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ممكن احد يساعدني فحال هالاساله؟؟؟؟

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    مهندسة اتصلات
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    تاريخ التسجيل: Mar 2006
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    ممكن احد يساعدني فحال هالاساله؟؟؟؟

    السلام عليكم000


    ابي احد يساعدني فحل هالاساله لاني ادرس هندسة اتصلات وابغي المساعده اذا ممكن ؟؟
    ومحتاجه اجابتها باسرع وقت00

    Why modulation is necessary ?
    What different types of modulation schemes are available ?
    Advantages and disadvantages of these types ?
    Application of each type of modulation ?

    بلييييز ساعدوني وابي يكون 2000 كلمه

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  2. [2]
    TheTeck
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    تاريخ التسجيل: Sep 2005
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    ومن يتق الله يجعل له مخرجا

    http://www.PLC2PLC.com

  3. [3]
    Qatar Eng
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    تاريخ التسجيل: Sep 2006
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    اقتباس المشاركة الأصلية كتبت بواسطة مهندسة اتصلات مشاهدة المشاركة
    السلام عليكم000


    ابي احد يساعدني فحل هالاساله لاني ادرس هندسة اتصلات وابغي المساعده اذا ممكن ؟؟
    ومحتاجه اجابتها باسرع وقت00

    Why modulation is necessary ?
    What different types of modulation schemes are available ?
    Advantages and disadvantages of these types ?
    Application of each type of modulation ?

    بلييييز ساعدوني وابي يكون 2000 كلمه
    Modulation is addition of information to an electronic optical signal carrier. It’s to impress a message (Voice, Image, Data, Etc) on to carrier wave for transmission. A band limited range of frequency that comprises the message (Base band) is translated to higher range of frequency.

    Modulation types

    Amplitude modulation
    In amplitude modulation the amplitude of the carrier frequency is modified by the amplitude of a modulating by the amplitude of modulation signal. The side bands are displaced at either side of the carrier frequency by a magnitude of the modulation frequency, and the amplitude depends on the modulation amplitude. The fundamental equation of AM modulation is:
    (1+m sin(wmt))* sin (wct)
    BWam = 2fm
    Where fm is the modulation frequency in hertz, wc is the carrier frequency, and wm is the modulation frequency, both in radians/sec. m is the modulation index, (modulation voltage/carrier voltage), and produce excessive distortion when greater than 1

    Double sideband amplitude modulation

    The power consumed in a AM carrier transmits no information and is effectively wasted. It would make sense, therefore to transmit only the side frequencies. Which do contain the information. When both sidebands of an AM modulated carrier are transmitted without the carrier, the transmission is called double-sideband suppressed carrier (DSBSC) or (DSB).

    Double sideband suppressed carrier amplitude modulation (DSBSCAM)

    The frequencies produced by amplitude modulation are symmetrically spaced above and below the carrier frequency and the carrier level is reduced to the lowest practical level, ideally completely suppressed. Note: the DSBSC transmission is a special case of double sideband reduced carrier transmission.

    A general sinusoidal carrier can be represented mathematically as:
    c(t) = Ao cos (2fo t + ǿo)
    Where fo is known as the carrier frequency, since a sinusoid is completely specified by its amplitude, phase, and frequency, it follows that once the carrier frequency is specified, only tow parameters are to be varied in the modulation process the amplitude Ao and phase ǿo.


    Single sideband amplitude modulation (SSBAM)
    SSB is a form of amplitude modulation (AM) where the carrier is suppressed typically 40 to 60 dB below the carrier, and one of the sidebands is removed. The major disadvantage of SSB is that the carrier must be reinserted in the receiver, to the original modulating audio.
    Single sideband amplitude modulation scheme is similar to the double sideband suppressed carrier AM scheme. The idea of signal sideband AM is that only one side spectrums on either the upper or the lower sideband from the carrier signal can be used to demodulate the message signal. Only the difference between tow schemes is SSB AM modulated signal required half the bandwidth of DSBSC AM modulated signal therefore it increases the bandwidth efficiency compared to DSBSC AM on the bandwidth limited channel.

    Frequency Modulation (FM)
    In the FM the carrier frequency is changed according to the amplitude of the modulating signal. The higher the amplitude of the modulating signal the higher is the change in carrier frequency. The amount of the change in carrier frequency is called the frequency deviation fd. Maximum deviation occurs at the maximum amplitude of the modulating signal. The frequency of the modulating signal determines the frequency deviation rate or how many times per second the carrier frequency deviates above and below its centre frequency fc. Frequency modulation uses the information signal, Vm(t) to very the carrier frequency within some small range about its orignal value. Here are the three signals in mathematical form:
    1) Information: Vm(t)
    2) Carrier: Vc(t) = Vco sin ( 2 p fc t + f )
    3) FM: VFM (t) = Vco sin (2 p [fc + (Df/Vmo) Vm (t) ] t + f)

    Advantages of FM
    Noise immunity: the main benefit of FM over AM is its superior immunity to noise, made possible by the clipper limiter circuits in the receiver, which effectively strip off all of the noise variations, leaving a constant amplitude FM signal.
    Capture effect: another major benefit of FM is that interfering signal on the same frequency are effectively rejected because of the amplitude limiters and the demodulating methods used by FM receivers, a phenomenon known as the capture effect takes place when tow or more FM signal occur simultaneously on the same frequency.
    Transmitter efficiency: a third advantage of FM over AM involves efficiency recall that AM can be produced by both low level and high level modulation techniques.
    Disadvantages of FM
    Excessive spectrum use: perhaps the greatest disadvantage of FM is that it simply uses too much spectrum space. The bandwidth of an FM signal is, in general, considerably wider than that of an AM signal transmitting similar information.
    Circuit complexity: one major disadvantage of FM in the past involved the complexity of the circuits used for frequency modulation and demodulation in comparison with the simple circuit used for amplitude modulation and demodulation. Today this disadvantage has almost disappeared due to the use of integrated circuits.



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