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Amplitude modulation frequency component

  1. Sep 9, 2012 #1
    Hello Everyone!

    About Amplitude Modulation, Usually it is seen in the book or web that (e.g. http://www.ni.com/white-paper/3002/en )
    '''' The message signal can be represented by m(t) = Mb cos(2πfb + φ)
    and the carrier signal can be represented by c(t) = Ac cos(2πfc + φ),
    Now Modulated signal = m(t) * c(t) ''

    My question is does m(t) represents only the highest frequency component in such a case? Or this is the fundamental frequency component?
    Or, this represents the whole message in that all the frequency components/harmonics lying within m(t)? If, this represents the whole message, why only a single frequency component is written (i.e fb)?

    If, this represents the max frequency component, why we are only considering only this one? Provided that most of the information of the message is contained in the fundamental frequency? SO, why we are not considering the other ones?

    I understand these are very silly queries, I would highly appreciate your time answering me these.

  2. jcsd
  3. Sep 9, 2012 #2


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    Science Advisor

    I don't know why they wrote m(t) in such a limited way. In general, m(t) is an arbitrary function of time. However, it is generally expected that spectrum of m(t) will be dominated by frequencies much lower than fc.
  4. Sep 10, 2012 #3


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    Staff: Mentor

    m(t) = Mb cos(2πfb + φ) represents a single component of the modulating signal. If the modulating signal comprises more than one sinusoidal component, then the modulated signal would be the sum of all those m(t)*c(t) products.
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