Amplitute modulation
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Amplitute modulation

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Amplitude Modulation

Amplitude Modulation

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Amplitute modulation Amplitute modulation Presentation Transcript

  • 1. What is modulation?• Modulation is the process of putting information onto a high frequency carrier for transmission (frequency translation).
  • • Once this information is received, the low frequency information must be removed from the high frequency carrier. This process is known as “ Demodulation”.
  • 2. What are the reasons for modulation?1. Frequency division multiplexing (To support multiple transmissions via a single channel) To avoid interference
  • M1(f) M(f) Multiplexed f signal0 + M2(f) 0 f1 f2 f f0
  • 2. Practicality of Antennas Transmitting very low frequencies require antennas with miles in wavelength
  • ANALOG AND DIGITALAnalog-to-analog conversion is the representation ofanalog information by an analog signal. One may askwhy we need to modulate an analog signal; it isalready analog. Modulation is needed if the medium isbandpass in nature or if only a bandpass channel isavailable to us.Topics discussed in this section:Amplitude ModulationFrequency ModulationPhase Modulation
  • Figure Types of analog-to-analog modulation
  • Figure Amplitude modulation
  • Note The total bandwidth required for AM can be determined from the bandwidth of the audio signal: BAM = 2B.
  • Figure AM band allocation
  • NoteThe total bandwidth required for FM can be determined from the bandwidth of the audio signal: BFM = 2(1 + β)B.
  • 4. What are the Basic Types of Analogue Modulation Methods ?Consider the carrier signal below: sc(t ) = Ac(t) cos( 2πfc t + θ ) 1. Changing of the carrier amplitude Ac(t) produces Amplitude Modulation signal (AM) 2. Changing of the carrier frequency fc produces Frequency Modulation signal (FM) 3. Changing of the carrier phase θ produces Phase Modulation signal (PM)
  • 5. What are the different Forms of Amplitude Modulation ?1. Conventional Amplitude Modulation (DSB-LC) (Alternatively known as Full AM or Double Sideband with Large carrier (DSB-LC) modulation2. Double Side Band Suppressed Carrier (DSB-SC) modulation3. Single Sideband (SSB) modulation4. Vestigial Sideband (VSB) modulation
  • Conventional Amplitude Modulation (Full AM)
  • 6. Derive the Frequency Spectrum for Full-AM Modulation (DSB-LC)
  • 6. Derive the Frequency Spectrum for Full-AM Modulation (DSB-LC)1 The carrier signal is sc (t ) = Ac cos(ω c t ) where ω c = 2πf c2 In the same way, a modulating signal (informationsignal) can also be expressed as sm (t ) = Am cos ωm t
  • 3 The amplitude-modulated wave can be expressed as s (t ) = [ Ac + sm (t )] cos(ω c t )4 By substitution s (t ) = [ Ac + Am cos(ω mt )] cos(ω c t )5 The modulation index. Am m = Ac
  • 6 Therefore The full AM signal may be written as s (t ) = Ac (1 + m cos(ω mt )) cos(ω c t ) cos A cos B = 1 / 2[cos( A + B ) + cos( A − B )] mAc mAcs (t ) = Ac (cos ω ct ) + cos(ω c + ω m )t + cos(ω c − ω m )t 2 2
  • 7. Draw the Frequency Spectrum of the above AM signal and calculate the Bandwidth fc-fm fC fc+fm 2fm
  • 8. Draw Frequency Spectrum for a complex input signal with AM fc-fm fc fc+fm
  • Frequency Spectrum of an AM signalThe frequency spectrum of AM waveform containsthree parts: 1. A component at the carrier frequency fc 2. An upper side band (USB), whose highest frequency component is at fc+fm 3. A lower side band (LSB), whose highest frequency component is at fc-fmThe bandwidth of the modulated waveform is twice theinformation signal bandwidth.
  • • Because of the two side bands in the frequency spectrum its often called Double Sideband with Large Carrier.(DSB- LC)• The information in the base band (information) signal is duplicated in the LSB and USB and the carrier conveys no information.
  • Modulation Index (m) 9. What is the significance of modulation index ?• m is merely defined as a parameter, which determines the amount of modulation.• What is the degree of modulation required to establish a desirable AM communication link? Answer is to maintain m<1.0 (m<100%).• This is important for successful retrieval of the original transmitted information at the receiver end.
  • • If the amplitude of the modulating signal is higher than the carrier amplitude, which in turn implies the modulation index m ≥ 1.0(100%) . This will cause severe distortion to the modulated signal.
  • 10. Calculate the power efficiency of AM signals• The ratio of useful power, power efficiency : sidebands power m2 / 2 m2 = = total power 1 + m / 2 2 + m2 2• In terms of power efficiency, for m=1 modulation, only 33% power efficiency is achieved which tells us that only one-third of the transmitted power carries the useful information.