FDM principles

Frequency Division Multiplex (FDM) is the frequency translation of a number of individual standard telephony channels so that they can be stacked side by side and form a single wide band signal. This principle is illustrated in Figure 20.1.

Two identical telephony channels C_t and C₂ (0.3 to 3.4kHz) are each mixed with a carrier frequency and combined. Only the lower sideband is selected from the mixing process and thus the channel frequencies are inverted on the combined signal. The spacing of the carrier dictates the spacing between the combined channels and is normally 4kHz i.e. f₂ - f _l = 4kHz. The received path at B undergoes the opposite process.

As the carrier is not transmitted with the signal then t' and f₂' are separately generated at B. Any frequency difference between the carriers at A or B will impose a frequency shift on the recovered channel of f - f '.

The function of filters Fal and Fa2 is to select the wanted lower sideband and suppress the unwanted carrier signal, upper sideband etc. Fbl and Fb2 filters select the required set of frequencies from the broadband line frequency spectrum for demodulation.

In the higher orders of translation it is not normally economic to design Fb filters with sufficient selectivity, such that only the re­quired band of frequencies are selected. The signal presented to the demodulator in this case can contain some information from the adjacent channels. These unwanted signals are removed by the greater selectivity of the lower order demodulating equipments.

An additional requirement for Fb filters is to limit the total band of frequencies applied to the demodulator and thus keep the inter-modulation noise generated to a minimum. Fa and Fb filters are normally the same design.

The filters Fcl and Fc2 remove the unwanted products from the demodulation process before presentation to the user or to the next lower order of translation.

The FDM system represents the most efficient use of bandwidth. With only the lower sideband of the modulation products trans­mitted the system limitations of bandwidth and overload are maxi­mised.