Comtech EF Data CDM-425, Инструкция по установке и эксплуатации

Страница: 595 / 744

CDM-425 Advanced Satellite Modem
Revision 0
Appendix E E–11 MN-CDM-425
E.11 G.703 Clock Extension
•
Chapter 6. FRONT PANEL OPERATION
There are some applications where it becomes necessary, at the distant end of a satellite link, to
provide a high-stability G.703 timing reference for timing equipment connected to the modem. For
example, in cellular backhaul applications, the BTS equipment may require such a reference
even though the satellite link itself may be operating at a data rate other than 1.544 Mbps or
2.048 Mbps
. This is sometimes accomplished by adding a specialized GPS receiver at the distant
end, which then provides the G.703 synchronizing signal. However, with the G.703 clock
extension mode this may become unnecessary, as the CDM-425 Advanced Satellite Modem –
operating at either end of the link, where the local modem has access to a high-stability G.703
signal – can provide an almost perfect copy of this signal at the distant end. The presence of
Doppler shift on the link is the only factor affecting the overall accuracy. If Doppler shift were not
present, the copy of the clock would be perfect.
This is accomplished by the use of a novel frequency synthesis and phase locking scheme. This
feature of the CDM-425 Advanced Satellite Modem permits the distant end to generate a G.703
synchronizing signal that, depending on a sufficiently accurate local reference, has short term
accuracy to within parts in 10
-8
, and is solely dependent on link Doppler shift.
The subsections and figures that follow illustrate three possible G.703 clock extension modes.
Details of how to set up the modems for these various operating modes are given in
Chapter 6.
FRONT PANEL OPERATION .
E.11.1 Clock Extension Mode 1
Figure E-7 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.