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Explain FDM with a neat block diagram. Give its application.

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

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  • 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 .

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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).

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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.

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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.
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