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Questions regarding AM and FM

  1. Feb 26, 2017 #1
    Q1. Why are higher frequencies assigned a higher bandwidth?
    Reasons for asking the question: My book quote "FM came late than AM waves and had to use higher frequencies than AM....... The greater range of frequencies available means that each station has a greater bandwidth......." If the equation is Bandwidth * Number of stations = Frequency Range, then it doesn't matter if the frequencies are of higher or lower order, for a given number of stations they both have the same bandwidth.

    Q2. Why do FM waves have an infinite number of frequencies?
    Reasons for asking the question: My book quotes " The frequency spectrum of a carrier is much more complex. In particular, there are many frequencies associated with an FM wave." Many more sites state that FM waves a theoretical limit of infinite frequencies.

    Q3. Why do FM waves are given a higher bandwidth than AM waves?
    Reasons for asking the question: My book quotes "In particular, there are many frequencies associated with an FM wave. This means that FM waves need a greater bandwidth for each station." I find a little flaw with this argument. If I were to use an AM wave with a signal having a high frequency, the AM wave will also be having a high bandwidth. Then why associate high bandwidths with FM?
    Last edited by a moderator: Feb 26, 2017
  2. jcsd
  3. Feb 26, 2017 #2


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    Amplitude Modulation means exactly that. You are modulating the amplitude of the carrier signal. If the signal strength varies, as it always will to some degree, you get information degradation. With Frequency Modulation, the amplitude can vary all over the place and as long as it is at least strong enough to be properly received, then there is no information loss because the frequency is not affected by amplitude variations.
  4. Feb 26, 2017 #3
    But the attained quality doesnt have anything to do with bandwidth right in these cases?
  5. Feb 26, 2017 #4


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    @Faiq I do not wish to be rude, but given the number of simple questions you are asking about radio, it seems to me that you just need to do a little more reading. Perhaps try Googling a few of these topics to get some extended information that your text doesn't seem to be providing you.

    Just as one example, FM broadcasts at higher frequencies because AM came first and other frequencies were used for other things, so FM was assigned higher frequencies. Also, FM needs a higher bandwidth BECAUSE it is Frequency Modulation. With AM, you are just transmitting one frequency so you only need to worry about sideband interference with other channels. With FM, you are transmitting different frequencies as the actual method of sending information so you need a higher bandwidth.
  6. Feb 26, 2017 #5


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    You really need to just read up some more on just what AM and FM basically ARE. Of course quality has to do with bandwidth on FM. With too narrow a bandwidth your ability to transmit information is constrained because it is the bandwidth that contains the information. Suppose you had a 100Mhz signal but only allowed 2Hz bandwidth. Great. You can transmit anything all the way from 99,999,999Hz to 100,000,001 Hz. Just how much information do you think you could pack into that?
  7. Feb 26, 2017 #6
    Can you site any source that will explain AM FM from basic?
  8. Feb 26, 2017 #7


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    I don't have any particular good ones but I think probably if you just google "FM radio", "AM radio", "radio bandwidth" and so forth you'll likely get it all. Sorry I don't have a more concise recommendation.
  9. Feb 26, 2017 #8


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    The Wikipedia article is a good introduction, with lots of links to other sources for further reading...


  10. Feb 27, 2017 #9


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    You are best advised to do your own searching for something like this. It is such a wide field and the range of presentations available is huge. Choose one to suit yourself - and then check what it says against other sites (there is a fair amount of rubbish talked about this topic). Dot edu addresses are fairly reliable.
    You have asked a big range of questions and the answer to 'why', in the case of Engineering topics is that there are always compromises in the choices made. History and politics play a big part and the choices are not always for the best. Take the BBC's rush into Digital Audio Broadcasting, which landed us with a system that's far from optimal - just for the same of being first on the scene.
  11. Mar 1, 2017 #10
    Try contacting your local amateur radio club. You'find they'll be able to help you out.
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