Q-Flex Satellite Modem Installation and Operating Handbook
7-6
7.1.2.4 Internal Clock
When
Internal
is selected, the modem’s Rx timing is phase locked to the modem’s
internal clock but is run at the Rx data rate frequency. The Rx Doppler buffer is enabled.
The clock and data appear on Receive Timing (RT) and Receive Data (RD) respectively.
Modem Rx
Data Interface
Rx Buffer
DDS
Rx Data
Rx Clock (RT)
Data from
decoder/deframer
Ref. TCXO
Figure 7-7 Internal Rx Clock Mode
7.1.3 Guidelines for Clocking Configuration
The following needs to be ascertained:
•
Incoming Rx carrier:
o
Is your Tx clock looped back at the remote end?
o
If not, what is the accuracy of the clock transmitted by the remote end?
•
Outgoing Tx carrier:
o
Is the incoming Rx clock being looped (Tx clock = Rx, or clock looped by
the equipment or via cables)?
o
If not, what is the accuracy of the Tx clock?
The various Tx and Rx clocking modes of the modem are described in the following
sections.
7.1.3.1 Clock Loop at One End
The end
with the clock loop
requires the buffer to be disabled (i.e. the Rx clock to be set to
Satellite
). If there is either a cabled clock loop or the equipment loops the Rx clock back to
the modem Tx input then set Tx clock =
Tx Clk In
. If there is no physical clock loop set the
Tx clock =
Rx
.
The end
without the loop
requires the buffer to be activated (i.e. the Rx clock to be set to
something other than
Satellite
). The buffer size should be set to twice the Doppler shift.
This in turn depends on the the accuracy with which the satellite orbit is maintained. As a
