Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

AM modulation.

  1. Feb 24, 2008 #1
    i hav got some doudts regarding AM (amplitude modulation)
    1) the amplitude of carrier wave is changed according to the signal --- defn of modulation
    wats the amplitude of modulated wave is it A+B or other ... if "A" is the amplitude of carrier & "B" is the amplitude of signal
    ......



    to be ctnd after the reply as my doubt is regarding modulating factor
     
  2. jcsd
  3. Feb 24, 2008 #2

    cepheid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If the carrier wave is C(t) and the signal is A(t), then the amplitude modulated signal is given by

    f(t) = A(t)C(t) (the carrier wave is simply multiplied by the signal).

    Example:

    [tex] C(t) = \sin(2\pi \nu t) [/tex]

    then

    [tex] f(t) = A(t)\sin(2 \pi \nu t) [/tex]

    Another way to think of this is as follows: the standard form for a sinusoidal signal of amplitude A is (ignoring arbitrary initial phase factors for simplicity):

    [tex] f(t) = A\sin(2 \pi \nu t) [/tex]

    Now imagine if, instead of that amplitude being fixed, it is time-varying (A(t)). Then we have:

    [tex] f(t) = A(t)\sin(2 \pi \nu t) [/tex]

    An amplitude-modulated signal is just a sinusoid with a time-varying amplitude, where the variation in time is such that the modulating signal contains information.
     
  4. Feb 24, 2008 #3
    agreed,
    i mean, what would be the maximum amplitude of the modulated signal.??
    the modulating factor which is defined as ratio of maximum change of amplitude of a carrier wave to amplitude of unmodulated carrier wave.
    if signal amplitude is equal to carrier amplitude then modulating factor is _______*
    *my book suggested that it is 1, but i am not able to get in to a conclusion (see how modulating factor is defined ) ------> the maximum change :confused:
     
    Last edited: Feb 24, 2008
  5. Feb 24, 2008 #4

    NoTime

    User Avatar
    Science Advisor
    Homework Helper

    Yes, it is 1 or 100%.
    If you modulate >1, then you can not recover the original signal.
     
  6. Feb 24, 2008 #5
    Ideally, you want the abs( m(t) ) <= 1, otherwise you're overmodulating the signal. http://en.wikipedia.org/wiki/Overmodulation
     
  7. Feb 25, 2008 #6
    i was jus trying to work with in the limits of formulae for modulating factor.

    agrred with u all that if m>1 (overmodulated), m=1(perfect modulation), m<1 (under modulated)

    ( if new modulated amplitude is "X"; "Y" amplitude of unmodulated carrier wave)
    the change is X-Y then modulating factor is (X-Y)/Y)...
    i jus wanna :uhh: to know what is the amplitude of modulated signal...then i can take the difference between and work out...
     
  8. Feb 25, 2008 #7
    If 100% modulated, the minimum value of the carrier is 0% so the maximum is 200%.

    Off topic, but to get a more punchy sound, rock stations overmodulate the positive peaks... you can't over modulate the negative peaks coz it cuts off the transmitter...
     
  9. Feb 25, 2008 #8

    NoTime

    User Avatar
    Science Advisor
    Homework Helper

    I think the FCC would come knocking on their door if they did that.
    What they can do (especially for commercials) is to process the input so that the average modulation is higher.
     
  10. Feb 26, 2008 #9
    Same thing.
     
  11. Feb 26, 2008 #10

    NoTime

    User Avatar
    Science Advisor
    Homework Helper

  12. Feb 27, 2008 #11
    This is what I was referring to...

    HTH.
     
  13. Feb 27, 2008 #12

    NoTime

    User Avatar
    Science Advisor
    Homework Helper

    Hmmm, the net effect of signal processing like this would seem to be equivalent to increasing the voltage to your final (barring the inherent distortion induced into the original signal).
    Calling this over modulation is at best suspect.
    It does not quite conform to the definition of the term.
     
  14. Feb 28, 2008 #13
    It seems weird to me too, but hey! it's LOUDER so it must be good... :eek:)

    Or something...
     
  15. Feb 28, 2008 #14

    NoTime

    User Avatar
    Science Advisor
    Homework Helper

    :rofl: If you can't be good, be loud.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?