Quantum and Evolution Series Installation and Operating Handbook
8-2
8.1.1 Transmit
Clocking
There are three transmit clocking modes that are used in the modem.
8.1.1.1 Internal
Clock
When
Internal
is selected, the modem, which is always configured to be a DCE (Data
Communications Equipment), supplies the clock. This is provided by the modem to the
DTE (Data Terminal Equipment) via the ETC (External Transmit Clock) signal line for
using in clocking the data through to the modem. It is important that the terrestrial data
coming into the modem uses the ETC clock looped back through the DTE equipment.
Modem Tx
Data Interface
FIFO
DDS
PLL
VCXO
TCXO
Tx Data Tx Clock (TT) Internal Tx Clock (ST)
Internal Tx Clock Mode
8.1.1.2 Tx Clock In
When
Tx Clock In
is selected, the DTE equipment is responsible for providing the clock.
The modem provides a signal on ST (Send Timing) in this mode and requires a clock
signal to be provided on TT (Terminal Timing). The ST signal is provided in order to allow
the DTE to be operated with a modem supplied clock, in a similar way to when
Internal
Clock
is selected but the TT and SD (Send Data) phase relationship will be correctly
aligned. If the external clock is missing for any reason then the modem will substitute an
internal clock. Note that the G.703 interface should always use external clocking.
