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                          Modulation
Modulation is a process that causes a shift in the range of
frequencies in a signal.
 • Signals that occupy the same range of frequencies can be
   separated
 • Modulation helps in noise immunity, attentuation - depends on
   the physical medium
Figure 1 shows the different kinds of analog modulation schemes
that are available



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                                          Carrier Modulation
        Communication System




       Base−band        Carrier         Amplitude         Angle
       Modulation       Modulation         (AM)


                                                    Frequency     Phase
                                                          (FM)        (PM)


           Figure 1: A broad view of communication system

   • Amplitude Modulation It is the process where, the amplitude of
     the carrier is varied proportional to that of the message signal.
 – Amplitude Modulation with carrier
   Let m(t) be the base-band signal, m(t) ←→ M (ω) and c(t)
   be the carrier, c(t) = Ac cos(ωc t). fc is chosen such that

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   fc >> W , where W is the maximum frequency component
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    of m(t).
    The amplitude modulated signal is given by


                  s(t) = Ac [1 + ka m(t)] cos(ωc t)


                        Ac
            S(ω) = π        (δ(ω − ωc ) + δ(ω + ωc )) +
                         2
                       ka Ac
                             (M (ω − ωc ) + M (ω + ωc ))
                         2




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    m(t)
                                                 M( ω)




                                     −f m               fm        ω
                 t

s(t)

                                                  S(ω )


                                        A c /2

                                       1/2 A/2 k M(0)
                                               a


                              − fc                           fc
                                                                      ω
                 t
                              2fm



           Figure 2: Amplitude modulation

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    Figure 2 shows the spectrum of the Amplitude Modulated
    signal.
– ka is a constant called amplitude sensitivity. ka m(t) < 1 and
  it indicates percentage modulation.
– Modulation in AM: A product modulator is used for
  generating the modulated signal as shown in Figure 3.




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                      A c cos(2 π fc t)


                                          +
                                               s(t)
                  Product
    m(t)          Modulator




     Figure 3: Modulation using product modulator

– Demodulation in AM: An envelope detector is used to get
  the demodulated signal (see Figure 4).

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     r
                               C                v (t)
                                         R       m
         +
         −



    Figure 4: Demodulation using Envelope detector

– The voltage vm (t) across the resistor R gives the message
  signal m(t)



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     Double Side Band - Suppressed Carrier
            (DSB-SC) Modulation
• In AM modulation, transmission of carrier consumes lot of
  power. Since, only the side bands contain the information
  about the message, carrier is suppressed. This results in a
  DSB-SC wave.
• A DSB-SC wave s(t) is given by


             s(t) =    m(t)Ac cos(ωc t)
                         Ac
            S(ω) =     π    (M (ω − ωc ) + M (ω + ωc ))
                         2



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    s(t)                                      S(f)

                                             1/2 A M(0)
                                                  c




                     t               −fc                  fc   ω

                                    2fm




                Figure 5: DSB-SC modulation

• Modulation in DSB-SC: Here also product modulator is used as
  shown in Figure 3, but the carrier is not added. Figure 6 shows
  the spectrum of the DSB-SC signal.


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                                                   LPF


                                        1/2 A M(0) cos(φ )
                                             c




                    −2f             0                    2fc
                      c


        Figure 6: Spectrum of Demodulated DSB-SC signal

  • Demodulation in DSB-SC: A coherent demodulator is used.
    The local oscillator present in the demodulator generates a
    carrier which has same frequency and phase(i.e. φ = 0 in
    Figure 7) a as that of the carrier in the modulated signal (see
    Figure 7)
  a Clearly  the design of the demodulator for DSB-SC is more complex than

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that vanilla AM
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                            v(t)                v o (t)
       Product                            LPF
s(t)   Modulator




                     cos(2 π fc t + φ )


        Local
        Oscillator



       Figure 7: Coherent detector




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              v(t) = s(t). cos(ωc t + φ)
                   = m(t)Ac cos(ωc t) cos(ωc t + φ)
                       m(t)
                   =        Ac [cos(2ωc t + φ) + cos(φ)]
                        2

• If, the demodulator (Figure 7) has constant phase, the original
  signal is reconstructed by passing v(t) through an LPF.




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Description: Modulation is converted into the baseband signal transmission signal technology. It will sample the analog signal after quantization, a binary digital signal "1" or "0" on-off modulated light carrier, and the pulse code (PCM). Digital modulation has the advantage of strong anti-interference relay when the impact of noise and dispersion does not accumulate, so you can achieve long distance transmission. The disadvantage is the need to wide band devices also are complex.