9.1.2

Modulation

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Modulation

Radiowaves can be altered by a process called modulation in order to transmit information.

Illustrative background for Amplitude modulation (AM)Illustrative background for Amplitude modulation (AM) ?? "content

Amplitude modulation (AM)

  • Amplitude modulation or AM radio waves are used to carry commercial radio signals in the frequency range from 540 to 1600 kHz.
  • A carrier wave which has the frequency used to tune in to the radio station is varied or modulated in amplitude by an audio signal.
  • The resulting wave has a constant frequency, but a varying amplitude.
Illustrative background for Properties of AMIllustrative background for Properties of AM ?? "content

Properties of AM

  • AM waves can go over and around rather large obstacles.
  • A very large antenna is needed to efficiently broadcast typical AM radio with its carrier wavelengths on the order of hundreds of meters.
  • AM waves are susceptible to noise as the addition of other waves adds to the amplitude of the wave.
Illustrative background for Frequency modulation (FM)Illustrative background for Frequency modulation (FM) ?? "content

Frequency modulation (FM)

  • Frequency modulation or FM radio waves are also used for commercial radio transmission but in the frequency range of 88 to 108 MHz.
  • A carrier wave which has the frequency used to tune in to the radio station is modulated in frequency by the audio signal.
    • This produces a wave of constant amplitude but with varying frequency.
Illustrative background for Properties of FMIllustrative background for Properties of FM ?? "content

Properties of FM

  • FM radio is less subject to noise from stray radio sources than AM radio.
  • FM antennas are much smaller than AM antennas.
  • However, FM is best received when there is a line of sight between the broadcast antenna and receiver, so they are often sent from very tall structures.
Illustrative background for Sideband frequenciesIllustrative background for Sideband frequencies ?? "content

Sideband frequencies

  • If a carrier wave of frequency fcf_c is amplitude modulated by a simple sine wave of frequency fsf_s, the resulting wave is found to be composed of three frequencies:
    • fcfs,fcf_c-f_s,f_c and fc+fsf_c+f_s
  • The two frequencies fcfsf_c-f_s and fc+fsf_c+f_s are known as sideband frequencies.
Illustrative background for BandwidthIllustrative background for Bandwidth ?? "content

Bandwidth

  • The bandwidth of this wave is equal to the range of the frequencies carried by the wave:
    • (fc+fs)(fcfs)=2fs(f_c+f_s)-(f_c-f_s) = 2f_s

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