External Reference; Receive Clocking; Buffer Disabled (Rx Satellite); Buffer Enabled, Tx=Rx (Tx Terrestrial Or External Clock) - Comtech EF Data CDM-600L Installation And Operation Manual

CDM-600L Satellite Modem
Clock Modes and Drop and Insert (D&I)
10.1.6

External Reference

When enabled, the External Reference input is applied to a PLL that phase locks the
modems internal 10 MHz reference oscillator to a 1, 2, 5, 10, or 20 MHz external
reference. Unlike the External Clock input, which serves as a direct substitute for the
transmit ST clock, the External Reference input becomes a reference for both the carrier
synthesizers and the data clocking since the internal 10 MHz reference oscillator is phase
locked to the external reference. If INTERNAL is selected for the transmit clock, and
External Reference is also enabled, the ST clock will be locked to the external reference.
10.2

Receive Clocking

There are three receive clocking modes in the CDM-600L, plus an additional setting used
for Drop and Insert only – see later section.
10.2.1

Buffer Disabled (RX Satellite)

When the buffer is disabled, the receive clock (Receive Timing, or RT) is derived directly
from the demodulator, and hence will be subject to plesiochronous and Doppler offsets.
In certain instances, this may be acceptable. There is still a minimum buffer in use to de-
jitter the effects of removing overhead framing.
Note: G.703 - Applicable.
10.2.2

Buffer Enabled, TX=RX (TX Terrestrial or External Clock)

In this instance, it is required that buffer be enabled, so that the clock and data appearing
on Receive Timing and Receive Data (RT and RD respectively) are synchronous with the
transmit clock or the external reference input. This is a relatively simple case, as the
output clock for the buffer is derived directly from either ST, TT or the external source.
Note: G.703 - Applicable.
10.2.3 Buffer Enabled, RX<>TX (TX Terrestrial or External Clock)
This is an uncommon case, where the receive data rate does not equal the transmit or
external reference. The modem will generate a phase-locked buffer output clock which
uses the selected reference, regardless of its frequency in relation to the receive data rate.
Note: G.703 - Applicable.
10.3

X.21 Notes

For X.21 operation, use the RS-422 pins, but ignore Receive Clock if the Modem is DTE,
and ignore Transmit clocks if the Modem is DCE.
10–3
Revision 2
MN/CDM600L.IOM