Frequency Division Multiplexing (FDM) System

The operation of frequency division multiplexing (FDM) is based on sharing the available bandwidth of a communication channel among the signals to be transmitted .

This means that many signals are transmitted simultaneously with each signal occupying a different frequency slot within a common bandwidth .

Each signal to be transmitted modulates a different carrier . The modulation can be AM,SSB, FM or PM .

The modulated signals are then added together to form a composite signal which  is transmitted over a single channel .

The spectrum of composite FDM signal has been shown in fig.1 .

Fig.1 : Spectrum of FDM Signal

Generally, the FDM systems are used for multiplexing the analog signals .

FDM Transmitter

Fig.2 shows the block diagram of an FDM transmitter .

Fig. 2 : FDM Transmitter

The signals which are to be multiplexed will each modulate a separate carrier .The type of modulation can be AM, SSB, FM or PM .

The modulated signals are then added together to form a complex signal which is transmitted over  a single channel .

Working Operation of the FDM Transmitter

Each signal modulates a separate carrier . The modulator outputs will contain the sidebands of the corresponding signals .

The modulator outputs are added together in a linear mixer or adder .

The linear mixer is different from the normal mixers. Here the sum and difference frequency components are not produced . But only the algebraic addition of the modulated outputs will take place .

Different signals are thus added together i the time domain but they have a separate identity in the frequency domain . This is as shown in fig.2 .

The composite signal at the output of mixer is transmitted over the single communication channel as shown in fig.2 . This signal can be used to modulate a radio transmitter if the FDM signal is to be transmitted through air .

FDM Receiver

The block diagram of an FDM receiver is  shown in fig.3 .

Fig.3 : FDM Receiver

The composite signal is applied to a group of bandpass filters (BPF) .

Each BPF has a center frequency corresponding to one of the carriers. The BPFs have an adequate bandwidth to pass all the channel information without any distortion .

Each filter will pass only its channel and rejects all the other channels .

The channel demodulator then removes the carrier and recovers the original signal back .