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Trying to understand Modulation

  1. Jun 1, 2004 #1


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    Hi guys,
    Need some help and guidance here, I'm having some difficulty(and confused) trying to understand Modulation in telecommunication systems.

    From what I understand (pls correct me if I am wrong);
    AM - the amplitude changes, but not the frequency.
    FM - the frequency changes, but not the amplitude.

    Is it true that the spectrum for AM would be the total(sum) of the spectra formed by each of the modulating signals? :confused:

    What about FM ? :confused:
  2. jcsd
  3. Jun 1, 2004 #2
    In reality there is am to pm modulation that effects the non-linear amplifiers.

    As for FM, I suspect that FM-AM convertion occurs also as there are no perfectly flat amplifiers
  4. Jun 2, 2004 #3


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    Thanks for the reply, but I still dun get it.

    Can anyone else help me (to understand) ?? :confused:
  5. Jun 4, 2004 #4
    When a carrier is amplitude modulated, NEW frequencies are actually formed. Suppose we amplitude modulate a 1 Mhz carrier with a 1000 hertz tone. We now have the carrier and 2 sidebands. Each 'sideband' is 1000 hertz on each side of the carrier. If you amplitude modulate a carrier with a complex signal such as speech then you will have upper and lower sidebands (many) that are direct copies of the speech. An amplitude modulator behaves very similarly to a mixer.

    Frequency modulation is a little different. Suppose we frequency modulate a 1 Mhz carrier with a 1000 hertz tone. How hard we drive the modulator determines how far the main carrier swings. Commercial FM broadcasts swing the carrier a maximum of 75 Khz each way. This is called deviation. So suppose we drive our theororetical modulator until we get 75 Khz of deviation. You will see on a spectrum analyzer 'sidebands' every 1000 hertz starting at the main carrier frequency all the way out to 75 Khz each side of the carrier. This would be considered a fairly high modulation index. The modulation index is the deviation / modulating frequency. The higher the index, the more individual sidebands there are and typically the better the signal to noise ratio will be. At certain modulation indexes, certain sidebands disappear and sometimes even the main carrier disappears. I believe this was discovered or explained or something by a man named Bessel. Hence the term 'bessel null'. If you want to set the deviation to exactly 75 Khz you drive the modulator with a certain frequency that Bessel functions can explain and adjust the drive to the modulator until you see the carrier disappear on a spectrum analyzer and you know you have exactly the deviation you want.

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