Comparison Between Amplitude Modulation and Angle Modulation

Angle modulation has following advantages over amplitude modulation (AM):

  1. The amplitude of FM is constant. It is independent of depth of modulation. Hence transmitter power remains constant in FM whereas in FM it varies in AM.
  2. Since amplitude of FM is constant, the noise interference is minimum in FM. Any noise superimposing an amplitude can be removed with the help of amplitude limits. Whereas it is difficult to remove amplitude variations due to noise in AM.
  3. The depth of modulation have limitation in AM. But in FM the depth of modulation can be increased to any value by increasing the deviation. This does not cause any distortion in FM signal.
  4. Since guard bands are provided in FM, there is less possibility of adjacent channel interference.
  5. Since space waves are used for FM, the radius of propagation is limited to line of sight. Hence it is possible to operate several independent transmitters on same frequency with minimum interference.
  6. Since FM uses UHF and VHF ranges, the noise interference is minimum compared to AM which uses MF and HF ranges.

There are some disadvantages of FM compared to AM as follows :

  1. The bandwidth requirement of FM is much higher than that of AM.
  2. The FM transmitting and receiving equipment is more complex and costly.
  3. Since FM uses UHF and VHF range of frequencies, its area of reception is limited only to line of sight. This is much lower than area covered by AM.

Table shows the comparison between FM and AM

Amplitude Modulation
  • Amplitude of the carrier is varied according to amplitude of modulation signal.
  • AM has poor fidelity due to narrow bandwidth.
  • Most of the power is in carrier hence less efficient.
  • Noise interference is more.
  • Adjacent channel interference is present.
  • AM broadcast operates in MF and HF range.
  • In AM only carrier and two sidebands are present.
  • The transmission equipment is simple.
  • Transmitted power varies according to modulation index.
  • Depth of modulation have limitation. It can not be increased above 1.
Frequency Modulation
  • Frequency of the carrier is varied according to amplitude of the modulating signal,
  • Since the bandwidth is large, fidelity is better.
  • All the transmitted power is useful.
  • Noise interference is minimum.
  • Adjacent channel interference is avoided due to wide bandwidth.
  • FM broadcast operates in VHF and UHF range.
  • Infinite number of sidebands of sidebands are present.
  • The transmission equipment is complex.
  • Transmitted power remains constant irrespective of modulation index.
  • Depth of modulation have no limitation. It can be increased by increasing frequency deviation.

Comparison between FM and PM

Table shows the comparison between FM and PM

Frequency Modulation
  • The maximum frequency deviation depends upon amplitude of modulating voltage and modulating frequency.
  • Frequency of the carrier is modulated by modulating signal.
  • Modulation index is increased as modulation frequency is reduced and vice versa.
Phase Modulation
  • The maximum phase deviation depends only upon the amplitude of modulating voltage.
  • Phase of the carrier is modulated by modulating signal.
  • Modulation index remains same if modulating frequency is changed.

Actually in FM, when frequency is changed, phase is also varied. FM is basically obtained from phase modulation as we see next.

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Santhakumar Raja

Hi, This blog is dedicated to students to stay update in the education industry. Motivates students to become better readers and writers.

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