The operation of 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 carrier.

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

The signals which are to be multiplexed with 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 signals which is transmitted over a single channel.

Each signal modulating a separate carrier, the modulator outputs will contain the side bands of the corresponding signals.

The modulator output are added together in a linear mixer or added the linear mixer is different from the normal mixers.

Here the sum and differences frequency components are not produced but only the algebraic addition of the modulated outputs will take place. Different signals are thus added together in the time domain but they have a separate identity in the frequency domain.

The composite signal at the output of mixer is transmitted over the single communication channel.

This signal can be used to modulate a radio transmitter if the FDM signal is to be transmitted through air.