Page 277 - Satellite Communications, Fourth Edition

Analog Signals 257

Figure 9.3 (a) Three-channel frequency-division multiplex scheme; (b) simplified repre-
sentation.

arrangement shown in Fig. 9.3a. The four 3-channel-multiplexed sig-
nals, termed pregroups, are then combined to form the 12-channel group.
This approach eases the filtering requirements but does require an
additional mixer stage, which adds noise to the process.
The main group designations in the CCITT modulation plan are:
Group. As already described, this consists of 12 VF channels, each
occupying a 4-kHz bandwidth in the multiplexed output. The overall
bandwidth of a group extends from 60 to 108 kHz.
Supergroup. A supergroup is formed by FDM five groups together.
The lower sidebands are combined to form a 60-VF-channel super-
group extending from 312 to 552 kHz.
Basic mastergroup. A basic mastergroup is formed by FDM five
supergroups together. The lower sidebands are combined to form a
300-VF-channel basic mastergroup.
Allowing for 8-kHz guard bands between sidebands, the basic mas-
tergroup extends from 812 to 2044 kHz.