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Modulation of waves (Communication Systems)

  1. Dec 26, 2014 #1
    Please look at the attachments below.
    I shall use the word "modulation" instead of "amplitude modulation" since I've been taught about AM only.
    So, my question is, in the first image (that I've attached), the messenger wave (which is modulated with the high frequency carrier wave) as well as the modulated wave, both are shown to be varying with time - and their phase is also changing (just like ALTERNATING CURRENT).

    So, this is the wave which is transmitted.

    But, in the second image - which shows the fate of the transmitted wave after being received by the receiver antenna- the output wave is shown to be varying with time - but its phase is not changing (i.e. The variations are limited to one quadrant only - above the axis only ) - just like time varying DC.

    So, 1) Can the arrangement be used to convert AC into DC?
    2) Is the change acceptable? I mean - the input wave's phase was changing - but it is not so with the output wave. So, will it affect the efficiency? Like- in the case of a voice signal?


    20141226_174517.jpg 20141226_174406.jpg
     
    Last edited: Dec 26, 2014
  2. jcsd
  3. Dec 26, 2014 #2

    ehild

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    The amplitude modulated signal received with the antenna, goes through a rectifier which transmits only the positive half of the signal. (The rectifier can be a diode, which transmits that part of the signal which makes the diode open ...) The result is a fast varying signal, with slowly changing height. Using a low-pass filter, it removes the high frequency component and you get the demodulated signal, that corresponds to the modulating one.
     
  4. Dec 26, 2014 #3

    NascentOxygen

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    There is no changing phase shift in the AM signal that I can make out, not like in FM. The demodulation block diagram applies to the AM signal, and shows the negative half of the waveform being removed, followed by filtering. The filtering function can even be considered to be performed by the audio amplifier or the loudspeaker, since neither will respond to the RF component in the rectified wave.
     
    Last edited: Dec 26, 2014
  5. Dec 26, 2014 #4
    I got the point. But still I think I didn't get the answer of the 'original' question. Actually, I'm trying to say that in the second diagram, all the oscillations are confined to one quadrant only - I mean, the wave is not "crossing" the time axis...Whereas in the first one, it can be seen actually crossing the time axis and entering in a different quadrant (like a sine wave).
     
    Last edited: Dec 26, 2014
  6. Dec 26, 2014 #5
    And please help me with my first question too...(marked as 1) in the original post).
    If this question seems to be pointless - then please explain the working of a transducer..Since It's not clear to me.
    Is an electrical transducer used only in the case of a communication system involving cables (eg. The 'basic' telephones - landline) or is it also used in the systems involving EM waves transmission (eg. Mobile phones)?
     
  7. Dec 26, 2014 #6

    NascentOxygen

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    A capacitor will remove the DC component and return the recovered message wave to being centred on zero. Most audio amplifiers have capacitive coupling, so the DC component is removed there, if not before.
     
  8. Dec 27, 2014 #7

    NascentOxygen

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    Sorry, I don't understand this. Perhaps you mean "modulator" (or maybe "detector") where you wrote "transducer"?
     
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