Q-Flex Satellite Modem Installation and Operating Handbook
6-14
6.2.4.7 Tx Clock Source
Tx Clock In
The external clock supplied on the interface
Clock In
line will be
used. Should this clock fail then the modem will switch to an
internal backup clock.
Internal
The Tx clock is generated from an internal frequency reference.
This is also output on the
Int Tx Clock Out
line for use by external
equipment.
Receive reference
The Tx clock is generated from the Rx clock. This is only of any
practical use when the Rx clock is set to
Satellite
or
Station
. This
setting should always be used when using
Paired Carrier™
with
IP traffic as explained in the information point below.
Station
The Tx clock is generated from the station clock input.
Table 6-6 Tx Clock Source
When using Paired Carrier™ with IP traffic, it is highly advisable to
set the Tx clock source to ‘Receive reference’ at one end of the
link in order to fix the relative clocking for the overlapped carriers,
thereby preventing any similar patterns in the respective satellite
frames (particularly frame headers) from interfering with each
other. Frame header clashes may show up as periodic Paired
Carrier™
outages potentially many hours apart.
6.2.4.8 Rx Clock Source
Satellite
This is the clock from the satellite. This is converted to the rate
required at the terrestrial port.
Tx Clock In
This causes the Tx clock to be used to clock Rx data to the
terrestrial port. This is a plesiochronous mode and requires the
receive buffer to be set to a value sufficient to accommodate both
the difference in the clocks at each end of the link and any Doppler
shift, while also providing the required interval between buffer
slips.
Internal
This uses an internal reference clock to be used to clock data to
the terrestrial port. This requires the receive buffer to be set as per
the
Tx Clock In
option.
Station
The Rx clock is generated from the station clock input. If the
station clock frequency is the same as the Rx data rate then the
station clock is used directly. If the station clock is a different
frequency to the Rx data rate then an internal conversion to the
