A.M Modulation

Discussion in 'Homework Help' started by omerysmi, Oct 28, 2014.

  1. omerysmi

    Thread Starter Member

    Oct 10, 2014
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    Suppose that I have an AM modulated wave while Am = 2v
    If it goes to envelope detector, I will get in the out all the time 2v?
    Does it matter if the modulation index is 1 or 0.5 or 0.2 for example? or that in each situation i will get 2v?

    In simple words I want to understand if the modulation index (ma) effect the amplitude of the message wave (Am) in the output of the envelope detector. (Or if the accuracy of the detector depends on the modulation index)

    Thanks!
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    The output from an envelope detector is directly proportional to the modulation level and modulation index (which is just a measure of how close you go to the clipping modulation level). That's how AM modulation works.
     
  3. omerysmi

    Thread Starter Member

    Oct 10, 2014
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    0
    I didn't understand you properly,
    Does it more good that that modulation index will be close to 1 or close to zero for getting better
    accuracy in the output of the detector?

    For example, if i have the following data:
    1 situation: Am=2v, Ac=2v (ma=1)
    2 situation: Am=2v, Ac=4v(ma=0.5)

    The detector will show the same result for the two situations?
     
    Last edited: Oct 28, 2014
  4. WBahn

    Moderator

    Mar 31, 2012
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    The answer is: It depends. There are all kinds of envelope detectors and each has its advantages and disadvantages. In general, it is probably reasonable to say that the smaller the modulation index the more linear the transfer characteristic will be, but the larger the modulation index the better the SNR will be. At some point in between is probably where you will find best overall performance.
     
  5. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Attached is a 1 volt AM carrier with the three modulations as specified in your question.

    Then think about the diode and low pass filter that demodulates these signals.

    The second picture is the situation you described. Bear in mind. most receivers have AGC ... Automatic gain control.
     
    Last edited: Oct 28, 2014
  6. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hello there,

    Yes this partly depends on the type of receiver being used. There's a power loss associated with a modulation from 0 to 1 but over modulation can allow more useful power to reach the receiver with the drawback being that a more complicated 'coherent' type detector is required which of course is a lot more than a (non coherent) rectifier diode detector.
     
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Regulatory agencies typically specify modulation shall not exceed 100 percent.

    Overmodulation typically causes spurious emissions.

    So, now the emitter would have two violations of the rules and regulations.
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    I missed something -- what's the other violation? Or are you saying that the first is overmodulation and the second would by any out-of-tolerance spurious emissions?
     
  9. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    more than 100 percent modulation
    causing spurious emissions ... and possibly
    interfering with other services.

    I just demonstrated I can't count to three. LOL

    Even FM has a definition that equates to 100 percent modulation, a figure that "shall not be exceeded". The FCC had a big crackdown about 20 years ago and sent notices out reminding the broadcasters of the 100% modulation limits ... which the engineering staff quickly lowered the modulation levels in the transmitter to prevent any overmodulation.

    But more importantly ... Mr Al, do you have a lab exercise that demonstrates more than 100 pct modulation is desired?

    I did find a lab exercise, which I attached below, as an example of coherent detection in the AM scheme, but all I see there is clipping, not overmodulation, where the instructions state it's an AM signal. The input modulation signal exceeding the input carrier signal doesn't necessarily mean overmodulation within some devices.

    I'll have to buy some AD633s to confirm or deny that experiment.
     
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  10. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    I would be interested in hear of it Joe...!
     
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