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The L-Carrier FDM System Heirachy
The L-Carrier Frequency Division Multiplexing heiarchy consists of four basic levels, three with an associated multiplexer function:
Level Multiplexer # Channels
----------- -------------- ----------
VF Channel None 1
Group Channel Mux 12
Supergroup Group Mux 60
Mastergroup Supergroup Mux 600
The Channel Multiplexer
In the L-Carrier system, a basic 4 KHz voice channel (0 – 4 KHz frequency range) is applied first to a channel modulator. The channel modulator would multiplex (using AM SSB-SC) each of 12 channels with 12 different carrier frequencies in the range of 64 – 108 KHz. The Lower Sideband (LSB) is used. The resulting aggregate multiplexed signal occupies a frequency range of 60 – 108 KHz (48 KHz of bandwidth).
Channel Carrier Frequency LSB Frequency Range
------- ----------------- -------------------
1 108 KHz 104 - 108 KHz
2 104 KHz 100 - 104 KHz
3 100 KHz 96 - 100 KHz
4 96 KHz 92 - 96 KHz
5 92 KHz 88 - 92 KHz
6 88 KHz 84 - 88 KHz
7 84 KHz 80 - 84 KHz
8 80 KHz 76 - 80 KHz
9 76 KHz 72 - 76 KHz
10 72 KHz 68 - 72 KHz
11 68 KHz 64 - 68 KHz
12 64 KHz 60 - 64 KHz
A pilot signal of frequency 104.08 KHz is injected into the aggregate signal, occupying space in the guard band between channels 1 and 2. Since this pilot is multiplexed in the Group and Supergroup multiplexers, a Baseband Analyzer or Frequency Selective Voltmeter can be used to check for the presence of an individual Group Pilot within the Supergroup or Mastergroup baseband levels.
Usually, the Group Pilot is transmitted at a level of 20 dB below signal level. A Group Pilot Alarm is declared when the received pilot is no longer detected or has drifted +/-5 dB in level.
The Group Multiplexer
If more than 12 channels (1 group) are required, a Group multiplexer is required. This multiplexer takes the 60 – 108 KHz signals from up to 5 Group multiplexers to form a 60-channel Supergroup.
The aggregate consists of a signal in the frequency range of 312 – 552 KHz (240 KHz of bandwidth).
Group Carrier Frequency LSB Frequency Range
----- ----------------- -------------------
1 420 KHz 312 - 360 KHz
2 468 KHz 360 - 408 KHz
3 516 KHz 408 - 456 KHz
4 564 KHz 456 - 504 KHz
5 612 KHz 504 - 552 KHz
The Supergroup Multiplexer
A Supergroup multiplexer consolidates up to 10 Supergroups for the generation of a 600 VF-channel aggregate.
The aggregate consists of a signal in the frequency range of 60 – 2540 KHz (2480 KHz of bandwidth).
Supergroup Carrier Frequency LSB Frequency Range
---------- ----------------- -------------------
1 612 KHz 60 - 300 KHz
2 Direct 312 - 552 KHz
3 1116 KHz 564 - 804 KHz
4 1364 KHz 812 - 1052 KHz
5 1612 KHz 1060 - 1300 KHz
6 1860 KHz 1308 - 1548 KHz
7 2108 KHz 1556 - 1796 KHz
8 2356 KHz 1804 - 2044 KHz
9 2604 KHz 2052 - 2292 KHz
10 2852 KHz 2300 - 2540 KHz
Frequency Synthesis and Locking
With SSB-SC, the receiver must demultiplex the signal using an accurate frequency for proper reproduction of the original channel’s contents. Therefore, the Carrier Generator circuits in each multiplex unit accept an 8 KHz pulse to lock the carrier frequency PLLs (Phase Locked Loops).
At one end of the link, a MASTER station provides a 96 KHz Synchronization Reference, which is inserted into the baseband at the output of the Supergroup Multiplexer. The MASTER station’s carrier frequencies are all locked to this clock (via the 8 KHz timing pulse reference).
At the other end of the link, a SLAVE station tracks the 96 KHz Synchronization Reference and applies it to a divider circuit that generates the 8 KHz timing pulse reference used to lock its carrier frequency PLLs.