receive frequencies of the DMD10 locked to the External Reference, but
the modem’s internal SCT oscillator is locked to the External Reference as well. SCT can then be
used as an ultra-stable clock.
Transmit Timing
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 PLL, in concert with the Dejitter FIFO,
reduces the input jitter. Jitter reduction exceeds the jitter transfer specified in CCITT G.821.
RS-422 or V.35 Interface
EXT CLK as TX Clock Source
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 and 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.
SCTE or SCT
If SCT is selected then only data -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 so a
return clock is not necessary. The TX CLK PHASE should be set to AUTO.