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How do Radio Waves work when used for FM or AM Radio transmission?

  1. Feb 8, 2013 #1
    I would like to know about radio waves used for FM or AM radio. I have read many articles that say, "The sound is encoded and rides on a carrier radio wave". How is that encoded? I am under the assumption that waves just have a frequency and wavelength and that is all. If a wave is transmitted at 101 Mhz then isn't that wave just a continuous 101Mhz? How does anything get encoded onto a wave of continuous frequency? Aren't wavelength and frequency locked together so that neither can be modified without modifying the other? If that's that the case, how can anything be "encoded"?

    I am lost, please help!
  2. jcsd
  3. Feb 8, 2013 #2


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    Start by reading about superhetrodyne radio.

    FM is a different, you can do your own search for that.
  4. Feb 8, 2013 #3


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    AM = Amplitude Modulation

    FM = Frequency Modulation
  5. Feb 8, 2013 #4


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    In both AM and FM, the signal is not a fixed, pure frequency. The signal is a superposition of the carrier frequency (the one that is indicated by your AM/FM tuner) and a range of frequencies on either side. The total width of an AM "channel" in the US is 10 kHz, including guard bands between channels. FM channels are 0.2 MHz wide, again including guard bands.

    In AM, the "extra" frequencies are basically a byproduct of the modulation scheme. In FM, the "extra" frequencies are the fundamental nature of the modulation scheme.
  6. Feb 8, 2013 #5
    Thanks jtbell. Simple, clear and to the point. So many things I read refused to just say it.
  7. Feb 9, 2013 #6
    One way to look at the difference in AM and FM is that with an AM signal the modulation is in the power (Amplitude) of the carrier wave where in FM the modulation is in the frequency. On broadcast FM, I might be off , but I believe the bandwidth used in most countries it is up to 75 kHz on either side of the carrier depending on level of audio on the signal.
  8. Feb 10, 2013 #7
    There is one common misunderstanding about frequencies:

    Pure frequency applicable only for infinite constant wave.

    Otherwise, even if there is a pure sinusoid for some interval, signal still has a frequency band, which width inverse to the packet size.

    This is a basic principle of information storage in the signal: More signal variations in time lead to wider frequency band.
