Comtech EF Data CDM-700 - Manual (Page 133)

Chapter 8. CLOCK MODES
8.1 Introduction
The CDM-700 allows the simultaneous operation of multiple ports on data interfaces plugged
into the two slots at the rear of the modem. For the most part, each port operates independently
and generally does the clocking for each port. When dealing with satellite modems, the subject of
clocking often becomes a complex issue. This section describes the various clocking options that
are available with the CDM-700. Refer to the interface appendix in the manual for further
information about the interface.
The CDM-700 is always Data Communications Equipment (DCE) and is normally connected to
Data Terminal Equipment (DTE).
Figure 8-1 presents a diagram of a typical interface; a general description of the clocking and
clock modes is provided below. The particular clocking modes available depend upon the data
interface and its characteristics.
Ext Ref
This signal is applied to J7 on the rear of the modem; it is located on the main part of
the modem – not on a data interface module. The modem locks its internal IF
synthesizers and signal processing circuitry to the Ext Ref signal. It is a low-phase
noise, highly stable signal.
When the Ext Ref is used, it replaces the internal reference (Int Ref) oscillator (10 MHz)
inside the modem and it is the master reference for all signals in the modem. It is normally
the source for ST clock.
Ext Clk
This input appears on some data interface modules. It is associated only with the
clock circuitry in the data interface and is not linked to the internal IF synthesizers.
When an Ext Clk signal is used in conjunction with a G.703 port, this is the signal
used to derive a signal to clock out the Rx Buffer.
Int Clk
The Int Clk or Internal Clock is the actual signal used on the data interface module.
It is derived from either the Ext Ref or Int Ref oscillator in the main part of the modem
– not from the data interface.
Tx-Terr
The signal sent to the modem by equipment external to the modem is the Tx-Terr
clock. It is SD on a G.703 interface, and TT on the HSSI interface. The modem
dejitters and phase locks to this signal and uses it to clock data into the modem.
8–1