Comtech EF Data CDM-425, Installation And Operation Manual

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CDM-425 Advanced Satellite Modem
Revision 0
Appendix E E–12 MN-CDM-425
The only thing that affects the overall accuracy and stability of the copy of the clock is the Doppler
shift of the link itself. This will be very dependent on the particular satellite used, and the accuracy
of the orbital station keeping (often referred to as orbital inclination). Typically the Doppler
variations are in the order of parts in 10
-8
, but more importantly it should be recognized that, over
a 24 hour period, the net error would be zero due to a fundamental characteristics of
geostationary orbits.
The T1 or E1 signal, available on the rear panel of the modem on the G.703 connectors, is now
used to provide a synchronizing source for equipment connected to the modem. The form of this
is an ‘all ones’ signal, which provides the maximum transition density in the AMI signal.
This scheme is sufficiently flexible to permit an E1 signal to be used at the local
end, and a T1 signal to be reproduced at the distant end, or vice versa
E.11.2 Clock Extension Mode 2
Figure E-8 shows Clock Extension Mode 2. This is for situations where clock extension needs to
be performed, but there is no local G.703 reference. In this case, the local modem now operates
in an EXTERNAL clock mode and the accuracy of the TX Clock is determined solely by the
accuracy of the equipment connected to the modem.
At the distant end, an E1 or T1 synchronizing signal is generated regardless of the link data rate,
as in Mode 1.
E.11.3 Clock Extension Mode 3
Figure E-9 shows Clock Extension Mode 3. This is very similar to Mode 1 but now, instead of the
EIA-530/V.35 serial interface being used, everything is based around the 10/100 Base T Ethernet
interface.
At the distant end, an E1 or T1 synchronizing signal is generated regardless of the link data rate,
as in Mode 1.