Loop-Timed, Rx=Tx; Loop-Timed, Rx<>Tx (Asymmetric Loop Timing); Receive Clocking; Buffer Disabled - Comtech EF Data CDM-550T Manual

Clocking
10.1.3

Loop-Timed, RX=TX

In certain circumstances, a terminal at the distant-end of a satellite link may be required
to provide a clock to the DTE equipment which is locked to the receive satellite signal.
This is similar to Internal Clock mode, in that the modem will source Send Timing (ST)
to the DTE, but now the timing is derived from the demodulator. The DTE then clocks
from this source, and gives the modem transmit data (Send Data, or SD), synchronous
with this clock. It is optional whether the DTE also returns the clock (Terminal Timing,
or TT) - the modem can accept if it is present, but uses ST if it is not. If the demodulator
loses lock, the modem's internal clock will be substituted, so an accurate and stable clock
is present on ST, rather than a clock which may jitter and wander in a random fashion.
10.1.4 Loop-Timed, RX<>TX (Asymmetric Loop Timing)
The CDM-550T incorporates circuitry which permits loop timing when the TX and RX
data rates are not the same. In this case the clock frequency appearing at ST will be
whatever the TX data rate is programmed to, but phase-locked to the demodulator's
receive symbol clock. In all other respects the operation is the same as for 'standard' loop
timing.
10.2

Receive Clocking

There are three receive clocking modes in the CDM-550T.
10.2.1

Buffer Disabled

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, and in the case of Loop Timing, it is
essential that the buffer is disabled (which is done automatically).
10.2.2

Buffer Enabled, RX=TX

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. This is a relatively simple case, as the output clock for the buffer is
derived directly from either ST or TT.
10–2
CDM-550T Satellite Modem
Rev. 1.3