Lec am modulation


Published on

Published in: Technology, Business

Lec am modulation

  1. 1. Chapter 2: Modulation
  2. 2. Communication System Chart Communication System Continuous Wave Digital Wave Amplitude Modulation (AM) Pulse Modulation (PM) Angle Modulation Frequency Modulation (FM) Analogue Pulse Modulation Digital Pulse Modulation
  3. 3. Modulation is defined as the process of modifying a carrier signal (radio wave) systematically by the modulating signal (audio)” This process makes the signal suitable for the transmission and compatible with the channel. The resultant signal is called the modulated signal MODULATION
  4. 4. MODULATION Modulated signal Carrier signal ( a transmitted electromagnetic pulse or wave high frequency of alternation on which information can be imposed by increasing signal strength, varying the base frequency, varying the wave phase, or other means) Baseband signal
  5. 5. Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave
  6. 6. Types of Modulation <ul><li>Three main type of modulations: </li></ul><ul><li>Analog Modulation </li></ul><ul><ul><li>Amplitude modulation </li></ul></ul><ul><ul><ul><li>Example: Double sideband with carrier (DSBWC), Double sideband suppressed carrier (DSBSC), Single sideband suppressed carrier (SSBSC), Vestigial sideband (VSB) </li></ul></ul></ul><ul><ul><li>Angle modulation (frequency modulation & phase modulation) </li></ul></ul><ul><ul><ul><li>Example: Narrow band frequency modulation (NBFM), Wideband frequency modulation (WBFM), Narrowband phase modulation (NBPM), Wideband phase modulation (NBPM) </li></ul></ul></ul>
  7. 7. Types of Modulation <ul><li>Pulse Modulation </li></ul><ul><ul><li>Example: Pulse Amplitude Modulation (PAM), Pulse width modulation (PWM) , Pulse Position Modulation (PPM) </li></ul></ul><ul><li>Digital Modulation </li></ul><ul><ul><li>Modulating signal is analog </li></ul></ul><ul><ul><ul><li>Example: Pulse Code Modulation (PCM), Delta Modulation (DM), Adaptive Delta Modulation (ADM), Differential Pulse Code Modulation (DPCM), Adaptive Differential Pulse Code Modulation (ADPCM) etc. </li></ul></ul></ul><ul><ul><li>Modulating signal is digital (binary modulation) </li></ul></ul><ul><ul><ul><li>Example: Amplitude shift keying (ASK), frequency Shift Keying (FSK), Phase Shift Keying (PSK) etc. </li></ul></ul></ul>
  8. 8. Modulation 1 Analogue Modulation Amplitude Modulation
  9. 9. Amplitude Modulation ~ DSBFC (Full AM) <ul><li>“ Amplitude Modulation is the process of changing the amplitude of the radio frequency (RF) carrier wave by the amplitude variations of modulating signal” </li></ul><ul><li>The carrier amplitude varied linearly by the modulating signal which usually consist of a range of a audio frequencies. The frequency of the carrier is not affected </li></ul><ul><li>Application of AM - Radio broadcasting, TV pictures </li></ul><ul><li>(video), facsimile transmission </li></ul><ul><li>Frequency range for AM - 535 kHz – 1700 kHz </li></ul><ul><li>Bandwidth - 10 kHz </li></ul>AMPLITUDE MODULATION
  10. 10.
  11. 11. Figure AM band allocation
  12. 12. <ul><li>In amplitude modulation, the amplitude of the carrier varies proportional to the instantaneous magnitude of modulating signal </li></ul><ul><li>Assuming </li></ul><ul><ul><li>Modulating signal : v m (t) = V m cos w m t </li></ul></ul><ul><ul><li>v m (t) = instantenous value of the sine wave voltage </li></ul></ul><ul><ul><li>V m (t) = peak value of the sine wave </li></ul></ul><ul><ul><li>Carrier signal : v c (t) = V c cos w c t </li></ul></ul>
  13. 13. AMPLITUDE MODULATION modulating Signal v m (t) Modulated Signal Carrier wave V c cos w c t
  14. 14. Envelope – the imaginary line on the carrier waveform Vc max V modulated signal v am
  15. 15. Carrier signal Modulating signal
  16. 16. The amplitude-modulated wave can then be expressed as
  17. 17. where notation m is termed the modulation index . It is simply a measurement for the degree of modulation and bears the relationship of V m to V c Therefore the full AM signal may be written as
  18. 18. Modulation Index m (Coefficient of Modulation/Modulation Factor/Degree 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.
  19. 19. Modulation Index m The modulation index can be determined by measuring the actual values of the modulation voltage and the carrier voltage and computing the ratio.
  20. 20. Modulation Index m Vm Vc
  21. 21. If the amplitude of the modulating signal is higher than the carrier amplitude, which in turn implies the modulation index . This will cause severe distortion to the modulated signal. Modulation Index m
  22. 22. The ideal condition for amplitude modulation (AM) is when m=1 , which also means V m =V c . This will give rise to the generation of the maximum message signal output at the receiver without distortion. Modulation Index m
  23. 23. Carrier component Upper sideband component Lower sideband component Using
  24. 24. <ul><li>   The frequency spectrum of AM waveform contains 3 parts : </li></ul><ul><ul><li>A component at the carrier frequency f c </li></ul></ul><ul><ul><li>An upper sideband (USB), whose highest frequency component is at f c +f m </li></ul></ul><ul><ul><li>A lower sideband (LSB), whose highest frequency component is at f c -f m </li></ul></ul><ul><ul><li>The bandwidth of the modulated waveform is twice the information signal bandwidth. </li></ul></ul># sideband is a component above and below centre frequency # Every sideband contains all the original message, but not the carrier
  25. 25. Amplitude Modulation <ul><li>Various forms of Amplitude Modulation </li></ul><ul><ul><li>Conventional Amplitude Modulation (Alternatively known as Full AM or Double Sideband Large carrier modulation (DSBLC) /Double Sideband Full Carrier (DSBFC) </li></ul></ul><ul><ul><li>Double Sideband Suppressed carrier (DSBSC) modulation </li></ul></ul><ul><ul><li>Single Sideband (SSB) modulation </li></ul></ul><ul><ul><li>Vestigial Sideband (VSB) modulation </li></ul></ul>
  26. 26. DSBFC Frequency Spectrum With single frequency f m B = Maximum freq. - minimum freq. = (fc+fm)-(fc-fm) = fc+fm-fc+fm = 2fm f C f c +f m f c -f m 2f m freq
  27. 27. <ul><ul><li>If f m consists of a range frequencies f 1 to f 2 , the component of the sidebands become: </li></ul></ul><ul><ul><li>Upper sideband (USB) range is from (f c +f 1 ) to (f c +f 2 ) </li></ul></ul><ul><ul><li>Lower sideband (LSB) range is from (f c -f 2 ) to (f c -f 1 ) </li></ul></ul>Amplitude Modulation ~ DSBFC (Full AM) f 1 f 2 f c -f 2 f c -f 1 f c +f 1 f c +f 2 Amplitude,V Amplitude,V Baseband signal lower sideband upper sideband Modulated signal freq freq
  28. 28. <ul><li>The previous modulated signal (DSBFC) has two drawbacks; it waste power and bandwidth </li></ul><ul><li>Power sent as the carrier contains no information and each sideband carries the same information independently </li></ul>Amplitude Modulation ~ Double Sideband Suppress Carrier (
  29. 29. f c -f m f c +f m LSB USB freq freq Frequency spectrum of a DSBSC system Total power in DSBSC <ul><li>Although, the power is improved, the bandwidth remain unchanged, </li></ul><ul><li>that is BW = 2B = 2 fmax </li></ul>Amplitude Modulation ~ DSBSC <ul><li>The double sideband suppressed carrier (DSBSC) is introduced to eliminate carrier hence improve power efficiency </li></ul><ul><li>It is a technique where it is transmitting both the sidebands without the carrier (the carrier is being suppressed) </li></ul>
  30. 30. <ul><li>The suppressed carrier is further improved by sending only one sideband </li></ul><ul><li>This not only uses less power but also only half of the bandwidth and it is called single sideband suppressed carrier (SSBSC) </li></ul>Amplitude Modulation ~ SSBSC
  31. 31. <ul><li>As both DSB and standard AM waste a lot of power and occupy large bandwidth, SSB is adopted </li></ul><ul><li>SSB is a process of transmitting one of the sidebands of the standard AM by suppressing the carrier and one of the sidebands (only transmits upper or lower sideband of AM) </li></ul><ul><li>Reduces bandwidth by factor of 2 </li></ul><ul><li>There are two possible of SSBSC </li></ul><ul><ul><li>the lower sideband V LSB = V m cos (w c -w m )t </li></ul></ul><ul><ul><li>the upper sideband V USB = V m cos (w c +w m )t </li></ul></ul>Frequency spectrum of a SSB system Total power in SSB Amplitude Modulation ~ Single Sideband (SSB) f c LSB USB LSB f c USB
  32. 32. <ul><li>SSB Applications: </li></ul><ul><li>SSB is used in the systems which require minimum bandwidth such as telephone multiplex system and it is not used in broadcasting </li></ul><ul><li>Point to point communications at frequency below 30 MHz – mobile communications, military, navigation radio etc where power saving is needed </li></ul>Amplitude Modulation ~ Single Sideband (SSB)
  33. 33. <ul><li>VSB is a technique AM transmission where the carrier, one sideband and a part of the other sideband are transmitted </li></ul>VSB application: VSB is mainly used in TV broadcasting for their video transmissions. TV signal consists of: Audio signal – is transmitted by FM Video signal – is transmitted by VSB Amplitude Modulation ~ Vestigial Sideband
  34. 34. A video signal consists of range of frequencies and maximum frequency is as high as 4.5Mhz. If it is transmitted using the conventional AM system, the required bandwidth is 9.0 Mhz (B=2fm). But according to the standardization, TV signal is limited to 6MHz only. So, to reduce to 6Mhz bandwidth, a part of the LSB is not transmitted. In this case SSB transmission is not applied as it is very difficult to suppress a sideband accurately at high frequency. Amplitude Modulation ~ Vestigial Sideband
  35. 35. Frequency spectrum of a Vestigial Sideband Amplitude Modulation ~ Vestigial Sideband Carrier for video Audio Signal (FM) Upper sideband Lower Side band f c -1.25 f c f c +4.5 4.5 MHz Carrier for audio
  36. 36. Conclusion <ul><li>Only sidebands contain the information </li></ul><ul><li>Lower and upper sideband are identical. Only one sideband is enough to recover the original signal </li></ul><ul><li>Carrier component does not contain any information but constitute 2/3 of the total power, at full modulation (m a =1) </li></ul>
  37. 37. Advantages and Disadvantages of AM <ul><li>Advantages: </li></ul><ul><ul><li>simple with proven reliability </li></ul></ul><ul><ul><li>low cost </li></ul></ul><ul><li>Disadvantages: </li></ul><ul><ul><li>wastage of power as most of the transmitted power are in the carrier component which does not contain information. When m a =1, 2/3 of the power is wasted </li></ul></ul><ul><ul><li>AM requires a bandwidth which is double to audio frequency </li></ul></ul><ul><ul><li>Noisy </li></ul></ul>
  38. 38. The total transmitted power in AM is the sum of the carrier power and the power in the sidebands. Carrier power : Sideband power: AM Power Distribution
  39. 39. Thus, at optimum operation (m = 100%), only 33% of power is used to carry information From previous equation, total current flow in AM is
  40. 40. <ul><li>As most of the signals are complex and can be represented by combination of various sine waves, m can be determined by </li></ul><ul><li>Thus, total power for this complex signal is </li></ul>