The subchapter presents the technique of Frequency - Division Multiplexing , the block scheme of a translating equipment and the signal spectrum before and after the demodulation, the hybrid circuits who make the connection from 2 wire baseband circuit to 4 wire FDM  system circuit, multichannel carrier systems used in telecommunications and the hierarchy of FDM systems.


In frequency - division multiplexing (FDM) transmission a number of baseband channels are sent over a common wideband transmission path by using each channel to modulate a different carrier frequency.

In FDM telephone systems is used SSBSC amplitude modulation to translate the signal spectrum of each baseband channel in a well-defined position of the line useful bandwidth. A channel translating equipment is shown in Figure 1.18.

At the sending end, for each channel, the incoming baseband signal from an audio frequency circuit, with a limited spectrum between 0.3 £fb£3.4 kHz, is applied to a balanced modulator supplied with the appropriate carrier Fn. The output of this modulator is a double - sideband suppressed carrier signal, as shown in Figure 1.10.c. This signal is applied to a band - pass filter which suppresses the upper sideband and transmits the lower sideband (Fn- 3.4) ¸ (Fn- 0.3) kHz. The signals obtained in this way from all the N channels are added to give a composite output signal, the wideband signal, containing the signal of each telephone channel translated to a different portion of the frequency spectrum.
At the receiving end, the incoming wideband signal is applied to a bank of band - pass filters, each of, which selects the frequency band containing the signal of one channel. This signal is applied to a modulator (called also demodulator) supplied with the appropriate carrier, the same frequency (Fn) as it was used in the sending part, and the output of this modulator consists of the baseband signal 0.3 ¸ 3.4 kHz and unwanted high - frequency components (2Fn- 3.4) ¸ (2Fn-0.3) kHz (Figure 1.19).

To connect the 2 - wire baseband circuit to the 4 - wire circuit of the FDM system a device, called hybrid, is used for each baseband channel. This hybrid is usually realised with differential transformer or with resistors (Figure 1. 20).

The attenuation introduced by a hybrid between two of its ports should be as small as possible, preferable no attenuation, when these are adjacent ports, and as much as possible (preferable infinite) between opposite ports. Such hybrids operate as a balanced bridge. If the balancing impedance Zb, connected at the port 3 is equal with the baseband circuit input impedance, at the port 1, then the attenuation between the opposite ports is, theoretically, infinite and between adjacent ports (like 1 - 2 or 4 - 1) is 0.345 N for the hybrid realised with transformer or 0.69 N for the hybrid realised with resistors. Practically is not possible to realise a perfect balancing and the attenuation between adjacent ports is finite.

Depending of the size of the useful bandwidth of the transmission line such multichannel carrier systems multiplex 12 telephone channels (12 - 60 kHz), 24 telephone channels (12 - 108 kHz), 60 telephone channels (12 - 252 kHz), 120 telephone channels (12 - 552 kHz) or 180 telephone channels (12 - 804 kHz) on multipair cables, and up to 10,800 telephone channels on coaxial cables. The radio relay lines may be equipped with multichannel carrier systems realising thousands telephone channels.

Early carrier systems transmitted 12 channels producing a line signal in the frequency range 12 - 60 kHz. Later, 24 - channel systems were introduced operating in the frequency range 12 - 108 kHz. 24 - channel carrier systems use two sets of 12 - channel frequency - translating equipment. One equipment is connected directly to the line, thus using the frequency band from 60 to 108 kHz (CCITT basic group B). The other equipment, identical with the first, is connected to the line through a stage of group modulation, as shown in Figure 1.21.

By using a carrier of 120 kHz and selecting the lower sideband, the group of channels is translated from the band 60 to 108 kHz (group B), shown in Figure 1.21.b, to the band 12 to 60 kHz (CCITT basic group A), shown in Figure 1.21.c.

Wideband signals for larger numbers of channels are obtained using further stages of modulation. For example, five 60 - to - 108 kHz groups are applied to group modulators supplied with carriers having frequencies of 420, 468, 516, 564 and 612 kHz respectively. The lower sidebands are selected and are combined to form the basic supergroup (called also secondary group). This accommodates 60 voice channels in the frequency band from 312 to 552 kHz.

The hierarchy of FDM systems, showing the different types of groups is as follows:

Grouping step
Group type
Frequency band


Number of voice channels
Number of groups from anterior step of translation
voice channel
0.3 - 3.4 
primary group (B)
60 - 108
secondary group
312 - 552
tertiary group
812 - 2044
quaternary group
8516 - 12,388

In this way the analogue part of the telephone network offers voice channels and large band channels, like primary group links, secondary group links and so on, for the transmission of the other kind of signals (data, TV, radio). These channels are analogue channels.