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Angle Modulation – Frequency Modulation Consider again the general carrier  represents the angle of the carrier.  There are two ways of varying the angle of the carrier. ,[object Object],[object Object],1
Frequency Modulation In FM, the message signal  m ( t ) controls the frequency  f c   of the carrier. Consider the  carrier  then for FM we may write:  FM signal  ,where the frequency deviation will depend on  m ( t ). Given that the carrier frequency will change we may write for an instantaneous  carrier signal where   i   is the instantaneous angle =  and  f i  is the instantaneous frequency. 2
Frequency Modulation Since  then  i.e.  frequency is proportional to the rate of change of angle. If  f c   is the unmodulated carrier and  f m  is the modulating frequency, then we may  deduce that  f c   is the peak deviation of the carrier.  Hence, we have  ,i.e.   3
Frequency Modulation After integration  i.e.   Hence for the FM signal,  4
Frequency Modulation The ratio  is called the  Modulation Index  denoted by     i.e.   Note – FM, as implicit in the above equation for  v s ( t ), is a non-linear process –  i.e.   the principle of superposition does not apply. The FM signal for a message  m ( t ) as a  band of signals is very complex. Hence,  m ( t ) is usually considered as a 'single tone  modulating signal' of the form  5
Frequency Modulation The equation  may be expressed as Bessel series (Bessel functions) where  J n (  ) are Bessel functions of the first kind. Expanding the equation for a few  terms we have: 6
FM Signal Spectrum. The amplitudes drawn are completely arbitrary, since we have not found any value for  J n (  ) – this sketch is only to illustrate the spectrum. 7
Generation of FM signals – Frequency Modulation. ,[object Object],[object Object],[object Object],In these devices (V/F or VCO), the output frequency is dependent on the input voltage  amplitude. 8
V/F Characteristics. Apply V IN  ,  e.g.  0 Volts, +1 Volts, +2 Volts, -1 Volts, -2 Volts, ... and measure the  frequency output for each V IN  . The ideal V/F characteristic is a straight line as  shown below. f c , the frequency output when the input is zero is called the undeviated or nominal  carrier frequency.  The gradient of the characteristic  is called the  Frequency Conversion Factor ,  denoted by    per Volt. 9
V/F Characteristics. Consider now, an analogue message input,  As the input  m ( t ) varies from  the output frequency will vary from a  maximum, through  f c ,  to a minimum  frequency.  10
V/F Characteristics. For a straight line,  y  =  c + mx , where  c  = value of  y  when  x  = 0,  m  = gradient, hence  we may say  and when V IN  =  m ( t )  ,i.e.  the deviation depends on  m ( t ). Considering that maximum and minimum input amplitudes are +V m  and -V m   respectively, then on the diagram on the previous slide.  The peak-to-peak deviation is  f max  – f min , but more importantly for FM the peak  deviation   f c  is  Peak Deviation ,  Hence,  Modulation Index ,  11
Summary of the important points of FM ,[object Object],[object Object],where  J n (  ) are Bessel coefficients and Modulation Index ,  ,[object Object],Frequency Conversion Factor ,     per Volt ,[object Object],[object Object],12
FM Signal Waveforms. The diagrams below illustrate FM signal waveforms for various inputs  At this stage, an input digital data sequence,  d ( t ), is introduced –  the output in this case will be FSK, (Frequency Shift Keying). 13
FM Signal Waveforms. Assuming  the output ‘switches’ between  f 1  and  f 0 .  14
FM Signal Waveforms. The output frequency varies ‘gradually’ from  f c  to ( f c  +   V m ), through  f c  to  ( f c  -   V m )  etc.   15
FM Signal Waveforms. If we plot  f OUT  as a function of  V IN :  In general,  m ( t ) will be a ‘band of signals’,  i.e.  it will contain amplitude and frequency  variations. Both amplitude and frequency change in  m ( t ) at the input are translated to  (just) frequency changes in the FM output signal,  i.e.  the amplitude of the output FM  signal is constant. Amplitude changes at the input are translated to deviation from the carrier at the  output. The larger the amplitude, the greater the deviation. 16
FM Signal Waveforms. Frequency changes at the input are translated to rate of change of frequency at the  output. An attempt to illustrate this is shown below: 17
FM Spectrum – Bessel Coefficients. The FM signal spectrum may be determined from The values for the Bessel coefficients,  J n (  ) may be found from  graphs or, preferably, tables of ‘Bessel functions of the first kind’.   18
FM Spectrum – Bessel Coefficients. In the series for  v s ( t ),  n  = 0 is the carrier component,  i.e.   , hence the  n  = 0 curve shows how the component at the carrier frequency,  f c , varies in amplitude,  with modulation index   .  19  J n (  )    = 2.4    = 5
FM Spectrum – Bessel Coefficients. Hence for a given value of modulation index   , the values of  J n (  ) may be read off the  graph and hence the component amplitudes ( V c J n (  )) may be determined. A further way to interpret these curves is to imagine them in 3 dimensions  20
Examples from the graph    = 0:  When    = 0 the carrier is unmodulated and  J 0 (0) = 1, all other  J n (0) = 0,  i.e.    = 2.4:  From the graph (approximately)  J 0 (2.4) = 0,  J 1 (2.4) = 0.5,  J 2 (2.4) = 0.45 and  J 3 (2.4) = 0.2 21
Significant Sidebands – Spectrum. As may be seen from the table of Bessel functions, for values of  n  above a certain  value, the values of  J n (  ) become progressively smaller. In FM the sidebands are  considered to be significant if  J n (  )    0.01 (1%). Although the bandwidth of an FM signal is infinite, components with amplitudes  V c J n (  ), for which  J n (  ) < 0.01 are deemed to be insignificant and may be ignored. Example:  A message signal with a frequency  f m  Hz modulates a carrier  f c  to produce  FM with a modulation index    = 1. Sketch the spectrum. 22
Significant Sidebands – Spectrum. As shown, the bandwidth of the spectrum containing significant  components is 6 f m , for    = 1.  23
Significant Sidebands – Spectrum. The table below shows the number of significant sidebands for various modulation  indices (  ) and the associated spectral bandwidth. e.g.  for    = 5, 16 sidebands  (8 pairs). 24
Carson’s Rule for FM Bandwidth. An approximation for the bandwidth of an FM signal is given by BW = 2(Maximum frequency deviation + highest modulated  frequency) Carson’s Rule   25
Narrowband and Wideband FM From the graph/table of Bessel functions it may be seen that for small   , (      0.3)  there is only the carrier and 2 significant sidebands,  i.e.  BW = 2 fm . FM with       0.3 is referred to as  narrowband FM  (NBFM) (Note, the bandwidth is  the same as DSBAM). For    > 0.3 there are more than 2 significant sidebands. As    increases the number of  sidebands increases. This is referred to as  wideband FM  (WBFM). Narrowband FM NBFM Wideband FM WBFM 26
VHF/FM VHF/FM (Very High Frequency band = 30MHz – 300MHz) radio transmissions, in the  band 88MHz to 108MHz have the following parameters: Max frequency input ( e.g.  music) 15kHz  f m   Deviation 75kHz  Modulation Index   5  For    = 5 there are 16 sidebands and the FM signal bandwidth is 16 fm  = 16 x 15kHz = 240kHz. Applying Carson’s Rule BW = 2(75+15) = 180kHz. 27
Comments FM ,[object Object],[object Object],FM signal,  ,[object Object],[object Object],[object Object],where  thus the component at the carrier frequency depends on  m ( t ), as do all the  other components and none may be suppressed. 28
Comments FM ,[object Object],[object Object],[object Object],[object Object],means that a large bandwidth is required – called  WBFM. ,[object Object],[object Object],[object Object],[object Object],29
Power in FM Signals. From the equation for FM we see that the peak value of the components is  V c J n (  ) for the  n th  component.  Single normalised average power =  then the  n th  component is  Hence, the total power in the infinite spectrum is Total power  30
Power in FM Signals. By this method we would need to carry out an infinite number of calculations to find  P T . But, considering the waveform, the peak value is  V ­c , which is constant. Since we know that the RMS value of a sine wave is  and power = ( V RMS ) 2  then we may deduce that Hence, if we know  V c  for the FM signal, we can find the total power  P T  for the infinite  spectrum with a simple calculation. 31
Power in FM Signals. Now consider – if we generate an FM signal, it will contain an infinite number of  sidebands. However, if we wish to transfer this signal,  e.g.  over a radio or cable, this  implies that we require an infinite bandwidth channel. Even if there was an infinite  channel bandwidth it would not all be allocated to one user. Only a limited bandwidth is available for any particular signal. Thus we have to make the signal spectrum fit into the available channel bandwidth. We can think of the signal spectrum as a ‘train’ and  the channel bandwidth as a tunnel – obviously we make the train slightly less wider  than the tunnel if we can. 32
Power in FM Signals. However, many signals ( e.g.  FM, square waves, digital signals) contain an infinite  number of components. If we transfer such a signal via a limited channel bandwidth,  we will lose some of the components and the output signal will be distorted. If we put  an infinitely wide train through a tunnel, the train would come out distorted, the  question is how much distortion can be tolerated? Generally speaking, spectral components decrease in amplitude as we move away  from the spectrum ‘centre’. 33
Power in FM Signals. In general distortion may be defined as With reference to FM the minimum channel bandwidth required would be just wide  enough to pass the spectrum of significant components. For a bandlimited FM  spectrum, let  a  = the number of sideband pairs,  e.g.  for    = 5,  a =  8 pairs  (16 components). Hence, power in the bandlimited spectrum  P BL  is = carrier power + sideband powers. 34
Power in FM Signals. Since  Distortion  Also, it is easily seen that the ratio  = 1 – Distortion  i.e.  proportion p f  power in bandlimited spectrum to total power =  35
Example  Consider NBFM, with    = 0.2. Let  V c  = 10 volts. The total power in the infinite  spectrum  = 50 Watts,  i.e.   = 50 Watts.  From the table – the significant components are i.e.  the carrier + 2 sidebands contain  or 99% of the total power  36
Example Distortion =  or 1%.  Actually, we don’t need to know  V c ,  i.e.  alternatively Distortion =  ( a  = 1)  D =  Ratio  37
FM Demodulation –General Principles. ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],38
FM Demodulation –General Principles. ,[object Object],[object Object],[object Object],[object Object],We define  V o  as the output when  f IN  =  f c , the nominal input frequency.  39
FM Demodulation –General Principles. The gradient  is called the voltage conversion factor  i.e.   Gradient = Voltage Conversion Factor ,  K  volts per Hz. Considering  y  =  mx  +  c   etc . then we may say  V OUT  =  V 0  +  Kf IN  from the frequency  modulator, and since  V 0  =  V OUT  when  f IN  =  f c  then we may write where  V 0  represents a DC offset in  V OUT .  This DC offset may be removed by level  shifting or AC coupling, or the F/V may be designed with the characteristic shown next 40
FM Demodulation –General Principles. The important point is that  V OUT  =  K  V IN . If  V IN  =  m ( t ) then the output contains the  message signal  m ( t ), and the FM signal has been demodulated.  41
FM Demodulation –General Principles. Often, but not always, a system designed so that  , so that  K   = 1 and  V OUT  =  m ( t ).  A complete system is illustrated.  42
FM Demodulation –General Principles. 43
Methods Tuned Circuit  – One method (used in the early days of FM) is to use the slope of a  tuned circuit in conjunction with an envelope detector. 44
Methods ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],45
Foster-Seeley Discriminator  This gives the composite characteristics shown. Diode  D 2  effectively inverts the  f 2   tuned circuit response. This gives the characteristic  ‘S’  type detector.  46
Phase Locked Loops PLL ,[object Object],[object Object],[object Object],[object Object],[object Object],47
Phase Locked Loops PLL ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],48

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Ref angle modulation

  • 1.
  • 2. Frequency Modulation In FM, the message signal m ( t ) controls the frequency f c of the carrier. Consider the carrier then for FM we may write: FM signal ,where the frequency deviation will depend on m ( t ). Given that the carrier frequency will change we may write for an instantaneous carrier signal where  i is the instantaneous angle = and f i is the instantaneous frequency. 2
  • 3. Frequency Modulation Since then i.e. frequency is proportional to the rate of change of angle. If f c is the unmodulated carrier and f m is the modulating frequency, then we may deduce that  f c is the peak deviation of the carrier. Hence, we have ,i.e. 3
  • 4. Frequency Modulation After integration i.e. Hence for the FM signal, 4
  • 5. Frequency Modulation The ratio is called the Modulation Index denoted by  i.e. Note – FM, as implicit in the above equation for v s ( t ), is a non-linear process – i.e. the principle of superposition does not apply. The FM signal for a message m ( t ) as a band of signals is very complex. Hence, m ( t ) is usually considered as a 'single tone modulating signal' of the form 5
  • 6. Frequency Modulation The equation may be expressed as Bessel series (Bessel functions) where J n (  ) are Bessel functions of the first kind. Expanding the equation for a few terms we have: 6
  • 7. FM Signal Spectrum. The amplitudes drawn are completely arbitrary, since we have not found any value for J n (  ) – this sketch is only to illustrate the spectrum. 7
  • 8.
  • 9. V/F Characteristics. Apply V IN , e.g. 0 Volts, +1 Volts, +2 Volts, -1 Volts, -2 Volts, ... and measure the frequency output for each V IN . The ideal V/F characteristic is a straight line as shown below. f c , the frequency output when the input is zero is called the undeviated or nominal carrier frequency. The gradient of the characteristic is called the Frequency Conversion Factor , denoted by  per Volt. 9
  • 10. V/F Characteristics. Consider now, an analogue message input, As the input m ( t ) varies from the output frequency will vary from a maximum, through f c , to a minimum frequency. 10
  • 11. V/F Characteristics. For a straight line, y = c + mx , where c = value of y when x = 0, m = gradient, hence we may say and when V IN = m ( t ) ,i.e. the deviation depends on m ( t ). Considering that maximum and minimum input amplitudes are +V m and -V m respectively, then on the diagram on the previous slide. The peak-to-peak deviation is f max – f min , but more importantly for FM the peak deviation  f c is Peak Deviation , Hence, Modulation Index , 11
  • 12.
  • 13. FM Signal Waveforms. The diagrams below illustrate FM signal waveforms for various inputs At this stage, an input digital data sequence, d ( t ), is introduced – the output in this case will be FSK, (Frequency Shift Keying). 13
  • 14. FM Signal Waveforms. Assuming the output ‘switches’ between f 1 and f 0 . 14
  • 15. FM Signal Waveforms. The output frequency varies ‘gradually’ from f c to ( f c +  V m ), through f c to ( f c -  V m ) etc. 15
  • 16. FM Signal Waveforms. If we plot f OUT as a function of V IN : In general, m ( t ) will be a ‘band of signals’, i.e. it will contain amplitude and frequency variations. Both amplitude and frequency change in m ( t ) at the input are translated to (just) frequency changes in the FM output signal, i.e. the amplitude of the output FM signal is constant. Amplitude changes at the input are translated to deviation from the carrier at the output. The larger the amplitude, the greater the deviation. 16
  • 17. FM Signal Waveforms. Frequency changes at the input are translated to rate of change of frequency at the output. An attempt to illustrate this is shown below: 17
  • 18. FM Spectrum – Bessel Coefficients. The FM signal spectrum may be determined from The values for the Bessel coefficients, J n (  ) may be found from graphs or, preferably, tables of ‘Bessel functions of the first kind’. 18
  • 19. FM Spectrum – Bessel Coefficients. In the series for v s ( t ), n = 0 is the carrier component, i.e. , hence the n = 0 curve shows how the component at the carrier frequency, f c , varies in amplitude, with modulation index  . 19  J n (  )  = 2.4  = 5
  • 20. FM Spectrum – Bessel Coefficients. Hence for a given value of modulation index  , the values of J n (  ) may be read off the graph and hence the component amplitudes ( V c J n (  )) may be determined. A further way to interpret these curves is to imagine them in 3 dimensions 20
  • 21. Examples from the graph  = 0: When  = 0 the carrier is unmodulated and J 0 (0) = 1, all other J n (0) = 0, i.e.  = 2.4: From the graph (approximately) J 0 (2.4) = 0, J 1 (2.4) = 0.5, J 2 (2.4) = 0.45 and J 3 (2.4) = 0.2 21
  • 22. Significant Sidebands – Spectrum. As may be seen from the table of Bessel functions, for values of n above a certain value, the values of J n (  ) become progressively smaller. In FM the sidebands are considered to be significant if J n (  )  0.01 (1%). Although the bandwidth of an FM signal is infinite, components with amplitudes V c J n (  ), for which J n (  ) < 0.01 are deemed to be insignificant and may be ignored. Example: A message signal with a frequency f m Hz modulates a carrier f c to produce FM with a modulation index  = 1. Sketch the spectrum. 22
  • 23. Significant Sidebands – Spectrum. As shown, the bandwidth of the spectrum containing significant components is 6 f m , for  = 1. 23
  • 24. Significant Sidebands – Spectrum. The table below shows the number of significant sidebands for various modulation indices (  ) and the associated spectral bandwidth. e.g. for  = 5, 16 sidebands (8 pairs). 24
  • 25. Carson’s Rule for FM Bandwidth. An approximation for the bandwidth of an FM signal is given by BW = 2(Maximum frequency deviation + highest modulated frequency) Carson’s Rule 25
  • 26. Narrowband and Wideband FM From the graph/table of Bessel functions it may be seen that for small  , (   0.3) there is only the carrier and 2 significant sidebands, i.e. BW = 2 fm . FM with   0.3 is referred to as narrowband FM (NBFM) (Note, the bandwidth is the same as DSBAM). For  > 0.3 there are more than 2 significant sidebands. As  increases the number of sidebands increases. This is referred to as wideband FM (WBFM). Narrowband FM NBFM Wideband FM WBFM 26
  • 27. VHF/FM VHF/FM (Very High Frequency band = 30MHz – 300MHz) radio transmissions, in the band 88MHz to 108MHz have the following parameters: Max frequency input ( e.g. music) 15kHz f m Deviation 75kHz Modulation Index  5 For  = 5 there are 16 sidebands and the FM signal bandwidth is 16 fm = 16 x 15kHz = 240kHz. Applying Carson’s Rule BW = 2(75+15) = 180kHz. 27
  • 28.
  • 29.
  • 30. Power in FM Signals. From the equation for FM we see that the peak value of the components is V c J n (  ) for the n th component. Single normalised average power = then the n th component is Hence, the total power in the infinite spectrum is Total power 30
  • 31. Power in FM Signals. By this method we would need to carry out an infinite number of calculations to find P T . But, considering the waveform, the peak value is V ­c , which is constant. Since we know that the RMS value of a sine wave is and power = ( V RMS ) 2 then we may deduce that Hence, if we know V c for the FM signal, we can find the total power P T for the infinite spectrum with a simple calculation. 31
  • 32. Power in FM Signals. Now consider – if we generate an FM signal, it will contain an infinite number of sidebands. However, if we wish to transfer this signal, e.g. over a radio or cable, this implies that we require an infinite bandwidth channel. Even if there was an infinite channel bandwidth it would not all be allocated to one user. Only a limited bandwidth is available for any particular signal. Thus we have to make the signal spectrum fit into the available channel bandwidth. We can think of the signal spectrum as a ‘train’ and the channel bandwidth as a tunnel – obviously we make the train slightly less wider than the tunnel if we can. 32
  • 33. Power in FM Signals. However, many signals ( e.g. FM, square waves, digital signals) contain an infinite number of components. If we transfer such a signal via a limited channel bandwidth, we will lose some of the components and the output signal will be distorted. If we put an infinitely wide train through a tunnel, the train would come out distorted, the question is how much distortion can be tolerated? Generally speaking, spectral components decrease in amplitude as we move away from the spectrum ‘centre’. 33
  • 34. Power in FM Signals. In general distortion may be defined as With reference to FM the minimum channel bandwidth required would be just wide enough to pass the spectrum of significant components. For a bandlimited FM spectrum, let a = the number of sideband pairs, e.g. for  = 5, a = 8 pairs (16 components). Hence, power in the bandlimited spectrum P BL is = carrier power + sideband powers. 34
  • 35. Power in FM Signals. Since Distortion Also, it is easily seen that the ratio = 1 – Distortion i.e. proportion p f power in bandlimited spectrum to total power = 35
  • 36. Example Consider NBFM, with  = 0.2. Let V c = 10 volts. The total power in the infinite spectrum = 50 Watts, i.e. = 50 Watts. From the table – the significant components are i.e. the carrier + 2 sidebands contain or 99% of the total power 36
  • 37. Example Distortion = or 1%. Actually, we don’t need to know V c , i.e. alternatively Distortion = ( a = 1) D = Ratio 37
  • 38.
  • 39.
  • 40. FM Demodulation –General Principles. The gradient is called the voltage conversion factor i.e. Gradient = Voltage Conversion Factor , K volts per Hz. Considering y = mx + c etc . then we may say V OUT = V 0 + Kf IN from the frequency modulator, and since V 0 = V OUT when f IN = f c then we may write where V 0 represents a DC offset in V OUT . This DC offset may be removed by level shifting or AC coupling, or the F/V may be designed with the characteristic shown next 40
  • 41. FM Demodulation –General Principles. The important point is that V OUT = K  V IN . If V IN = m ( t ) then the output contains the message signal m ( t ), and the FM signal has been demodulated. 41
  • 42. FM Demodulation –General Principles. Often, but not always, a system designed so that , so that K  = 1 and V OUT = m ( t ). A complete system is illustrated. 42
  • 43. FM Demodulation –General Principles. 43
  • 44. Methods Tuned Circuit – One method (used in the early days of FM) is to use the slope of a tuned circuit in conjunction with an envelope detector. 44
  • 45.
  • 46. Foster-Seeley Discriminator This gives the composite characteristics shown. Diode D 2 effectively inverts the f 2 tuned circuit response. This gives the characteristic ‘S’ type detector. 46
  • 47.
  • 48.