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F1 x F2 

Sum and Mixing of Frequencies. F1 x F2 . eam=EcSin(Wct)+mEc/2Cos(Wc-Wm)t-mEc/2Cos(Wc+Wm)t. USB. LSB. Carrier. f USB = fc + fm and f LSB = fc − fm. Figure 3-8: The relationship between the time and frequency domains. Sidebands and the Frequency Domain. E max − E min. Ea =.

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F1 x F2 

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  1. Sum and Mixing of Frequencies F1 x F2 

  2. eam=EcSin(Wct)+mEc/2Cos(Wc-Wm)t-mEc/2Cos(Wc+Wm)t USB LSB Carrier fUSB = fc + fm and fLSB = fc − fm

  3. Figure 3-8: The relationship between the time and frequency domains. Sidebands and the Frequency Domain

  4. Emax−Emin Ea = 2 Calculatiom of modulation index by envelope m =Ea / Ec Ec = Emax - Ea

  5. Ec mEc/2 mEc/2 Fc Fc+Fm Fc-Fm BW = fUSB−fLSB=2fm Ec/2 Ec/2 mEc/4 mEc/4 mEc/4 mEc/4 -Fc Fc-Fm Fc Fc+Fm -Fc-Fm -Fc+Fm Two sided spectrum

  6. Property of Active Device

  7. Block Diagram of a Simple AM Transmitter

  8. Power relations in AM PT = (IT)2R where IT is measured RF current and R is antenna impedance PT=PC+PUSB+PLSB

  9. Power relations in AM in terms of current PT=PC+PUSB+PLSB PT = (IT)2R Pc = (Ic)2R But

  10. Transmission Efficiency Useful Power /Total power Percent efficiency

  11. Modulation by several sinewaves Two modulating signals are given by X1(t)=Em1CosWm1t X2(t)=Em2CosWm2t ec=Ec CosWct Carrier wave eam=A CosWct where A=Ec+X1(t)+X2(t) eam= (Ec+Em1CosWm1t + Em2CosWm2t) CosWct eam= Ec(1+Em1/Ec CosWm1t + Em2/ Ec CosWm2t) CosWct eam= Ec(1+m1 CosWm1t + m2 CosWm2t) CosWct eam= Ec CosWct+m1Ec/2 Cos(Wc+Wm1)t + m1Ec/2 Cos(Wc-Wm1)t+m2Ec/2 Cos(Wc+Wm2)t+m2Ec/2 Cos(Wc-Wm2)t Ec m2Ec/2 m1Ec/2 m1Ec/2 BW=2fm2 m2Ec/2 Fc-fm2 Fc Fc+fm2 Fc+fm1 Fc-fm1

  12. Total power in AM Wave= Pt=PUSB1+PUSB2+PLSB1+PLSB2 Modulation Index

  13. Power relations in AM PT = (IT)2R where IT is measured RF current and R is antenna impedance PT=PC+PUSB+PLSB

  14. Power relations in AM in terms of current PT=PC+PUSB+PLSB PT = (IT)2R Pc = (Ic)2R But

  15. Transmission Efficiency Useful Power /Total power Percent efficiency

  16. Modulation by several sinewaves Two modulating signals are given by X1(t)=Em1CosWm1t X2(t)=Em2CosWm2t ec=Ec CosWct Carrier wave eam=A CosWct where A=Ec+X1(t)+X2(t) eam= (Ec+Em1CosWm1t + Em2CosWm2t) CosWct eam= Ec(1+Em1/Ec CosWm1t + Em2/ Ec CosWm2t) CosWct eam= Ec(1+m1 CosWm1t + m2 CosWm2t) CosWct eam= Ec CosWct+m1Ec/2 Cos(Wc+Wm1)t + m1Ec/2 Cos(Wc-Wm1)t+m2Ec/2 Cos(Wc+Wm2)t+m2Ec/2 Cos(Wc-Wm2)t Ec m2Ec/2 m1Ec/2 m1Ec/2 BW=2fm2 m2Ec/2 Fc-fm2 Fc Fc+fm2 Fc+fm1 Fc-fm1

  17. Total power in AM Wave= Pt=PUSB1+PUSB2+PLSB1+PLSB2 Modulation Index

  18. Amplitude Modulators • There are two types of amplitude modulators. They are low-level and high-level modulators. • Low-level modulators generate AM with small signals and must be amplified before transmission. • High-level modulators produce AM at high power levels, usually in the final amplifier stage of a transmitter. • Modulators are class C amplifiers and at output tank circuit.

  19. Low level AM Transmitter 540KHz to 1640KHz

  20. 540KHz to 1640KHz modulator

  21. Amplifier Classes Class A - bias point is set so that the amplifier conducts through a complete cycle (360 deg) of the input waveform. This class has low efficiency (~35%) but high linearity. Class AB - bias point is set so that the amplifier conducts through at least 180 deg but less than 360deg of the input waveform. This class has better efficiency (~55%) but lower linearity. Class B - bias point is set so that the amplifier conducts through a half cycle (180 deg) of the input waveform. This class has higher efficiency (~60%),but poor linearity. Class C - bias point is set so that the amplifier conducts through less than 180 deg of the input waveform. This class has higher efficiency (~70%), but even poorer linearity

  22. Use of Tank circuit

  23. Low level Class C Grid Modulator

  24. High level Plate Modulator

  25. Low level Transistor Modulator

  26. Low level and High level AM Transmitter 540KHz to 1640KHz modulator

  27. Advantages of DSBFC = 1.Transmitters are less complex 2.Receivers are simple, detection is easy. 3.Cost efficient. Disadvantages = 1.Power wastage - carrier doesn’t carry any information and USB & LSB contains same information. 2. Needs larger Bandwidth 3. Gets affected by noise.

  28. Balanced Modulator Modulating signal • Types of AM= • DSBFC • DSBSC • SSB • ISB • VSB 180 phase shift DSBSC carrier

  29. mEc/2 mEc/2 Fc Fc+Fm Fc-Fm • DSB-SC Generation Methods • Ring Balanced Modulator • Lattice Balanced Modulator • Push pull Balanced modulator eam=mEc/2Cos(Wc-Wm)t-mEc/2Cos(Wc+Wm)t 180 phase shift Ec BW=2fm

  30. Balanced Modulator 1.Ring modulator 2.Lattice-type balanced modulator.

  31. Lattice Modulator + - - +

  32. Push Pull Balanced Modulator Drain Current inet = aem + 2becem Modulating signal Two side bands

  33. AM Waveforms

  34. mEc/2 mEc/2 mEc/2 mEc/2 Fc Fc+Fm Fc-Fm Fc Fc+Fm Fc-Fm • SSB Generation Methods • Filter Method • Phase shift method • Third method (Weaver method) BW=fm

  35. SSB Circuits Figure 4-31 An SSB transmitter using the filter method.

  36. This technique can be used at relatively low carrier frequencies. At high frequencies, the Q of the filter becomes unacceptably high. The required Q necessary to filter off one of the sidebands can be approximated by:

  37. SSB Circuits Figure 4-33 An SSB generator using the phasing method.

  38. SSB phase shift

  39. Important points= • Sharp cutoff Filters are not required • Freq Up conversion is not required • Easy to switch between sidebands. Simply change the oscillator position. • Designing a phase shift network for AF range is dificult.

  40. Weaver Method or Third method LSB

  41. Independent side band transmitter 10MHz to 30MHz

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