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Frequency modulation

The process of changing the frequency of a carrier wave in accordance with the audio frequency signal is known as frequency modulation.
  • Audio quality of AM transmission is poor. There is always a need to eliminate amplitude sensitive noise. This is possible if we eliminate amplitude variation. (i.e., a need to keep the amplitude of the carrier constant). This is precisely what we do in FM.
  • In FM the overall amplitude of FM wave remains constant at all times.
  • In FM, the total transmitted power remains constant.
Fig. 12

Frequency deviation

The maximum change in frequency from mean value (vc) is known as frequency deviation. This is also the change or shift either above or below the frequency vc and is called as frequency deviation.
∴ 78021.png
where kf is the constant of proportionality. It determines the maximum variation in frequency of the modulated wave for a given modulating signal.

Carrier swing (CS)

The total variation in frequency from the lowest to the highest is called the carrier swing,
i.e., CS = 2 × Δf

Frequency modulation index (mf)

The ratio of maximum frequency deviation to the modulating frequency is called modulation index.

Frequency spectrum

FM side band modulated signal consist of infinite number of side bands whose frequencies are
The number of side bands depends on the modulation index mf.
Fig. 13
In FM signal, the information (audio signal) is contained in the side bands. Since the side bands are separated from each other by the frequency of modulating signal fm, so
Bandwidth = 2n × fm
where n = number of significant side band pairs

Deviation ratio

The ratio of maximum permitted frequency deviation to the maximum permitted audio frequency is known as deviation ratio. Thus,
Deviation ratio 78051.png

Percent modulation

The ratio of actual frequency deviation to the maximum allowed frequency deviation is defined as percent modulation. Thus,
Percent modulation, 78057.png

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