Mumbai University > Electronics > Sem 4 > Priniciples of Communication Engineering

• The operation of frequency division multiplexing (FDM) is based on sharing the available bandwidth of a communication channel among the signals to be transmitted.
• The spectrum of composite FDM signal has been shown in figure.1 .

Figure 1: FDM Spectrum

FDM Transmitter

• Each signal that needs to be sent over a communication channel undergoes modulation with various carrier frequencies, as shown clearly in the diagram below. There are different kinds of modulation such as amplitude modulation, pulse modulation, frequency modulation etc.
• As the name suggests, the modulation done here is frequency modulation by the FDM transmitter.
• These modulated signals are then added up using a linear adder or a mixer, forming a composite signal which gets transmitted over a communication channel (single channel).

Figure 2: FDM Transmitter

FDM Receiver

• At the receiving end, the single composite signal is received by the FDM receiver.
• The receiver then passes the composite signal through various band pass filters.
• Each of these band pass filters has a frequency corresponding to the frequencies of one of the carrier waves.
• Each band pass filter will accept the signal whose frequency matches with the frequency of the carrier signal and rejects all other channels.
• The signals coming out of band pass filters pass through a demodulator.
• The demodulator does the work of separating the original signal from the carrier signal.

Figure 3: FDM Receiver

Guard Band

• Frequency overlap is a serious issue when it comes to frequency division multiplexing and it must be completely avoided.
• Two frequency ranges can be separated by using some narrow frequency ranges called guard bands.
• The guard bands avoid signal interference and enhance the quality of communication.

Applications of FDM 

• FDM is used for FM & AM radio broadcasting. Each AM and FM radio station uses a different carrier frequency. In AM broadcasting, these frequencies use a special band from 530 to 1700 KHz. All these signals/frequencies are multiplexed and are transmitted in air. A receiver receives all these signals but tunes only one which is required. Similarly FM broadcasting uses a bandwidth of 88 to 108 MHz
• FDM is used in television broadcasting.
• First generation cellular telephone also uses FDM.