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Amplitude Modulation ppt

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basics of amplitude modulation

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Amplitude Modulation ppt

  1. 1. INTRODUCTION What is communication Transmitter Channel Receiver A communication system Process of conveying message or information Transmitter modifies the message signal into a form which is suitable for transmission over channel which is achieved by MODULATION Information source
  2. 2. WHAT IS MODULATION MODULATION is the basic requirement for transmitting the message signal through free space  It is the process of transmission of information signal (low frequency audio signal) using a high frequency carrier signal
  3. 3. WHY MODULATION? Carrying one signal to another : uses carrier (having high frequency , smaller wavelength) Modulated signal is transmitted Problems with transmitting baseband signal/ Need of modulation Height of transmitting and receiving antenna Noise and interference from other sources at low frequencies: Multiplexing Narrow banding
  4. 4. PRACTICABILITY OF ANTENNAS h=λ/4, for efficient transmission. For f=30 Hz => h= 2500 km f=3kHz => h= 25 km f=3MHz => h= 25 m Thus as Frequency increases height of the antenna decreases
  5. 5. MODULATION Defined as “ The process by which some characteristics of a signal called carrier varied in accordance with the instantaneous value of another signal called modulating signal “  The information bearing signal is called modulating signal The signal resulting from process of modulation is known as modulated signal
  6. 6. TYPES OF MODULATION Continuous wave Modulation : carrier is continuous in nature (usually sinusoidal) (AM,FM,PM) Pulse Modulation :Carrier is pulse type waveform
  7. 7. TYPES OF MODULATION  Sine wave (carrier) described by 3 parameters: amplitude, frequency and phase.  Let carrier signal be: v(t) = A sin (ωt + φ) So can have  – Amplitude modulation (AM)  – Frequency modulation (FM)  – Phase modulation (PM) Frequency and phase combined are known as Angle Modulation
  8. 8. AMPLITUDE MODULATION The amplitude of the carrier is changed in accordance with the instantaneous value of modulating signal Carrier : c(t) = Vc cos (2πfct + φ) modulating signal v(t) = Vm cos (2πfmt) Information is contained in the envelop
  10. 10. AMPLITUDE MODULATION Modulated signal: v(t) = Vc cos (2πfct) {1 + m cos (2πfm t)}  Vc = unmodulated peak carrier amnplitude fc= carrier frequency fm = modulation frequency m = modulation index (“degree” of modulation) m must be between 0 and 1 If m > 1 get overmodulation (bad …distortion)
  11. 11. VARYRING MODULATION INDEX m =Vmax – Vmin / Vmax + Vmin
  12. 12. AMPLITUDE SPECTRUM Modulation produces two new components called sidebands, at frequencies above and below the carrier The spacing in frequency between carrier and sidebands is equal to fm (the modulating frequency) Bandwidth requirement : 2 fm
  14. 14. WHERE DO THE SIDEBANDS COME FROM Expand v(t) = Vc cos (2π fct) {1 + m cos (2π fm t)} Using trig identities to get: v(t) = Vc cos (2π fct) +0.5m Vc cos (2π [fc- fm ]t) +0.5m Vc cos (2π [fc+ fm ]t)  This expression consists of 3 sine waves at frequencies of carrier (fc), lower sideband (fc-fm) and upper sideband (fc+fm). fc= carrier frequency
  15. 15. EFFICIENCY  For a fully modulated carrier (m=1), 2/3 of the power is in the carrier, the rest in the sidebands (33.33% efficient )  Total power Pt = Pc (1 + m2 /2)  Carrier Power (Pc) = Vc 2 / 2  Side band Power =Plsb=Pusb= m2 Pc / 4  Information in side band : Power gets wasted in carrier AM is bandwidth inefficient (2 fm) Gets effected due to noise
  16. 16. VARIATIONS OF AM  Double Sideband with carrier (AM): This is the most widely used type of AM modulation. In fact, all radio channels in the AM band use this type of modulation. Double Sideband Suppressed Carrier (DSBSC): This is the same as the AM modulation above but without the carrier. Single Sideband (SSB): In this modulation, only half of the signal of the DSBSC is used. Vestigial Sideband (VSB): This is a modification of the SSB to ease the generation and reception of the signal.