CIE A Level Physics (9702) 2019-2021

Revision Notes

20.1.2 Modulation (AM & FM)

Defining Modulation

  • Modulation is defined as:

The process of changing the characteristics of the carrier signal

  • The carrier signal is simply the wave that is to be transmitted
  • In modulation, the carrier signal is combined with a modulating signal that contains the information to be transmitted
  • Reasons why a transmitted wave is modulated rather than transmitting the information signal directly as a radio wave are so :
    • More than one radio station can operate in the same region so less interference
    • A shorter aerial can be used
    • Less attenuation and less power required
    • Less distortion
  • There are two types of modulation:
    • Amplitude modulation (AM)
    • Frequency modulation (FM)
  • These were first commonly used in radio broadcasting
  • The difference between them is in how the carrier wave is modulated

Amplitude Modulation

  • In amplitude modulation (AM), the amplitude (or strength) of the signal is varied to incorporate the sound information
  • AM is defined as:

The amplitude of the carrier wave varies in synchrony with the displacement of the information signal

  • Amplitude modulation is distinguishable from the power of the radio signal varying with frequency

Frequency Modulation

  • Frequency modulation (FM) is defined as:

The frequency of a carrier wave varies in synchrony with the displacement of the information signal

  • In frequency modulation (FM), unlike AM, the amplitude of the carrier is kept constant, but the frequency of the signal is varied to incorporate the sound information

Defining Bandwidth

  • The bandwidth of a signal is defined as:

The total range of frequencies required to pass a modulated signal to carry the information without distortion or loss of data

  • Bandwidth is measured in Hertz (Hz)
  • In other words, the bandwidth is the difference between the highest and lowest frequencies of a transmitted signal
  • Optical fibres have the highest bandwidth out of all transmission media

AM and FM Transmissions

  • Advantages of FM over AM:
    • Less prone to interference
    • Has a greater bandwidth with a better quality signal
  • Disadvantages of FM over AM:
    • FM signals are impacted by physical barriers
    • They are short-range signals, so more transmitters are needed
    • The circuitry is more complex and requires a greater expense

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