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Homework Help: Communication systems -- Find the Modulation Depth...

  1. Jul 14, 2015 #1
    1. The problem statement, all variables and given/known data
    Hello, the plot in the image shows a modulated signal s(t), with one period of the modulating signal m(t). The mean value of m(t) is equal to zero . Its asked me to find the modulated index, the amplitude of the carrier, the equation of the message(m(t)) and the kind of modulation (DSB-SC or AM).

    2. Relevant equations
    s(t)=A*m(t)*cos*2*pi*fc*t) (dsb-sc modulation)
    s(t)= (1+ka*m(t))*cos*2*pi*fc*t) (am modulation)
    3. The attempt at a solution
    I don't know, I'm really stuck in this exercise.

    Attached Files:

    • com.png
      File size:
      110.5 KB
  2. jcsd
  3. Jul 14, 2015 #2
    Have you tried plotting those questions with various values of the constants to help learn their meanings?
  4. Jul 14, 2015 #3
    Yes, I do. But I can't still figure out a solution for it.

    I tried to solve s(0)= (1+ka*m(0)), but I don't know who is m(t)
  5. Jul 17, 2015 #4


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    Staff: Mentor

    The image seems to not show m(t). You are making this difficult for yourself if you ignore the assistance a sketch of the modulating signal will likely give you.

    If you are not shown m(t), perhaps you can figure it out, or even "guess" it?
  6. Jul 17, 2015 #5

    rude man

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    Homework Helper
    Gold Member

    I suggest:
    take the two s(t) signals in "relevant equations". Take one at a time.
    Can you imagine s(t)=A*m(t)*cos*2*pi*fc*t) (dsb-sc modulation) giving you the shown signal?
    Can you imagine s(t)= (1+ka*m(t))*cos*2*pi*fc*t) (am modulation) giving you the shown signal?
    Only one of those two can give you the shown signal.
    And a little thinking should get you the answers you seek.
    Hint: only one of your s(t) can produce a signal with differing maxima (5 and 15).
    P.S. obviously, m(t) is a sinusoid.
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