Quantum and Evolution Series Installation and Operating Handbook
8-6
8.1.2.4 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 RT (Receive Timing) and RD (Receive Data) respectively.
Modem Rx
Data Interface
Rx Buffer
DDS
Rx Data
Rx Clock (RT)
Data from
decoder/deframer
Ref. TCXO
Internal Rx Clock Mode
8.1.3 Guidelines for Clocking Configuration
The following need to be ascertained:
•
Incoming
Rx
carrier:
o
Is your Tx clock looped back to you at the distant end?
o
If not, what is the accuracy of the clock transmitted by the distant end?
•
Outgoing Tx carrier:
o
Are you looping the incoming Rx clock (Tx Clock = Rx, or clock looped by
the equipment or cables your end)?
o
If not, what is the accuracy of your Tx clock?
The various Tx and Rx clocking modes of the modem are described in the following
sections.
8.1.3.1 Clock Loop At One End
The end
with the clock loop
requires Buffer disable (Rx Clock =
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 active and set for 2 x Doppler shift only as
there is no Plesiochronous effect with a clock loop. The Buffer size required depends on the
satellite orbit (or rather the accuracy with which it's position is maintained), currently 2ms
(end to end) buffering is enough for the Doppler shift from 1 satellite hop on current Intelsat
satellites. For 2 x Doppler shift 4ms is required.
