Comtech EF Data CDM-570A - Manual (Page 513)

CDM-570A/570AL Satellite Modem with Optional Packet Processor MN-CDM570A
Appendix F Revision 2
F–7
F.5.1 Clock Extension Mode 1
Figure F-3 shows Clock Extension Mode 1. The local modem is assumed to be operating on
INTERNAL clock. A T1 or E1 G.703 signal is applied to the rear panel connector of the modem,
where the clock is recovered.
The G.703 signal is not intended to convey data – its function is only to provide a
synchronizing clock. The data is transferred using the EIA-530/V.35 serial interface.
The internal clock reference generator locks – in both frequency and phase – to this recovered
clock, and a special synthesizer generates an ST clock of ANY ARBITRARY FREQUENCY over the
range 2.4 kHz to 9.98 MHz with a resolution of 1 Hz. The synthesis is exact – there is no
approximation or residual error. For example, if you select 168.231 kbps as the transmit data
rate, and an E1 reference, there will be exactly 168,231 clock cycles generated for every
2,048,000 cycles of the E1 reference.
The internal ST clock is now used, as in the standard Internal Clock mode, to provide the timing
reference for the externally-connected equipment. The data is then transmitted at the desired
data rate to the distant end (or distant ends – this works for broadcast applications as well).
Now, at the distant end modem (timing mode: RX Satellite), the RX signal is received,
demodulated, and the clock is recovered. A second synthesizer, very similar to the one used at
the local modem, is now used to generate an E1 or T1 timing signal. Again, it should be
emphasized that the synthesis is exact. The net result is that the E1 or T1 timing signal used at
the local end is reproduced at the distant end, regardless of the link data rate.
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.