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Chapter 2 amplitude_modulation


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Chapter 2 amplitude_modulation

  3. 3. TOPICS <ul><li>Need for Modulation </li></ul><ul><li>Principles of AM </li></ul><ul><li>Modulation Index and Signal Power </li></ul><ul><li>AM Circuits </li></ul><ul><li>Single Sideband Suppressed Carrier (SSBSC) </li></ul><ul><li>SSB Circuits </li></ul>
  4. 4. NEED FOR MODULATION <ul><li>It is because modulation makes the information signal more compatible with the medium. </li></ul><ul><li>Modulation = Imposing information at low frequency onto a higher frequency signal. </li></ul><ul><li>A technique for transmitting information efficiently from one place to another. </li></ul><ul><li>Simplest form of modulation is the amplitude modulation. </li></ul>
  6. 6. PRINCIPLES OF AM <ul><li>AM is defined as: </li></ul><ul><ul><li>Amplitude of carrier frequency change proportionately to the value of the modulation signal. </li></ul></ul><ul><li>Advantages: </li></ul><ul><ul><li>Simple modulator circuits </li></ul></ul><ul><ul><li>Cheap :low-quality form of modulation used for commercial broadcasting of audio & video signal. </li></ul></ul><ul><li>Disadvantages: </li></ul><ul><ul><li>Poor performance due to noise </li></ul></ul><ul><ul><li>Inefficient use of transmitter power. </li></ul></ul><ul><li>Application: </li></ul><ul><ul><li>2 way radio communications, broadcasting, aircraft comm. & citizen band (CB) radio. </li></ul></ul>
  7. 7. <ul><li>AM modulators are nonlinear devices </li></ul><ul><ul><li>2 input and 1 output: modulating signal and carrier signal. </li></ul></ul><ul><li>Several types of amplitude modulation </li></ul><ul><ul><li>AM DSBFC </li></ul></ul><ul><ul><li>DSB-SC </li></ul></ul><ul><ul><li>SSB </li></ul></ul><ul><li>AM generation is shown in Figure 2.1 </li></ul><ul><li>Modulated wave = AM envelope as shown in Figure 2.2 </li></ul>Figure 2.1: Block diagram of Amplitude Modulation Information, V m (t) Carrier, V c (t) Modulator V AM (t)
  8. 8. Figure 2.2 AM signal with the envelope
  9. 9. AM IN ACTION
  10. 10. <ul><li>AM begins with carrier v c ,  a sine wave with frequency  c & amplitude V c : </li></ul><ul><li>Modulating signal: </li></ul><ul><li>Then AM is: </li></ul>DERIVATION OF AM EQUATION <ul><li>Where m (modulation index) is defined as V m / V c , hence: </li></ul><ul><li>The voltage resulting AM wave envelope at any instant is: </li></ul>
  11. 11. <ul><li>This yield, the upper and lower sidebands – frequency & amplitude. </li></ul>Using Trigo ID Carrier LSB USB
  12. 12. AM FREQUENCY SPECTRUM & BANDWIDTH <ul><li>AM modulators are non-linear device => non-linear mixing occurs. </li></ul><ul><li>Output envelope is complex wave made up of DC voltage, carrier frequency, the sum ( f m + f c ) & difference ( f c – f m ) frequencies. </li></ul><ul><li>AM spectrum contains frequency component spaced f m Hz on either side of the carrier. </li></ul><ul><li>Figure 2.3 shows the frequency spectrum of AM wave. </li></ul>
  13. 13.
  14. 14. Figure 2.3: Frequency spectrum for AM wave f c f c + f m f c - f m Bandwidth = 2f m V c V m /2 V m /2
  15. 15. SPECTRUM PARAMETERS <ul><li>Center frequency = Carrier frequency = </li></ul><ul><li>Upper sideband freq. = carrier freq. + modulating freq. </li></ul><ul><li>Lower sideband freq. = carrier freq. - modulating freq. </li></ul><ul><li>Center frequency peak amplitude: </li></ul><ul><li>Upper and lower sideband voltages: </li></ul><ul><li>Bandwidth = Maximum freq. - minimum freq. </li></ul>
  16. 16. EXAMPLE 2.1 <ul><li>Q. Modulating signal f m =3 kHz frequency and a carrier frequency f c =1 M Hz. What is the upper & lower sideband frequency? Then find the bandwidth of the modulated signal. </li></ul><ul><li>A. 997 kHz, 1003 kHz, 6 kHz. </li></ul>
  17. 17. EXAMPLE 2.2 <ul><li>Q. A 1.4 MHz carrier is modulated by a signal with frequencies from 20Hz & 10KHz. Determine the range of frequencies generated for the upper and lower sidebands? </li></ul><ul><li>USB = 1.400020Hz, 1.410000Hz, </li></ul><ul><li>LSB = 1.390000Hz, 1.399980Hz </li></ul>